National Acoustic Laboratories

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The application of cortical auditory evoked
potential recordings in infant hearing aid fitting
Maryanne Golding, Harvey Dillon, Suzanne C
Purdy, John Seymour, Wendy Pearce, Lyndal
Carter, Richard Katsch, Mridula Sharma, Katrina
Agung, Kirsty Gardner-Berry
National Acoustic Laboratories Cooperative
Research Centre for Hearing Speech Science, The
University of Auckland, New Zealand Audiology,
Macquarie University
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Evaluation of aided functioning in infants

• Universal new born screening

Early fitting of hearing aids
Need for an evaluation method
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So baby, how does it sound?

• Objective hearing aid evaluation for
• young infants
• difficult-to-test people

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Why the rush?Language ability 6 months after
implantation
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Why use cortical responses?
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Why cortical responses to evaluate hearing aid
fitting in infants?

• Reliably present in awake young infants
• More likely to correlate well with perception
• Can be elicited by a range of speech phonemes
  close to desired outcomes
• Stimuli handled reasonably by hearing aids
• Can be very frequency specific if needed

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Where do cortical responses come from?
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The end of the road
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Auditory cortex and current sources
Sussman et al (2008)
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Cortical responses in adults with normal hearing
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Adult
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Adult grand mean waveforms at Cz
Speech
Tones
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Cortical responses in infants
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Infants
P
N
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Maturational effects on cortical evoked response
morphology

• N8-16 per grand mean
• Cz site
• stimulus 10 click train, 2 ms ISI _at_ 65 dB SL
• rate 1.3/s

• Ponton et al (2000)

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2 years
12 years
I
?
II
? Fewer neuro-filaments in young children,
especially in more superficial cortical layers
thought to generate N1 (Ponton, Moore
Eggermont 1999)
III
IV
V
Axonal neuro- filaments
Axonal neuro- filaments
VI
Cell bodies
Cell bodies
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Latency versus age
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(No Transcript)
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Maturation with time in sound

• Ponton and Eggermont 2007

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Auditory system maturity

• Ponton and Eggermont (2007)
• Latency matures consistent with the time in
  sound
• Sharma (2002)
• Provided implantation occurs by 4 years of age

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Cortical responses in infants to different speech
sounds
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Grand Average n 16 infants
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Number of subjects (out of 20) with significant
differences between responses
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Number of infants (N20) with significantly
different cortical responses to pairs of stimuli
m vs t m vs g t vs g
Based on MANOVA at Cz, 101 to 500 ms post-onset,
in eight bins each 50 ms
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Three stimuli
m
t
g
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Why not obligatory cortical responses?

• variable shape across ages
• variable shape with auditory experience
• variable shape from person to person
• variable shape from time to time (state of
  person, especially sleepiness)
• stimulus
• Inter-stimulus interval

An automated method of response detection and
response differentiation
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Automatic detection of cortical responses
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Desirable characteristics

• No reliance on a template
• Able to use information from contributing
  portions of waveform
• Able to discount non-contributing portions of
  waveform

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Analysis using Hotellings t2 statistic
X3
Voltage
Time

• Divide each record into 50 ms time bins
• Average data points within each time bin
• Use these averages as variables in
  Hotellings t2 analysis
• Result is probability of the
  waveform being random noise

X a1X1 a2X2 ........ a9X9 Test is
there any set of weighting coefficients for which
X ? 0?
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Presentation of average response in series
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Receiver Operating Characteristics Expert judges
Sensitivity
d
1 - Specificity
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d results - for 60 stimuli
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d results - for 200 stimuli
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Infants Hotellings versus experts

• Normal hearing infants aged 7 to 16 months

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Growth of amplitude with SLadults tonal stimuli
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Loudness growth above threshold
Hellman Meiselman, 1990
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Detectability (adults tonal stimuli)
P0.05
P0.01
P0.001
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Significant responses normal hearing and
hearing impaired Adults tonal stimuli (n100 or
200)
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Proportion with responses present - adults
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• .. but infants move around !

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Residual noise level

• rms noise standard deviation / vn

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Residual noise levels (for 100 epochs)
Adults
5
0
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Proportion with responses present (plt0.05)
normal hearing infants 100 epochs
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Cortical responses and functional performance

• Do cortical responses tell us about real-life
  auditory performance?

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Parents Evaluation of Aural/oral performance in
Children (PEACH) Questionnaire

• Parents are asked to describe their babys
  aural/oral
• skills based on real-life experiences
  (listening in quiet
• and in noise and alertness to environmental
  sound)
• Scores are assigned based on the number of
  observed
• behaviors and how frequently these occur.
• Final overall score of 0 40 can be calculated
  (and
• reported as a percentage).

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Functional deficit versus cortical score
All aided children rs 0.60 n24 p 0.001
SN only rs0.61 n12 p 0.02 MD
only rs0.82, n5 p 0.04 AN only rs0.36
N7 p 0.22
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Reducing measurement variability (random
electrical signals) ? Speeding up
measurements? Increasing validity of
interpretation
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Active electrodes
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Capacitive Coupling 50 Hz
Passive Electrodes
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Capacitive Coupling 50 Hz
Active Electrodes
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Capacitive Coupling 50 Hz
Active Electrodes
Passive Electrodes
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Finding thresholds with cortical responses

• What does an absent cortical response mean?

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Cortical auditory evoked responses traditionally
used for objective assessment of hearing
thresholds in adults

• In 1965 Hallowell Davis showed good agreement
  between cortical and pure tone thresholds in
  children
• For many years cortical response audiometry has
  been regarded as the gold standard for
  objective electrophysiological hearing assessment

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Davis (1965) Cortical evoked potential versus
behavioural thresholds
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From Rickards, F. et al (1996) Cortical Evoked
Response Audiometry in
noise induced hearing loss claims. Aust.
J. Otol. 2 (3)
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Clinical applications and implications
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Clinical applications of corticals

• For finding thresholds (when awake)
• Determining whether speech sounds are audible
• aided or unaided
• for patients who cant respond reliably by
  behavioral testing e.g., infants, multiply
  disabled people.

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Clinical implications of corticals

• Significant response is obtained to speech at 65
  dB SPL
• No significant response is obtained to speech at
  65 dB SPL or to speech at 75 dB SPL

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Clinical implications of corticals (cont)
No /t/ response
Mixed results (and noise is low)
No /g/ response
No /m/ response
Mixed results (and noise is high)
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Corticals for more advanced measurements
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Application for auditory neuropathy (AN)

• 15 of babies found to have hearing loss at birth
  in NSW have AN
• Management unclear (no device, hearing aid or
  cochlear implant)
• Rance showed close relationship between cortical
  response in older children and benefit from
  hearing aids
• Gap detection worse in people with AN
• Investigating gap detection by cortical responses

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/Ah/ 2 second duration
0msG.avg
5msG.avg

10msG.avg
20msG.avg
50msG.avg
5.0
2.5
Offset
0.0
Onset
µV
Gap
-2.5
-5.0
-7.5
-350.0
150.0
650.0
1150.0
1650.0
2150.0
2650.0
ms
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Summary

• Cortical responses
• For checking the audibility of speech sounds
• Indicate the maturity of the auditory system
• Automatic detection as good as experts
• Residual noise size critical
• For checking hearing thresholds when the patient
  is awake

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• Thanks for listening
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