Note: Descriptions are shown in the official language in which they were submitted.
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A SYSTEM AND METHOD FOR GENERATING AND RECORDING
AUDITORY STEADY-STATE RESPONSES WITH A SPEECH-LIKE
STIMULUS
TECHNICAL FIELD
The present application relates to a method of recording Auditory Evoked
Potentials
(AEP), in particular Auditory Steady State Responses (ASSR). The disclosure
relates specifically to a method of recording auditory evoked potentials of a
person
(a human being). The application also relates to a data processing system
comprising a processor and program code means for causing the processor to
perform at least some of the steps of the method.
The application furthermore relates to a system for recording auditory evoked
potentials of a person, and to its use.
Embodiments of the disclosure may e.g. be useful in applications such as
diagnostic instruments for verifying fitting of a hearing aid.
BACKGROUND
When transient sounds are presented to human subjects, the summed response
from many remotely located neurons in the brain can be recorded via non-
invasive
electrodes (e.g. attached to the scalp and/or located in an ear canal of a
person).
These auditory evoked potentials (AEPs) can be recorded from all levels of the
auditory pathway, e.g. from the auditory nerve (compound action potential,
CAP);
from the brainstem (auditory brainstem response, ABR); up to the cortex
(cortical
auditory evoked potential, CAEP), etc. These classical AEPs are obtained by
presenting transient acoustic stimuli at slow repetition rates. At more rapid
rates,
the responses to each stimulus overlap with those evoked by the preceding
stimulus
to form a steady-state response (as defined in [Picton et al., 1987]). In
early studies,
such auditory steady-state responses (ASSR) were also evoked by sinusoidally
amplitude modulated (AM) pure tones. Due to the tonotopic organisation of the
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inner ear (the cochlea) and auditory pathway, the carrier frequency for AM
tones
determines the region of the basilar membrane within the cochlea being
excited, but
producing evoked responses that follow the modulation frequency. In this way,
ASSR has proved to be an efficient tool for testing different frequency
locations
within the auditory pathway. AM tones are the simplest frequency specific
stimuli
used to evoke ASSRs, but they only stimulate a small number of auditory nerve
fibres resulting in relatively small response amplitude. This small response
can be
problematic for response resolution, so various methods have been developed to
increase area of excitation in the cochlea to recruit more auditory nerve
fibres, and
to increase response amplitude and hence response detection and accuracy.
W02006003172A1 (US 8,591,433 B2) describes the design of frequency-specific
electrical or acoustical stimuli for recording ASSR as a combination of a
series of
multiple spectral components (i.e. pure tones), designed to optimise response
amplitudes. This was achieved by pre-compensating for the frequency-dependent
delay introduced in the inner ear (cochlea) to achieve more synchronised
auditory
nerve firing across frequency. It is a basic characteristic of an auditory
evoked
response that the magnitude of the response depends on the number of auditory
units/nerve fibres that are activated synchronously by the stimulus (as
defined in
[Eggermont, 1977]). By compensating for the inherent cochlea frequency-
dependent delay, and specifically defining the magnitude and phase of each of
the
multiple spectral components, a repetitive frequency glide or chirp can be
created.
The repetition rate of this chirp-train is determined from the frequency
spacing
between the spectral components used in its generation, and the band-width is
controlled by the number of components chosen. In this way, a very flexible
and
efficient stimulus for recording ASSR can be created (cf. e.g. [Elberling,
2005],
[Elberling et al., 2007a], [Elberling et al., 2007b] and [CebuIla et al.,
2007]).
One of the major advantages of ASSR is the ability to perform simultaneous
multi-
band recordings, i.e. to present multiple stimuli at different carrier
frequencies to
both ears ¨ each with different repetition rates, and hence detection and
observation of these potentials are typically made in the frequency domain. By
creating the stimulus to be periodic, the stimulus and response structure in
the
frequency domain are well-defined and importantly predictable, thus ASSR lends
itself well to automatic detection algorithms based on sets of harmonics of
the
repetition rates.
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At present, ASSRs in the clinic and in research are stimulated using repeated
trains
of broad-band and narrow-band chirps, amplitude modulated tones, combined
amplitude and frequency modulated tones, and trains of clicks and tone-bursts.
As
a result of highly successful universal new-born screening programs in many
countries, paediatric audiologists are now routinely seeing patients within
the first-
few weeks of life (Chang et al., 2012). Therefore, it is advantageous to have
hearing
aid fitting protocols designed for young infants, as the sooner the
intervention then
the better the clinical outcomes. Thus the use of ASSR for estimating
audiometric
threshold is fast gaining ground for use with neonates referred from a
screening
programme. Determining these patients' thresholds via behavioral measures is
highly unreliable or impossible, hence the need for objective physiological
methods.
Regression statistics exist to calculate the expected difference between
physiologic
and behavioural thresholds at each sound level. Accurate threshold estimation
depends on being able to determine whether a small response exists in the
presence of residual background noise. In addition to infants, ASSR and
objective
measures are used with hard to test adults, i.e. adults with severe mental or
physical impairment.
Once a hearing aid is fitted to a particular user, then parameters need to be
adjusted, for example to ensure that gain is set such that the speech spectrum
is
amplified within the dynamic hearing range of the subject. For good practice,
a
verification of this fitting needs to be made to ensure that this is in fact
the case.
This is particularly important for infants because of their inability to take
part in
behavioral testing, and because the acoustic properties of infants' very small
ear
canals vary dramatically among individuals pagatto et al., 2002]. Hence, a
robust
objective method for doing this is important. Sound field auditory evoked
potentials
¨ ABR, CAEP and ASSR have all been proposed as potential methods for doing
this. It has been shown that ABRs are inappropriate, as the stimuli used to
evoke
them are typically very short (< 10 ms), and hearing aid processing distorts
the
stimulus, making the response waveform hard to interpret.
CAEPs are growing in popularity for verification of hearing aid fitting. In
particular,
CAEPs evoked from short-duration phonemes and speech-like stimuli are argued
to
reflect neural encoding of speech and provide objective evidence that the
amplified
speech has been detected. CAEPs have documented disadvantages that they are
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strongly affected by the attention of the subject, which is hard to control in
infants.
Also, objective detection of response waveforms is challenging as the
waveforms/evoked potentials vary significantly across subjects. Finally, even
though they are longer in duration than ABR stimuli, typical stimuli to evoke
CAEPs
are still relatively short, and hence are subject to the same distortion and
disadvantage as described above.
SUMMARY
The present disclosure relates to the field of recording Auditory Evoked
Potentials
(AEPs) from human participants. The present disclosure aims at reducing at
least
some of the disadvantages of prior art solutions. Specifically the disclosure
focuses
on a new technique for stimulating Auditory Steady State Responses with a more
speech-like signal, including amplitude variations over time that are similar
to free-
running speech.
This present disclosure is concerned with making ASSR stimuli more speech-
like,
while still retaining their advantageous properties. This may be used in the
verification of hearing aid fitting application described above, as it
circumvents some
of the challenges seen with cortical evoked responses. However, equally the
method may be used to record an unaided response, with the advantage of
exciting
the auditory system with a signal capable of assessing its ability to process
speech.
In agreement with the propositions of John & Picton (2004) the present
disclosure
employs spectrally shaping a multi-band ASSR stimuli to have a normal long-
term
speech spectrum. Imposed on this will be a low-frequency amplitude modulation
similar to the envelope seen in free-running speech (cf. e.g. [Plomp; 1984]).
For
sounds, which convey information, such as speech, much of the information is
carried in the changes in the stimulus, rather than in the parts of the sound
that are
relatively stable. Normal multi-band ASSR with different separate carrier
frequencies or bands will for each band have a different repetition rate. This
allows
multiple band detection of the response in the frequency domain. The different
repetition rates and bands summed up in the time domain will result in a
stimulus
that has a waveform that will vary over time. The present disclosure proposes
additionally several ways to apply a low-frequency amplitude modulation in the
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range of the normal modulation spectrum of speech (cf. e.g. [Plomp; 1984]...
<20
Hz), in order to make the stimulus more speech-like, i.e. to have realistic
amplitude
fluctuations similar to free-running speech. [John & Picton, 20043 suggested
applying a broadband envelope with magnitudes similar to those of a real
speech
5 envelope. However, real speech modulations are not the same across the whole
frequency range, as is clearly seen from the frequency-band-specific speech
modulation spectra presented by [Horube et at., 2010]. Therefore, the present
disclosure proposes to apply independent speech envelopes to each of the
stimulus
bands described above. Furthermore, it is proposed to use envelopes estimated
directly from real speech in order to create a stimulus more akin to real
speech.
How a hearing aid (HA) processes incoming auditory stimuli is extremely
important
if the desired application is using ASSR for verification of fitting. A HA is
a medical
device designed to amplify incoming sounds to make them audible for the
wearers,
and to improve their speech intelligibility. Modern digital HAs are complex
nonlinear
and non-stationary (time-varying) devices, that change their state or mode of
operation based on an on-going estimation of the type and characteristics of
the
sound being presented to them. For instance, the amount of dynamic gain being
applied may depend on whether the stimulus is speech, music, random noise or
dynamic environmental sounds, objectively estimated in the signal processing
algorithms of the HAs. In verification of fitting applications in infants and
hard to
test individuals (e.g. adults), we are predominantly interested in whether
that
individual's HAs are programmed such that speech at normal listening levels is
amplified to be within the listener's audible range. It is important then,
that any new
ASSR stimulus is processed by the HA in a similar way to real free-running
speech.
The present disclosure proposes to modify the ASSR stimuli to be more speech-
like, e.g. by introducing appropriate amplitude variations over time.
Insertion gain
measurements from different HAs can be made according to IEC 60118-15 (2012),
to verify that the stimuli are being processed in a speech-like manner.
The exact neuro-physiological generator site is ambiguous with ASSR, as the
whole
auditory system responds when excited. Changing the repetition rates of the
multi-
band ASSR stimulus shifts the region of the auditory system that dominates the
power of the response that is recorded at the surface of the scull. Auditory
steady
state responses at repetition rates < 20/s are believed to be predominantly of
cortical origin. For rates > 20/s, the responses are generally believed to be
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generated by both cortical and brainstem sources, with the strength of the
cortical
activity decreasing with increasing repetition rate. Above approximately 50 Hz
it is
generally accepted that brainstem sources dominate. Special mention should be
given to the rates close to 40/s, here the responses are generated both by the
brainstem, the primary auditory cortex and thalamocortical circuits (cf. e.g.
[Picton
et al.; 20031 and [Kuwada et al.; 2002]). Responses higher in the auditory
pathway
than the brainstem are affected by attention and state of arousal. Also there
is a
significant maturational effect for cortical responses ¨ with neonates' having
small
40/s ASSR amplitudes, due to this. However rates in the range around 70/s to
100/s produce robust and solid responses, that are not overly affected by
state of
arousal and certainly not attention, maturational effects attributed to neural
immaturity and developmental changes in the acoustics of the ear canal or
middle
ear. Therefore, by varying repetition rate it is possible to shift the
location of the
ASSR generation mechanism to different locations in the auditory pathway (i.e.
brainstem to cortex) depending on what is needed for the application in
question.
The modification to the ASSR stimulus proposed here presents a new approach to
obtain speech-like stimuli. In embodiments of the disclosure, information
about the
neural encoding of speech stimuli may be extracted, whilst retaining the
excellent
detection statistics in ASSR recording.
An object of the present application is to excite the auditory system with a
signal
capable of assessing the auditory system's ability to process speech. A
further
object of embodiments of the application is to create a stimulus for driving a
hearing
aid in a speech-like manner in order to objectively asses the hearing ability
of a user
while wearing the hearing aid. A further object of embodiments of the
application is
to create a stimulus, which in addition to the abovementioned advantages
allows
simultaneous estimation of the aided electrophysiological response to several
distinct speech levels. This information may be used to guide modifications to
the
gain settings of the hearing aid. Ultimately, this information may be used in
an
automated hearing-aid fitting procedure.
Objects of the application are achieved by the invention described in the
accompanying claims and as described in the following.
7
A method for recording auditory steady-state responses of a person:
In an aspect of the present application, an object of the application is
achieved by a
method for recording auditory steady-state responses of a person, the method
comprising a) providing an acoustic stimulus signal to an ear of the person,
b)
recording the auditory steady-state responses of the person originating from
said
acoustic stimulus signal. The method provides that, the acoustic stimulus
signal
comprises a speech-like stimulus. Preferably, the method comprises that the
speech-like stimulus is provided as a combination (e.g. a summation, or a
weighted
sum) of a series of frequency-specific stimuli, each having a specified (e.g.
predetermined) frequency bandwidth, presentation rate, amplitude, and
amplitude
modulation. Moreover, the individual frequency-specific stimuli are
independently
amplitude modulated by an envelope representing band-specific speech
modulations determined from a real speech signal, or an artificial speech
signal,
which is bandpass filtered into frequency bands corresponding to the stimulus
frequency bands, and where the speech envelopes are determined independently
for each band, providing that the amplitude modulations are quantized time
and/or
level, by respectively Quantizing envelope gains in time to make them constant
across each epoch to minimize temporal distortion to the individual stimuli,
and
Quantizing the envelope gains to represent specific desired stimulus levels.
An advantage of the disclosure is that it allows a clinical assessment of the
effect of
a hearing device in a normal mode of operation, i.e. when processing speech
stimuli.
An aim of the present disclosure is to provide a new way of evoking an ASSR,
using
a speech-like stimulus, yet retaining the advantages of traditional approaches
where
the stimulus and response structure in the frequency domain are well-defined
and
predictable. This is e.g. achieved by using automatic algorithms for detection
or
amplitude (power) estimation based on a set of harmonics of the repetition
rates
within the different frequency bands of the stimulus (such frequency bands are
in the
following termed 'stimulus frequency bands').
Date Regue/Date Received 2023-02-02
7a
In an embodiment, the method comprises
al) designing an electrical stimulus signal representing a speech-like signal;
a2) converting said electrical stimulus signal to an acoustic stimulus signal;
a3) applying said acoustic stimulus signal to an ear of the person.
Preferably, the application of the acoustic stimulus to an ear of the person
is
performed in a free-field configuration (from a loudspeaker, e.g. a
directional
loudspeaker that is located outside the ear canal of the person).
Altematively, the
acoustic stimulus may be applied to the eardrum of the person (only) by a
loudspeaker of a hearing device worn at or in an ear of the user. In the
latter case,
the stimuli may be transmitted to or generated in the hearing device.
Date Regue/Date Received 2023-02-02
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In an embodiment, the method comprises recording said auditory evoked
responses
of the person
= When the person is wearing a hearing device at the ear; as well as
= When the person is not wearing the hearing device at the ear.
It is anticipated that the person has a hearing impairment at one or both
ears, and
that the hearing device (e.g. a hearing aid) is configured for compensating
for a
hearing impairment of an ear of the person exhibiting such hearing impairment.
It is understood that the hearing device, when exposed to the acoustic
stimulus, is
turned on. In an embodiment, the person has a hearing impairment at one or
both
ears. Preferably, the hearing device (e.g. a hearing aid) is configured to
compensate for a hearing impairment of that ear of the person.
In an embodiment, the speech-like stimulus is created as an electric stimulus
signal,
which is converted to an acoustic signal (e.g. by an electro-acoustic
transducer, e.g.
a vibrator or a loudspeaker). In an embodiment, the speech-like stimulus is
created
as an acoustic signal composed of a number of individual acoustic signal
components that together (e.g. when mixed, e.g. added) represent the speech-
like
stimulus.
In an embodiment, the method comprises that the presentation rates of the
individual frequency-specific stimuli are configurable. In an embodiment, the
method
comprises that the presentation rates of the individual frequency-specific
stimuli are
different and chosen to be appropriate for the recording of the auditory
evoked
responses in response to multiple, simultaneous, frequency-specific stimuli,
and for
obtaining responses from the appropriate structures of the auditory pathway.
By
choosing different repetition rates for different stimulus frequency bands
(e.g. one-
octave wide) it is possible to simultaneously test different frequency regions
of the
auditory system. The ASSR response spectrum to a single band comprises a
series of harmonics at multiples of the repetition rate. Thus if multiple
bands are
presented simultaneously, but each band has its own unique repetition rate,
then
the physiological evoked response will contain multiple harmonic series, but
importantly at distinct frequencies. This will allow separation of the
responses from
the different bands.
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In an embodiment, the method comprises that a combined amplitude-spectrum of
individual frequency-specific stimuli corresponds to a long-term amplitude
spectrum
of normal speech. In an embodiment, the combined amplitude spectrum is a
combination of a multitude of (e.g. three or more, such as four or more, or
eight or
more) individual frequency-specific stimuli. In an embodiment, each of the
frequency-specific stimuli (for instance four one-octave wide Chirp stimuli,
see e.g.
[Elberling et al., 2007b]) is weighted such that the overall broadband
stimulus has a
long-term spectrum which approximates that of normal speech.
In an embodiment, the method comprises that either a combined broadband
stimulus of the individual frequency-specific stimuli or the individual
frequency-
specific stimuli are amplitude-modulated corresponding to a corresponding low-
frequency modulation which occurs in normal speech. In an embodiment, low-
frequency amplitude modulation similar to that seen in free-running speech is
applied to the broad-band speech spectrum shaped ASSR stimulus. The low
frequency amplitude modulation and long-term speech spectrum will preferably
ensure that hearing aids will process the combined stimulus in a way similar
to free-
running speech. In an embodiment, the term low-frequency modulation which
occurs in normal speech' is e.g. taken to mean the resulting stimulus having a
modulation spectrum comparable to normal speech, e.g. with a maximum around
4 Hz when measured in 1/3 octave bands (cf. e.g. [Plomp, 1984]). In an
embodiment, the low-frequency modulation which occurs in normal speech' is
e.g.
taken to mean the envelope determined from a real running speech signal, or an
artificial speech signal, such as the ISTS (Holube et al., 2010).
In an embodiment, the method provides that the long-term amplitude-spectrum
and
the low-frequency amplitude modulation of the individual frequency-specific
stimuli
correspond to speech spoken with a specific vocal effort. In an embodiment,
examples of speech spoken with a specific vocal effort are represented by
Soft,
Normal, Raised, and Shouted. In an embodiment, a combined broadband stimulus
having a long-term averaged spectrum is provided at levels defined in terms of
specific vocal effort according to ANSI S3.5. (1997). This is advantageous for
clinical applications aimed at testing at standardized speech levels.
In an embodiment, the method provides that the individual frequency-specific
stimuli
consist of band-limited chirps. In an embodiment, the individual frequency-
specific
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chirps are one-octave wide. In an embodiment, the individual frequency-
specific
chirps are configured to cover the frequency range relevant for speech
intelligibility,
e.g. adapted to the specific person, e.g. covering a frequency in the range
between
200 Hz and 8 kHz, e.g. a range between approximately 350 Hz to 5600 Hz. In an
5 embodiment, the frequency-specific stimuli consist of four, one-octave
wide chirps
having the center frequencies 500, 1000, 2000 and 4000 Hz (defining four
stimulus
frequency bands), respectively, thus covering the frequency range from
approximately 350 Hz to 5600 Hz. In an embodiment, the method provides that
the
individual frequency-specific stimuli are independently amplitude modulated by
an
10 envelope representing band-specific speech modulations. In an
embodiment, the
band-specific speech modulations can be determined from a real speech signal,
or
an artificial speech signal, which is band-pass filtered into frequency bands
corresponding to the stimulus frequency bands described above, and where the
speech envelopes are determined independently for each band.
In an embodiment, the method comprises providing that the amplitude
modulations
are quantized, e.g. in time and/or level.
In an embodiment, the method comprises providing that said frequency-specific
stimuli are arranged (or scaled) such that a combined amplitude-spectrum of
said
frequency-specific stimuli corresponds to a long-term amplitude spectrum of
normal
speech.
In a further embodiment, the method comprises
= providing speech-like modulations of said frequency-specific stimuli such
that a
modulation spectrum of said frequency-specific stimuli corresponds to a
modulation spectrum of normal speech, and
= providing that the imposed speech-like amplitude modulations are
quantized in
time and/or level.
In an embodiment, the method provides that the imposed speech-like modulations
are quantized in time and/or level. Applying speech-like amplitude modulations
to a
repetitive ASSR stimulus means that the individual repetitions of the basic
stimulus
will be presented at many different levels. This can be actively exploited to
carry out
ASSR estimation corresponding to several different stimulus levels at the same
time, from one on-going measurement. This is, however, crucially dependent on
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being able to sort incoming recording blocks ('epochs') of an ASSR measurement
into different 'level bins', corresponding to the different stimulus levels.
Having
realized that, further advantages can be obtained by:
1. Quantizing the (frequency band specific) envelope gains in time to make
them
constant across each epoch to minimize temporal distortion to the individual
stimuli (or minimize spectral splatter in the frequency domain).
2. Quantizing the envelope gains to represent specific desired stimulus
levels.
The quantization of the modulations in level, can, for instance, correspond to
Leg +
10 dB, Leq, Leq - 10 dB, and Leq -20 dB, where Leq is the long-term average
level
of the (potentially band-pass filtered) speech signal used to create the
envelope.
Thus, the quantization of the modulations in time as well as level, together
with
corresponding sorting of the measured response epochs in the detection stage
of
the ASSR method allows for simultaneous estimation of the electrophysiological
response to specific parts of the speech signal in terms of level, e.g. 'loud'
(Leq +
10 dB), 'average' (Leq), 'soft' (Leg ¨ 10 dB), and 'very soft' (Leq ¨20 dB).
In an embodiment, the term 'an epoch' is taken to comprise one full repetition
of the
periodic, combined frequency-specific stimuli, as defined for the frequency-
specific
stimuli before any amplitude modulations are imposed.
In an embodiment, the method provides that level-quantized stimulus which
yields
estimates of the electrophysiological response to speech at several distinct
levels is
used for automated fitting of a hearing aid. This is achieved by automatically
increasing hearing-aid gain in the frequency ranges where the
electrophysiological
response is below normative values and vice versa.
In an embodiment, the method provides that the recording of said auditory
evoked
responses comprises recording of auditory steady-state responses, ASSR.
A method of desionina stimuli:
In an aspect, a method of designing stimuli for an ASSR system is provided.
The
method comprises:
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= Defining a number of different frequency specific stimuli, each having a
specific
frequency or center frequency and bandwidth (and being located in different
stimulus frequency bands);
= Defining an individual repetition rate for each of the different
frequency specific
stimuli;
= Spectrally shaping the amplitude spectrum of each of the different
frequency
specific stimuli to provide spectrally shaped frequency specific stimuli;
= Amplitude modulating either each of the spectrally shaped frequency
specific
stimuli or the combined broad-band signal with a real or simulated envelope of
running speech;
= Combining the spectrally shaped frequency specific stimuli to provide a
combined broad-band signal;
Wherein the spectral shaping of the amplitude spectrum of each of the
different
frequency specific stimuli is configured to provide that the combined signal
corresponds to the long-term spectrum of running speech spoken with a specific
vocal effort.
It is intended that some or all of the process features of the method of
recording
auditory evoked responses described above, in the 'detailed description of
embodiments' or in the claims can be combined with embodiments of the method
of
designing stimuli for an ASSR system, when appropriate, and vice versa.
In an embodiment, the method comprises quantizing the amplitude modulations in
time and/or level.
In an embodiment, the number of frequency specific stimuli is larger than or
equal
to two, such as larger than three, e.g. equal to four. In an embodiment, the
number
and the frequencies or center frequencies of the frequency specific stimuli
are
adapted to cover a predefined the frequency range, e.g. a range of operation
of a
particular hearing device or a frequency range of importance to speech
intelligibility,
e.g. part of the frequency range between 20 Hz and 8 kHz, e.g. between 250 Hz
and 6 kHz.
In an embodiment, the method comprises applying independent speech envelopes
to each of the stimulus frequency bands. In other words, the method comprises
that
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the speech like envelope is extracted independently for the different stimulus
frequency bands.
In an embodiment, the method comprises that the resulting combined signal
comprising speech like stimuli is configured to ensure that a hearing device
enters a
speech mode of operation (e.g. a program specifically adapted to process
speech),
while at the same time allowing an ASSR measurement, when the speech-like
stimuli are received by the hearing device.
A stimulation system:
In an aspect, a stimulation system for designing and generating stimuli for an
ASSR
system is provided. The stimulation system comprises:
= A stimulation generator for generating a number of different frequency
specific
stimuli, each having a specific frequency or center frequency and bandwidth,
and (the stimulation generator being) configured to apply an individual
repetition
rate to each of the different frequency specific stimuli;
= A spectral shaping unit for spectrally shaping the amplitude spectrum of
each of
the different frequency specific stimuli to provide spectrally shaped
frequency
specific stimuli;
= A combination unit for combining the spectrally shaped frequency specific
stimuli to provide a combined broad-band signal;
= An amplitude modulation unit for amplitude modulating either each of the
spectrally shaped frequency specific stimuli or the combined broad-band signal
with a real or simulated envelope of running speech; and
wherein the spectral shaping of the amplitude spectrum of each of the
different
frequency specific stimuli is configured to provide that the combined signal
corresponds to the long-term spectrum of running speech spoken with a specific
vocal effort.
It is intended that some or all of the process features of the method of
designing
stimuli for an ASSR system described above, in the 'detailed description of
embodiments' or in the claims can be combined with embodiments of the system
of
designing stimuli for an ASSR system, when appropriate, and vice versa.
14
In an embodiment, the stimulation system comprises a unit for quantizing the
amplitude modulations, e.g. in time and/or level:
In an embodiment, the stimulation system is configured to apply independent
speech
envelopes to each of the stimulus frequency bands (a stimulation band
comprising a
frequency specific stimulus).
In an embodiment, the stimulation system is adapted to provide that the
resulting
combined signal comprising speech like stimuli is configured to ensure that a
hearing device enters a speech mode of operation (e.g. a program specifically
adapted to process speech), while at the same time allowing an ASSR
measurement,
when the speech-like stimuli are received by the hearing device.
A diagnostic system:
In an aspect, a diagnostic system for recording auditory steady-state
responses of a
person, the system comprising a stimulation unit for providing an acoustic
stimulus
signal to an ear of the person, and a recording unit for recording the
auditory steady-
state responses of the person origination from said acoustic stimulus signal
is
furthermore provided by the present application. The stimulation unit is
configured to
provide that the acoustic stimulus signal comprises a speech-like stimulus.
The speech-
like stimulus is preferably provided as a combination of a series of frequency-
specific
stimuli, each having a specified (e.g. predetermined) frequency bandwidth,
presentation
rate, amplitude and amplitude modulation. Moreover, the individual frequency-
specific
stimuli are independently amplitude modulated by an envelope representing band-
specific speech modulations determined from a real speech signal, or an
artificial
speech signal, which is bandpass filtered into frequency bands corresponding
to the
stimulus frequency bands, and where the speech envelopes are determined
independently for each band, providing that the amplitude modulations are
quantized
time and/or level, by respectively Quantizing envelope gains in time to make
them
constant across each epoch to minimize temporal distortion to the individual
stimuli, and
Quantizing the envelope gains to represent specific desired stimulus levels.
It is intended that some or all of the process features of the method
described
above, in the 'detailed description of embodiments' or in the claims can be
combined with embodiments of the diagnostic system, when appropriately
substituted
Date Regue/Date Received 2023-02-02
15
by a corresponding structural feature and vice versa. Embodiments of the
diagnostic
system have the same advantages as the corresponding methods.
In an embodiment, the stimulation unit comprises
= A stimulation generator for generating a number of different frequency
specific
stimuli, each having a specific frequency or center frequency and bandwidth
(and
being located in different stimulus frequency bands), and configured to apply
an
individual repetition rate to each of the different frequency specific
stimuli;
= A spectral shaping unit for spectrally shaping the amplitude spectrum of
each of
the different frequency specific stimuli to provide spectrally shaped
frequency
specific stimuli;
= A combination unit for combining the spectrally shaped frequency specific
stimuli to provide a combined broad-band signal;
= An amplitude modulation unit for amplitude modulating either each of the
spectrally shaped frequency specific stimuli or the combined broad-band signal
with a real or simulated envelope of running speech to provide said speech-
like
stimulus; and
wherein the spectral shaping of the amplitude spectrum of each of the
different
frequency specific stimuli is configured to provide that the combined signal
corresponds to the long-term spectrum of running speech spoken with a specific
vocal
effort.
In an embodiment, the stimulation unit comprises a unit for quantizing the
amplitude
modulations.
In an embodiment, the stimulation unit comprises a quantization unit for
quantizing
the amplitude modulations in time and/or level.
In an embodiment, the stimulation unit is configured to apply independent
speech
envelopes to each of the stimulus frequency bands.
In an embodiment, the combination unit comprises or is constituted by a SUM-
unit
for adding the individual spectrally shaped frequency specific (band-limited)
stimuli to
provide a combined broad-band signal. In an embodiment, the combination unit
comprises or is constituted by a summation unit.
Date Regue/Date Received 2023-02-02
16
In an embodiment, the diagnostic system is configured to record auditory
evoked
responses (e.g. ASSR) of the person
= When the person is wearing a hearing device at the ear; as well as
= When the person is not wearing the hearing device at the ear.
A data processinp system
According to a data processing system comprising a processor and a computer
readable medium having stored thereon instructions for execution by the
processor to
perform the steps of the method as defined herein.
A computer-readable medium
According to A tangible computer-readable medium having stored thereon
instructions
for execution by a data processing system to perform at least some, such as a
majority
or all, of the steps of the method as defined herein.
A combined system:
In an aspect, a combined system comprising a diagnostic system as described
above,
in the 'detailed description of embodiments' or in the claims, and a hearing
device for
compensating a user's hearing impairment is furthermore provided.
In an embodiment, the hearing device is adapted to provide a frequency
dependent
gain and/or a level dependent compression and/or a transposition (with or
without
frequency compression) of one or frequency ranges to one or more other
frequency
ranges, e.g. to compensate for a hearing impairment of a user. In an
embodiment,
the hearing device comprises a signal processing unit for enhancing input
signals
and providing a processed output signal. Preferably, the hearing device is
configured ¨ at least in a specific mode of operation ¨ to enhance speech
intelligibility of a user.
In an embodiment, the combined system is adapted to ensure that the hearing
device enters a speech mode of operation (e.g. a program specifically adapted
to
Date Regue/Date Received 2023-02-02
16a
process speech), while at the same time allowing an ASSR measurement, when
speech-like stimuli according to the present disclosure are received by the
hearing
device.
Date Regue/Date Received 2023-02-02
CA 02938690 2016-08-12
17
Use:
In an aspect, use of a diagnostic system as described above, in the 'detailed
description of embodiments' and in the claims, is moreover provided. In an
embodiment, use of a diagnostic system to verify a fitting of a hearing aid is
provided. In an embodiment, use of a diagnostic system to record ASSR on a
person wearing a hearing aid (configured to compensate for the person's
hearing
impairment) is provided (aided measurement). In an embodiment, use of a
diagnostic system to record ASSR on a person not wearing a hearing aid is
provided (unaided measurement).
A hearing device:
In an aspect, a hearing device is furthermore provided. The hearing device
comprises an input unit and an output transducer. The hearing device is
configured
to receive or generate different frequency specific stimuli generated as
defined by
the method of designing stimuli for an ASSR system as described above in the
'detailed description of embodiments' or in the claims. The hearing device is
furthermore configured to present the different frequency specific stimuli as
an
acoustic stimulus via said output transducer of the hearing device.
In an embodiment, the hearing device is configured to enter a speech mode of
operation (e.g. a program specifically adapted to process speech), while at
the
same time allowing an ASSR measurement, when speech-like stimuli according to
the present disclosure are received by the hearing device. Thereby it is
ensured that
the stimuli, when presented to the user have been processed by the hearing
device
in the same manner as the processing of normal speech signals. The recorded
signals evoked by the stimuli are thus representative of the user's perception
of
speech.
In an embodiment, the hearing device comprises or consists of a hearing aid.
In an embodiment, the input unit of the hearing device comprises one or more
microphones for picking up sound from the environment, including e.g. acoustic
stimulus from a loudspeaker of a diagnostic system according to the present
CA 02938690 2016-08-12
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disclosure. In an embodiment, the hearing device is configured to receive the
different frequency specific stimuli from the diagnostic system via a wireless
link. In
an embodiment, the hearing device comprises a combination unit allowing the
different frequency specific stimuli to be presented to the user via the
output
transducer (e.g. a loudspeaker), alone or in combination with electric sound
signals
picked up or received by the input unit.
Definitions:
The term 'a speech-like signal' or 'speech-like stimuli' is in the present
context taken
to mean a signal (or stimulus) that has a long-term spectrum and amplitude
variations over time that are similar to free-running speech (as e.g. defined
by IEC
60118-15. (2012)). Preferably, the speech-like signal (or stimulus) is
configured to
exhibit level fluctuations (low-frequency amplitude modulations) with a
dynamic
range over time in the free field corresponding to speech, as e.g. defined in
the
IEC60118-15 standard (e.g. when analyzed in 1/3-octave bands).
In the present context, a 'hearing device' refers to a device, such as e.g. a
hearing
instrument or an active ear-protection device or other audio processing
device,
which is adapted to improve, augment and/or protect the hearing capability of
a user
by receiving acoustic signals from the user's surroundings, generating
corresponding audio signals, possibly modifying the audio signals and
providing the
possibly modified audio signals as audible signals to at least one of the
user's ears.
A 'hearing device' further refers to a device such as an earphone or a headset
adapted to receive audio signals electronically, possibly modifying the audio
signals
and providing the possibly modified audio signals as audible signals to at
least one
of the user's ears. Such audible signals may e.g. be provided in the form of
acoustic
signals radiated into the user's outer ears, acoustic signals transferred as
mechanical vibrations to the user's inner ears through the bone structure of
the
user's head and/or through parts of the middle ear as well as electric signals
transferred directly or indirectly to the cochlear nerve of the user.
The hearing device may be configured to be worn in any known way, e.g. as a
unit
arranged behind the ear with a tube leading radiated acoustic signals into the
ear
canal or with a loudspeaker arranged close to or in the ear canal, as a unit
entirely
or partly arranged in the pinna and/or in the ear canal, as a unit attached to
a fixture
CA 02938690 2016-08-12
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implanted into the skull bone, as an entirely or partly implanted unit, etc.
The
hearing device may comprise a single unit or several units communicating
electronically with each other.
More generally, a hearing device comprises an input transducer for receiving
an
acoustic signal from a user's surroundings and providing a corresponding input
audio signal and/or a receiver for electronically (i.e. wired or wirelessly)
receiving an
input audio signal, a signal processing circuit for processing the input audio
signal
and an output means for providing an audible signal to the user in dependence
on
the processed audio signal. In some hearing devices, an amplifier may
constitute
the signal processing circuit. In some hearing devices, the output means may
comprise an output transducer, such as e.g. a loudspeaker for providing an air-
borne acoustic signal or a vibrator for providing a structure-borne or liquid-
borne
acoustic signal. In some hearing devices, the output means may comprise one or
more output electrodes for providing electric signals.
In some hearing devices, the vibrator may be adapted to provide a structure-
borne
acoustic signal transcutaneously or percutaneously to the skull bone. In some
hearing devices, the vibrator may be implanted in the middle ear and/or in the
inner
ear. In some hearing devices, the vibrator may be adapted to provide a
structure-
borne acoustic signal to a middle-ear bone and/or to the cochlea. In some
hearing
devices, the vibrator may be adapted to provide a liquid-borne acoustic signal
to the
cochlear liquid, e.g. through the oval window. In some hearing devices, the
output
electrodes may be implanted in the cochlea or on the inside of the skull bone
and
may be adapted to provide the electric signals to the hair cells of the
cochlea, to one
or more hearing nerves, to the auditory cortex and/or to other parts of the
cerebral
cortex.
BRIEF DESCRIPTION OF DRAWINGS
The aspects of the disclosure may be best understood from the following
detailed
description taken in conjunction with the accompanying figures. The figures
are
schematic and simplified for clarity, and they just show details to improve
the
understanding of the claims, while other details are left out. Throughout, the
same
CA 02938690 2016-08-12
reference numerals are used for identical or corresponding parts. The
individual
features of each aspect may each be combined with any or all features of the
other
aspects. These and other aspects, features and/or technical effect will be
apparent
from and elucidated with reference to the illustrations described hereinafter
in
5 .. which:
FIG. 1A schematically shows an embodiment of a method of generating a speech-
like stimulus signal, and FIG. 1B shows an embodiment of diagnostic system for
recording an auditory evoked potential according to the present disclosure,
FIG. 2A, 2B, 2C, 2D, 2E, 2F show exemplary individual signal components from
which a resulting speech-like stimulus signal shown in FIG. 2G according to
the
present disclosure is generated,
.. FIG. 3 shows an example of the IEC60118-15, (2012) method for determining
hearing aid insertion gain and appropriate level dynamic range for speech-like
stimuli,
FIG. 4A and 4B shows two exemplary setups of a diagnostic system for
(together)
verifying a fitting of a hearing aid, FIG. 4A illustrating an AEP measurement,
where
the user wears a hearing device in a normal mode (aided), and 4B illustrating
an
AEP measurement, where the user does not wear a hearing device (unaided),
stimulation being in both setups provided via a loudspeaker of the diagnostic
system,
FIG. 5A shows an embodiment of a diagnostic system, and
FIG. 5B shows an embodiment of a diagnostic system stimulating a hearing
device
while worn by a person, stimulation being provided via a loudspeaker of the
diagnostic system,
FIG. 6A shows an embodiment of a stimulation unit according to the present
disclosure,
FIG. 66 shows an example of stimulus envelope gains quantized in time and
level,
CA 02938690 2016-08-12
21
FIG. 7A shows a first scenario of an AEP measurement, where the user wears a
hearing device in a normal mode (aided), and where stimuli are provided to the
hearing device for being played to the user by a loudspeaker of the hearing
device,
FIG. 7B shows a second scenario of an AEP measurement, where the user wears a
hearing device in a normal mode (aided), and where stimuli are provided to the
hearing device for being played to the user by a loudspeaker of the hearing
device,
FIG. 7C shows a third scenario of an AEP measurement, where the user wears
first
and second hearing devices of a binaural hearing system in a normal mode
(aided),
and where stimuli are provided to the hearing devices for being played to left
and
.. right ears of the user by a loudspeaker of the hearing device.
FIG. 8 shows an embodiment of a diagnostic system stimulating a hearing device
while worn by a person, wherein stimulation is provided via a loudspeaker of
the
hearing device.
FIG. 9 shows a flow diagram for a method of designing stimuli for an AEP
system,
e.g. an ASSR system.
DETAILED DESCRIPTION OF EMBODIMENTS
The detailed description set forth below in connection with the appended
drawings
is intended as a description of various configurations. The detailed
description
includes specific details for the purpose of providing a thorough
understanding of
various concepts. However, it will be apparent to those skilled in the art
that these
concepts may be practiced without these specific details. Several aspects of
the
apparatus and methods are described by various blocks, functional units,
modules,
components, circuits, steps, processes, algorithms, etc. (collectively
referred to as
"elements"). Depending upon particular application, design constraints or
other
reasons, these elements may be implemented using electronic hardware, computer
program, or any combination thereof.
The electronic hardware may include microprocessors, microcontrollers, digital
signal processors (DSPs), field programmable gate arrays (FPGAs), programmable
logic devices (PLDs), gated logic, discrete hardware circuits, and other
suitable
CA 02938690 2016-08-12
22
hardware configured to perform the various functionality described throughout
this
disclosure. Computer program shall be construed broadly to mean instructions,
instruction sets, code, code segments, program code, programs, subprograms,
software modules, applications, software applications, software packages,
routines,
subroutines, objects, executables, threads of execution, procedures,
functions, etc.,
whether referred to as software, firmware, middleware, microcode, hardware
description language, or otherwise.
FIG. 1A shows an embodiment of a method of generating a speech-like stimulus
signal. FIG. 1B shows an embodiment of diagnostic system for recording an
auditory evoked potential according to the present disclosure.
FIG. 1A shows the principle and preferred embodiment of the stimulus
generation of
the present disclosure. To the left (block Octave-band chirps), as an example,
four
octave-band Chirps are generated with the center frequencies of 500, 1000,
2000,
and 4000 Hz. The stimuli are presented at different rates of stimulation (see
e.g.
FIG. 2A, 2B, 2C, 2D) and can be used for the simultaneous multiple frequency-
specific stimulation of the ASSR (cf. e.g. W02006003172A1, [Elberling et al.,
2007b]). The four Chirps are next (cf. block Spectral Shaping) spectrally
shaped so
.. the amplitude spectrum of the combined signal corresponds to the long-term
spectrum of running speech spoken with a specific vocal effort (here as an
example
the vocal effort is 'Normal' ¨ ANSI S3.5. (1997)).
Next (cf. blocks Modulation), the combined and spectrally shaped signal is fed
into
.. an amplitude modulator, which modulates either each of the band-limited
stimuli or
the combined broad-band signal with a real or simulated envelope of running
speech (cf. e.g. [Plomp, 1984]). Finally (cf. stage Simulated speech signal)
the
simulated speech signal is fed to a stimulus transducer (here a loudspeaker is
shown as an example) with a presentation level as required.
In FIG. 1A references are made to the detailed temporal waveforms in FIG. 2A-
2G.
FIG. 1A schematically illustrates an embodiment of a stimulation part
(represented
by STU and OT in FIG. 1B) of a diagnostic system. FIG. 1B schematically shows
an
embodiment of a diagnostic system (DMS) comprising a stimulation unit (STU),
an
output transducer (0T), and a recording unit (REC) in communication with a
number
CA 02938690 2016-08-12
23
of recording electrodes (rec-elec). FIG, 1B further includes a user (U)
wearing a
hearing device (HD) at a 1st ear (1st ear) and an ear plug (plug) at the 2"d
ear (2'd
ear). FIG. 1B illustrates 'free field', aided measurement with a diagnostic
system
according to the present disclosure. The hearing device is adapted for picking
up
sound from the environment to provide an electric input signal, and comprises
a
signal processing unit for providing an improved signal by applying a level
and
frequency dependent gain to the input signal to compensate for a hearing
impairment of the user's 1st ear, and an output unit for presenting the
improved
signal as output stimuli perceivable by the user as sound. The ear plug (plug)
is
adapted to block sound at the 2nd ear from evoking neurons in the auditory
system.
When electric stimuli (stim) generated by the stimulation unit (STU) and
converted
to acoustic stimuli (ac-stim) via output transducer (OT), the acoustic stimuli
(ac-
stim) are picked up by the input transducer of the hearing device(HD) at the
first ear
(13t ear) of the user (U), processed by the signal processing unit, and
presented to
the auditory system (Auditory system) of the user via the output unit of the
hearing
device. The stimuli from the output unit of the hearing device evokes
responses
(aep) from the auditory system (Auditory system). The evoked responses (aep)
are
recorded by the recording unit (REC) via recording electrodes (rec-el) mounted
on
the user's head (HEAD), e.g. attached to the skin and/or tissue of the user's
scalp
or ear canal. The recording (REC) and stimulation (STU) units are in
communication
(cf. signal coot), e.g. to control timing relations between the generation of
stimuli by
the stimulation unit and the detection and processing of evoked responses
(ASSRs)
by the recording unit.
FIG. 2A, 2B, 2C, 2D, 2E, 2F shows exemplary individual signal components from
which a resulting speech-like stimulus signal (as illustrated in FIG. 2G) is
generated.
FIG. 2A-2G shows the details of the time signals at the different stages of an
embodiment of the proposed invention. From top to bottom: First the four
.. frequency-specific stimuli are shown using a time scale of 100 ms (FIG. 2A-
2D).
The different rates of stimulation are indicated to the left and as an example
vary
from 84.0/s to 90.6/s. FIG. 2A shows a 500 Hz narrow band chirp with a
repetition
rate of 86.0 Hz. FIG. 2B shows a 1000 Hz narrow band chirp with a stimulation
rate
of 90.6 Hz. FIG. 2C shows a 2000 Hz narrow band chirp with a stimulation rate
of
84.0 Hz. FIG. 2D shows a 4000 Hz narrow band chirp with a stimulation rate of
88.6
Hz. Each of the narrow band chirps is generated by respective filtering (with
a 1
CA 02938690 2016-08-12
24
octave bandpass filter) of a broadband linear chirp between a minimum
frequency
(e.g. 350 Hz) and a maximum frequency (e.g_ 11.3 kHz) (cf. [Elberling & Don,
2010]). The spectrally shaped combined broad-band signal is shown in FIG. 2E
as
the SUM of weighted Chirp signals'. In FIG. 2A-2G, four frequency-specific
stimuli,
.. each comprising a periodically repeated (1 octave wide) narrow band chirp,
are
used to generate the combined broad-band signal. Alternatively, another number
of
narrow band chirps may be used, e.g. 12 (1/3 octave wide) narrow band chirps
covering the same frequency range from appr. 350 Hz to appr. 5600 Hz.
.. Next, using a time scale of 10 s, an example of a 'Simulated Speech
Envelope' is
shown in FIG. 2F, and finally the corresponding modulated output signal is
shown
as the 'Simulated Speech Signal' in FIG. 2G. The simulated speech envelope in
FIG. 2E is e.g. generated as an envelope of exemplary free-running speech.
.. FIG. 3 shows an example of the IEC60118-15, (2012) method for determining
hearing aid insertion gain and appropriate level dynamic range for speech-like
stimuli.
FIG. 3 gives an example of the IEC60118-15, (2012) method for determining
hearing aid insertion gain and appropriate level dynamic range ([dB SPL]
versus
frequency [Hz]) for speech-like stimuli. On the left figure (denoted Unaided)
is
shown the level variations of a standardized speech test-stimulus ([Holube et
al.,
2010]) recorded in a hearing aid test-box (Interacoustics TB25). The level
variation
for each 1/3-octave band is indicated by the 30, 65 and 99th percentiles of
the
corresponding distribution of the short-term (125 ms) amplitude values. Also
shown
is the long term amplitude speech spectrum (LTASS) in one-third octave bands.
The middle figure (denoted Aided) shows the output from a pediatric hearing
aid,
measured in the test-box using an occluded-ear simulator (IEC 60318-4, 2010).
On
the right figure (denoted EIG=Aided-unaided 65 dB SPL) is shown the estimated
insertion gain (EIG). It is observed that the estimated insertion gain of
signal
components having relatively lower input levels (represented by the 30%
percentile)
is larger than the estimated insertion gain of signal components having
relatively
higher input levels (represented by the 99% percentile). This is e.g. due to a
compression algorithms, which tend to amplify low input levels more than high
input
.. levels. A preferred embodiment of the present disclosure is to use the
methods set
CA 02938690 2016-08-12
down in the IEC 60118-15, (2012)-standard to demonstrate the speech-like
processing of the new ASSR stimuli with digital hearing aids.
FIG. 4A and 4B shows exemplary setups of a diagnostic system for verifying a
5 fitting of a hearing aid, FIG. 4A and 4B illustrating an AEP measurement,
where the
user wears a hearing device in a normal mode (aided), and where the user does
not
wear a hearing device (unaided), respectively. The diagnostic system comprises
the
components discussed in connection with FIG. 1B and is in FIG. 4A used in an
aided measurement where free field acoustic stimuli (ac-stim1) from the output
10 transducer (OT, here a loudspeaker) are picked up by a hearing device (HD1)
adapted for being located in or at a first ear (earl) of a user (U) (or fully
or partially
implanted in the head of the user). The hearing device comprises an input unit
(/U,
here a microphone is shown), a signal processing unit (not shown) for applying
a
level and frequency dependent gain to an input signal from the input unit and
15 presented such enhanced signal to an output unit (OU, here an output
transducer
(loudspeaker) is shown). The output transducer of the hearing device is in
general
configured to present a stimulus (based on the signal picked up by the input
unit
/U), which is perceived by the user as sound. The auditory system of the user
is
schematically represented in FIG. 4A and 4B by the ear drum and middle ear (M-
20 ear), cochlea (cochlea) and the cochlear nerve (neurons). The nerve
connections
from the respective cochlear nerves to the auditory centers of the brain (the
Primary
Auditory Cortex, shown as on center denoted PAC in FIG. 4A and 4B, and in FIG.
7A-7C) are indicated by the bold dashed curves in FIG. 4A and 4B. The
diagnostic
system comprises a stimulation unit (STU) adapted to provide an electric
stimulus
25 signal (stimi) comprising a number of individually repeated frequency
specific
stimuli, which are combined and spectrally shaped in amplitude to emulate a
long-
term spectrum of running speech (at a certain vocal effort), and amplitude
modulated in time to provide an envelope of the stimuli equivalent to that of
running
speech. The diagnostic system further comprises a recording unit (REC) for
recording the auditory evoked responses of the person originating from said
acoustic stimulus signal ac-stiml. In the scenario of FIG. 4B the free field
acoustic
stimulus signal ac-stim1 is received by the persons' ear and auditory system
(without hearing aid means, i.e. in an 'unaided' mode). In the scenario of
FIG. 4A
the free field acoustic stimulus signal ac-stiml is picked up, processed and
presented to the person's auditory system by the hearing device (i.e. an
'aided'
mode). In both the aided and unaided setup, the stimulation is provided at one
ear
CA 02938690 2016-08-12
26
(the right ear, earl) and the other ear (the left ear, ear2) is provided with
an ear plug
(plug) to block sound that ear from evoking neurons in the auditory system.
The
recording unit comprises or is operationally connected to electrodes (ACQ)
adapted
to pick up brainwave signals (recO, rec. 1, rec2) (e.g. AEPs) when
appropriately
located on the head of the user. In the embodiments of FIG. 4A and 46, three
electrodes (ACQ) are shown located on the scalp of the user (U). The recording
unit
and the stimulation unit are in communication with each other (signal cont),
e.g. to
control a timing between stimulation and recording. The recording unit
comprises
appropriate amplification, processing, and detection circuitry allowing
specific ASSR
data to be provided.
FIG. 5A shows an embodiment of a diagnostic system alone, and FIG, 5B shows an
embodiment of a diagnostic system stimulating a hearing device while worn by a
person.
FIG. 5A is a block diagram of a diagnostic system (DMS) as also illustrated
and
described in connection with FIG. 4A and 48 and in FIG. 1B. The diagnostic
system
comprises an electrode part (ACQ) comprising a number Ne of electrodes for
picking up evoked potentials recn from the auditory system and brain when
mounted
on the head of the user. The evoked potentials recn picked up by the
electrodes are
fed to the recording unit (REC) for processing and evaluation. Electric
stimuli stim
(e.g. controlled (e.g. initiated) by the recording unit (REC) via control
signal cont)
according to the present disclosure are generated by the stimulation unit (S
TIM)
and converted to (free field) acoustic stimuli ac-stim by an output transducer
(loudspeaker) of the system. FIG. 5B shows the diagnostic system (DMS) used in
an 'aided' mode (as illustrated and discussed in connection with FIG. 4A),
where a
person wearing a hearing device (HD) is exposed to the acoustic stimuli (ac-
stim) of
the diagnostic system at one ear. The acoustic stimuli (ac-stim) are picked up
by a
sound input (Sound-in) of the hearing device located at the ear. The acoustic
stimuli
(ac-stim) are converted to an electric input signal by a microphone of the
hearing
device and processed in a forward signal path of the hearing device to a
loudspeaker presenting the processed stimuli the user as an output sound
(Sound-
out). The forward path of the hearing device (HD) comprises e.g. an analogue
to
digital converter (AD) providing a digitized electric input signal (IN), a
signal
processing unit (SPU) for processing the digitized electric input, e.g. in a
speech
processing mode of operation, and providing a processed signal (OUT), which is
CA 02938690 2016-08-12
27
converted to an analogue signal by a digital to analogue converter (DA) before
it is
converted to sound signal by the loudspeaker of the hearing device. The output
sound (Sound-out) from the hearing device represents a processed version of
the
speech-like acoustic stimuli (ac-stim) from the diagnostic system (as
delivered by
the hearing device). The user's auditory system picks up the output sound
(Sound-
out) from the hearing device (HD) and evokes potentials (AEP) that are picked
up
by the electrodes (ACQ) of the diagnostic system (DMS). The diagnostic system
(DMS) and the hearing device (HD) together represent a combined system (CS).
The hearing device (HD) can be of any kind (type (air conduction, bone
conduction,
cochlear implant (or combinations thereof), style (behind the ear, in the ear,
etc.) or
manufacture) capable of enhancing a speech (or speech-like) input signal
according
to a user's needs. In an embodiment, the capability of the hearing device to
process
speech-like stimuli signals from the diagnostic system as ordinary speech is
verified
in a separate measurement (e.g. in a low-reflection measurement box), e.g.
according to the IEC 60118-15 (2012)-standard, cf. further below.
Example:
As an example, the ASSR stimulus according to the present disclosure may be
generated by four one-octave wide narrow-band (NB) chirp-train ASSR stimuli ¨
constructed according to the methods described in US 8,591,433 B2, and with
center frequencies 500, 1000, 2000, and 4000 Hz and repetition rates 86.0/s,
90.6/s, 84.0/s, and 88.6/s respectively. These examples are illustrated in FIG
2 (A-
D). To make the stimulus speech-like, the target sound pressure level should
preferably correspond to normal speech levels in the octave bands. The
stimulus
should preferably be presented in a room with only minor reflections (e.g.
anechoic).
Each band is then weighted according to ANSI S3.5. (1997) for normal vocal
effort
speech measured at a distance of 1 m from the source (e.g. a loudspeaker).
According to the ANSI-standard the octave-band sound pressure levels are then
set
to 59.8, 53.5, 48.8 and 43.9 dB SPL for the 500, 1000, 2000 and 4000 Hz octave
bands respectively. The bands are then combined (see FIG 2 E), such that the
sum
of the individual bands will result in a broad-band stimulus with a long-term
spectrum identical to speech at normal vocal effort, corresponding to a free-
field
sound pressure level of approximately 62.5 dB SPL.
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28
Next the broad-band stimulus is fed to a modulator and the simulated speech
envelope is applied. This is illustrated in FIG 2 F as a low-pass (4 Hz cut-
off)
filtered envelope of Gaussian white noise. The modulator multiplies the broad-
band
ASSR stimulus with the simulated speech envelope and the result is shown in
FIG 2
G.
When presented through a hearing aid, the co-modulation of envelope power
across bands and the fluctuation in band power will in principle excite the
device in
a mode of operation similar to speech. By using the I EC 60118-15 (2012)-
standard,
the appropriate acoustic measurements in a hearing aid analyzer can be made to
demonstrate that the stimulus is processed by the hearing aid in a manner
similar to
speech. Normal speech has inherent level fluctuations (amplitude modulation),
the
dynamic range of these over time in the free-field is an important
characteristic for
speech, and are analyzed in 1/3-octave bands in the IEC60118-15 standard. If
the
new ASSR stimulus has the same input dynamic range as a standardized speech
stimulus it is ensured that the hearing aid is stimulated correctly. The
output from
the hearing aid and the estimated insertion gain are also made to quantify
this
relationship and further demonstrate that the hearing aid is processing the
stimulus
in a speech-like manner. An example of this procedure is given in FIG 3.
In the present example the AM is applied to the combined broad-band stimulus
(cf.
FIG. 2A-2G). Alternatively, the AM can be applied in a way as to simulate the
co-
modulation in normal speech, i.e. have narrow band regions with common
modulation rather than across the full region of the broad-band stimulus. This
could
simply be done using a filter-bank and multiple modulators before combining
into a
single broad-band stimulus. This is illustrated in FIG. 6A.
FIG. 6A shows an embodiment of a stimulation unit (STU) according to the
present
disclosure. The stimulation unit (STU) of FIG. 6A comprises a generator of
frequency specific stimuli (FSSG), e.g. narrowband stimuli as shown in FIG.
1A, 1B,
2A-2G, but alternatively other frequency specific stimuli, e.g. stimuli
generated by
individual pure tones each tone amplitude modulated by a lower frequency
carrier.
The frequency specific stimuli generator (FSSG) provides stimuli signals fs-
stim.
The stimulation unit (STU) further comprises a spectrum shaping unit (SSU)
that
shapes the frequency specific stimuli fs-stim to provide that the amplitude
spectrum
of the resulting combined signal ss-stim corresponds to the long-term spectrum
of
CA 02938690 2016-08-12
29
running speech, e.g. spoken with a specific vocal effort. The stimulation unit
(STU)
further comprises an analysis filter-bank (A-FB) that splits the frequency
shaped
stimuli ss-stim in a number N of stimulus frequency bands, providing (time-
varying)
frequency shaped band signals shstl, shst2, shstN.
The stimulation unit (STU)
further comprises band-level modulators (denoted 'x' in FIG. 6A) for amplitude
modulating frequency shaped band signals shstl, shst2, shstN
with individual
band level modulation functions rsaml, rsam2, rsamN,
provided by a band level
modulation unit (BLM) configured to provide that the resulting amplitude
modulated
frequency shaped band signals smstl, smst2, smstN
have an envelope
equivalent to that of running speech. For example, the BLM may derive its
envelopes from a real speech signal, or an artificial speech signal such as
the ISTS
[Holube et al., 2010], by band-pass filtering the speech signal by
appropriately wide
band-pass filters centered at the N stimulus frequency bands of the A-FB and
extracting the frequency specific envelopes from these N band-limited signals.
The
.. stimulation unit (STU) further comprises a combination unit (here in the
form of a
SUM unit) to combine band level signals smstl, smst2, smstN
to provide a
resulting time-domain stimulation signal stim. The resulting electric stimuli
stim may
then be converted to acoustic stimuli (cf. ac-stim in FIG. 1B, 4 and 5) by an
electro-
acoustic transducer (cf. e.g. OT in FIG. 1B), e.g. a loudspeaker (cf. e.g.
speaker in
FIG. 1A, 4, 5).
The BLM may further comprise a quantization stage in which the N envelopes are
quantized in time and level. This may be achieved as follows. First, the
speech
signal from which the envelope is derived is time-stretched such that its
duration is
equal to an integer number of periods of the unmodulated stimulus. After
filtering
the speech signal into the N stimulus frequency bands and extracting the
envelopes, the envelopes are first quantized in time, such that each band
envelope
is constant across each period of the unmodulated stimulus. Secondly, the
envelopes are quantized in level, for instance, corresponding to Leq + 10 dB,
Leq,
Leq - 10 dB, and Leq - 20 dB, where Leq is the long-term average level of the
band-
pass filtered speech signal. FIG. 6B shows an example of the time trajectories
of
the envelope gains derived from the 60 seconds (s) long ISIS for octave-wide
stimulus frequency bands centered at 500, 1000, 2000, and 4000 Hz. In this
example, the period (epoch length) of the unmodulated stimulus is 136.53 ms
and
the entire sequence depicted in FIG. 66 extends over 440 epochs, corresponding
to
CA 02938690 2016-08-12
60.075 s. The level quantization corresponds to Leq + 10 dB, Leq, Leq - 10 dB,
and
Leq - 20 dB, with lower envelope gains remaining unmodified.
It should be noted that the epoch length chosen for this example is
substantially
5 shorter than what is typical for an ASSR stimulus. This is, however,
advantageous
in order to be able to represent modulations in the frequency range relevant
for
speech. The modulation spectrum of speech exhibits a characteristic broad peak
around 4 Hz. In order to represent modulations in this frequency range, the
envelope trajectories need to change at least every 1/4 Hz = 250 ms, which
sets an
10 upper limit to the epoch length.
The envelope trajectories depicted in FIG. 6B are staircase functions, which
implies
discontinuities at every step of the trajectory. It may be preferable to
introduce
smooth transitions between from one step to the next, e.g. by raised cosine
ramps,
15 in a compromise between spectral splatter to the stimuli within each
epoch and
transition artifacts between epochs.
The quantization levels for this example were chosen to provide level
categories
that would be clinically relevant for assessing access to various parts of
speech and
20 to allow meaningful adjustments to the settings of a hearing device.
However, other
level categories could be chosen just as well.
The division of the envelope epochs into the chosen categories was done
separately for each stimulus frequency band, by sorting all time-quantized
epoch
25 envelope values and then determining boundaries such that the mean
envelope
level within each set of boundaries (a 'level bin') was as close as possible
to
Leq + 10 dB, Leg, Leq - 10 dB, and Leq - 20 dB. All envelope gains within a
level
bin was then set to the bin's nominal level and returned to their original
time-wise
position in the trajectory.
In this way a fixed pattern of envelope trajectories was created across the
60.075 s
duration of the time-stretched speech signal from which the envelopes were
derived. This pattern of envelopes is supposed to be repeated until the ASSR
recording is terminated.
CA 02938690 2016-08-12
31
FIG. 7A, 7B, and 7C shows scenarios similar to those of FIG. 4A described
above.
A difference, though, is that the stimuli generated by the stimulation unit
(STU) of
the diagnostic system in the embodiments of FIG. 4A are transmitted (wired or
wirelessly) directly to the hearing device(s) or are generated in the hearing
device(s)
(instead of being played via a loudspeaker of the diagnostic system and picked
up
by the microphone(s) of the hearing device(s)). In both cases, the stimuli are
presented to the user (U) via a loudspeaker (07) of the hearing device (HD1).
FIG, 7A shows a first scenario of an AEP measurement, where the user (U) wears
a
hearing device (HD1) in a normal mode (aided), and where stimuli stim/ are
provided by a stimulation unit (STU) of the diagnostic system directly to the
hearing
device (HD1) for being played to the (U) user by a loudspeaker (07) of the
hearing
device (HD1). The connection between the diagnostic system and the hearing
device may be a wired connection of a wireless connection (e.g. based on
Bluetooth
or other standardized or proprietary technology).
FIG. 7B shows a second scenario of an AEP measurement, where the user (U)
wears a hearing device (HD1) in a normal mode (aided), and where stimuli stim/
are provided directly (electrically) to the hearing device for being played to
the user
by a loudspeaker of the hearing device. The embodiment of FIG. 7B is similar
to the
embodiment of FIG. 7A. A difference is though that the stimuli generated by
the
stimulation unit (STU) of the diagnostic system in FIG. 7A are generated in
the
hearing device (HD1) instead. The stimulation unit (STU) is located in the
hearing
device (HD1) and controlled by the diagnostic system via control signal cont
(here)
from the recording unit (REC) of the diagnostic system.
FIG. 7C shows a third scenario of an AEP measurement. The embodiment of FIG.
7C is similar to the embodiment of FIG. 7B. A difference is that in the
embodiment
of FIG. 7C, the user wears first and second hearing devices (HD1, HD2) of a
binaural hearing system in a normal mode (aided) (instead of a single hearing
device at one of the ears). Both hearing devices (HD1, HD2) comprise a
stimulation
unit (STU), which is controlled by the diagnostic system via control signal
cont
(here) from the recording unit (REC) of the diagnostic system.
An advantage of the embodiments of FIG. 7A, 7B and 7C compared to the
embodiment of FIG. 4A is that the stimuli are provided electrically to the
CA 02938690 2016-08-12
32
loudspeaker of the hearing device (not via intermediate electric to acoustic
transducer (loudspeaker of diagnostic system) and acoustic to electric
transducer
(microphone of hearing device)).
FIG. 8 shows an embodiment of a combined system (CS) comprising a diagnostic
system (DMS) stimulating a hearing device (HD) while worn by a person (U),
wherein stimulation stim is transmitted directly to the hearing device (HD)
and
provided via a loudspeaker (01) of the hearing device (HD). The embodiment of
FIG. 8 is similar to the embodiment of FIG. 5B. The forward path of the
hearing
device (HD) comprises an input transducer (here a microphone), an analogue to
digital converter (AD), signal processing unit (SPU), a combination unit (CU),
a
digital to analogue converter (DA), and an output transducer (here a
loudspeaker).
A difference to the embodiment of FIG. 5B is that in the embodiment of FIG. 8,
the
stimulation signal stirn is sent directly from a stimulation unit (STIM) of
the
diagnostic system (DMS) to a combination unit (CU) the hearing device (HD) via
an
interface (IF), (e.g. a wireless interface). The combination unit (CU) is
configured to
allow a stimulation signal stim received from the diagnostic system (DMS) to
be
presented to a user (U) via the loudspeaker (and DA-converter (DA)) of the
hearing
device (either alone or in combination (mixed with) the processed signal PS
from
the signal processing unit (SPU) of the forward path of the hearing device
(HD). The
combination unit may be controlled by the diagnostic system (e.g. by a signal
transmitted via the (e.g. wireless) interface (1.9).
FIG. 9 shows a flow diagram for a method of designing stimuli for application
in a
system for recording Auditory Evoked Potentials (AEP), in particular in a
system for
recording Auditory Steady State Responses (ASSR). The method comprises
al . Defining a number of different frequency specific stimuli, each having a
specific
frequency or center frequency and bandwidth defining respective stimulus
frequency bands;
S2. Defining an individual repetition rate for each of the different frequency
specific
stimuli;
S3. Spectrally shaping the amplitude spectrum of each of the different
frequency
specific stimuli to provide spectrally shaped frequency specific stimuli;
S4. Amplitude modulating either each of the spectrally shaped frequency
specific
stimuli or the combined broad-band signal with a real or simulated envelope of
running speech;
CA 02938690 2016-08-12
33
S5. Combining the spectrally shaped frequency specific stimuli to provide a
combined broad-band signal;
S6. Wherein the spectral shaping of the amplitude spectrum of each of the
different
frequency specific stimuli is configured to provide that the combined signal
corresponds to the long-term spectrum of running speech spoken with a specific
vocal effort.
In an embodiment, the method comprises applying independent speech envelopes
to each of the stimulus frequency bands. In an embodiment, the individual
frequency-specific stimuli are independently amplitude modulated by an
envelope
representing band-specific speech modulations.
In an embodiment, the method comprises providing that the imposed speech-like
modulations are quantized in time and/or level. In an embodiment, the method
comprises providing that the modulations in time are in synchrony with the
periodicity of unmodulated stimulus, so that the spectral properties of the
stimulus
within each period will remain undisturbed by the imposed speech-like
modulations.
In an embodiment, the method comprises providing a system for recording AEP,
e.g. ASSR, configured to apply the stimuli as designed by the method of
designing
stimuli as defined by steps S1-86.
It is intended that the structural features of the devices described above,
either in
the detailed description and/or in the claims, may be combined with steps of
the
method, when appropriately substituted by a corresponding process or vice
versa.
As used, the singular forms "a," "an," and "the" are intended to include the
plural
forms as well (i.e. to have the meaning "at least one"), unless expressly
stated
otherwise. It will be further understood that the terms "includes,"
"comprises,"
"including," and/or "comprising," when used in this specification, specify the
presence of stated features, integers, steps, operations, elements, and/or
components, but do not preclude the presence or addition of one or more other
features, integers, steps, operations, elements, components, and/or groups
thereof.
It will also be understood that when an element is referred to as being
"connected"
or "coupled" to another element, it can be directly connected or coupled to
the other
element but an intervening elements may also be present, unless expressly
stated
CA 02938690 2016-08-12
34
otherwise. Furthermore, "connected" or "coupled" as used herein may include
wirelessly connected or coupled. As used herein, the term "and/or" includes
any and
all combinations of one or more of the associated listed items. The steps of
any
disclosed method is not limited to the exact order stated herein, unless
expressly
stated otherwise.
It should be appreciated that reference throughout this specification to "one
embodiment" or "an embodiment" or "an aspect" or features included as "may"
means that a particular feature, structure or characteristic described in
connection
with the embodiment is included in at least one embodiment of the disclosure.
Furthermore, the particular features, structures or characteristics may be
combined
as suitable in one or more embodiments of the disclosure. The previous
description
is provided to enable any person skilled in the art to practice the various
aspects
described herein. Various modifications to these aspects will be readily
apparent to
those skilled in the art, and the generic principles defined herein may be
applied to
other aspects.
The claims are not intended to be limited to the aspects shown herein, but is
to be
accorded the full scope consistent with the language of the claims, wherein
reference to an element in the singular is not intended to mean "one and only
one"
unless specifically so stated, but rather "one or more." Unless specifically
stated
otherwise, the term "some" refers to one or more.
Accordingly, the scope should be judged in terms of the claims that follow.
CA 02938690 2016-08-12
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