Re: finger nails on blackboard

[Neil Todd <todd@xxxxxxxxxxxxxxxxx>]

>
> Date:    Tue, 31 Jul 2001 09:33:20 +1000
> From:    Michael Norris <michaeln@CSEE.UQ.EDU.AU>
> Subject: Re: finger nails on blackboard
>
> On Mon, 30 Jul 2001, Neil Todd wrote:
>
>> So why do sounds with frequencies between 1-2 kHz cause the effect? My guess
>> is that the effect is produced by activation of various myogenic reflex
>> responses including the stapedius response, the post-auricular response and
>> responses of other muscles innervated by the facial nerve (and possibly the
>> trigeminal nerve). It so happens that the tuning curves of stapedius
>> motorneurons have their best frequencies between 1-2 kHz with a threshold of
>> about 75 dB in the cat (see Kobler et al. (1992), J. Neurophysiol. 68,
>> 807-817).  (These should be distinguished from myogenic vestibular responses
>> mediated by the accessory nerve, which responds to frequencies less than 1
>> kHz.) In order for this to work then the scraping sound would have to be
>> above about 75 dB, but it's not clear from Halpern et al. what intensity
>> they presented the sounds to the subjects.  However, the proposed mechanism
>> would account for why the effect appears to be reflexive. It can't be very
>> pleasant having all those muscles twitching away!
>>
>> Neil
>
> The next question is:  is the temporal structure of the sound important?
> Halpern&al tried flattening the amplitude countour and found only a
> small difference, but that may not have much affected the amplitude
> contour in the 1-2KHz band. I'm thinking that if the effect is due to
> something like this muscle twitching, maybe the stimulus needs to be
> aperiodic to avoid adaptation.  ?  or is that a silly thing to say?
>
> -m.


I would suspect that in principle the temporal structure is important. The
middle ear reflex is generally associated with impulsive sounds. The
amplitude contour of the 1-2 kHz band may contain peak values (which may be
as much as 20 dB higher than the slow time-weighted SPL) above about 75 dB
at random intervals. If the mean interval was greater than a certain value
then I would also suspect that the typical time course of the scraping sound
would not be sufficient for significant adaptation to take place,
particularly for moderate intensities. However, Halpern et al.
note that the effect is lessened after repeated exposure. Whether this is
adaptation or habituation, or whether the randomness of the impulsivity of
the stimulus is important, I could not say. Clearly further experimentation
is required.

Neil