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Subject: Auditory illusions

Dear Nedra,
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ Although I am not an expert in this area, but I have done some preliminary (unpublished) work on the use of the McGurk effect for the evaluation of current day multichannel digital hearing aids. Few of the basic assumptions for the rationale and implications were as follows:
1)ÂÂÂÂÂ One of the main benefits of audio visual integration is in difficult listening situations when either the speech is
degraded or there is background noise
2)ÂÂÂÂÂ The visual cue in an auditory visual integration task is relatively unaffected by noise.
3)ÂÂÂÂÂ Hearing impaired listeners especially those with relatively larger auditory deprivation periods rely more on visual cues (the AV balance is slightly tilted towards visual dominance) in speech perception than normal hearing listeners, thus making them better speech readers and also relatively poor AV integrators.
4)ÂÂÂÂÂ The psycho-acoustical aspects of sensori-neural hearing loss suggests a reduction in the possibility of perceiving certain
classes of speech sounds especially in presence of noise.
5)ÂÂÂÂÂ Moreover some of the speech sounds may not be heard in noisy environments no matter how intensely they are amplified by the
hearing aids
6)ÂÂÂÂÂ The Digital multichannel hearing aids may also have some amount of internal distortion and delay due to digital processing and
7)ÂÂÂÂÂ In such scenarios the hearing aid user may benefit from the visual cues provided by a speakers facial and lip movements
which should relatively be unaffected by noise and thus boost speech perception
for energetic masking at poor signal to noise ratios as well as in
informational masking.
8)ÂÂÂÂÂ We presented hearing aid users (relatively homogenous group in terms of hearing aid used) the Mc Gurk stimuli in both
congruent and incongruent conditions in quiet and in noise (three different SNRs) at comfortable level at which the subjects scored > 70% on a screening test using PB words.
9)ÂÂÂÂÂ A criterion of 3/4th fusion responses was kept to determine the presence of Mc Gurk effect
10)ÂÂ The results indicated that subjects with normal hearing performed better than the subjects using HAâs in all conditions. The presence of auditory and visual information simultaneously in the congruent condition was beneficial for speech perception in quiet and in noise. Noise
reduced the subjects ability to perceive speech at poor SNR and had a more severe impact on the performance of the HA users than in normal subjects. The McGurk effect was absent in the HA users at the poorer SNRs compared to normal
listeners and in these conditions the HA users responses to the McGurk stimuli
were mainly visually dominated.
11)ÂÂ Background noise and increased listening effort are significant factors influencing
hearing-aidsatisfaction and one of the major reason for rejection of HAâs.
12)ÂÂ Testing McGurk effect in a noisy environment may be a useful way to understand auditory
visual speech perception in HA users and verify the benefits of aided AV speech
perception in noise in HA users.
13)ÂÂ Some of the implications in terms of hearing aid rehabilitation for the same may be
in terms of enhancing optimal HA fitting to achieve not only good auditory perception
in noise but also optimum auditory visual perception in noise and the emphasis
on auditory training and the use of speech reading skills. 
14)ÂÂ The study that I have mentioned above was in no way devoid of limitations like lesser sample size and the hearing aids were used at the same programs that the HA users were using in their everyday listening environments and thus all the user had different program settings. May be the rationale and implications might be of some interest to you though.

Imran Dhamani