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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?


----------------------- /\/\/\/- -------------------------------------
 The graph data may not convey the true nature of the screech -  the
 reader may add a visceral sense to the consideration of this report
 by chewing on a piece of aluminium foil while viewing the data.
                           -- Barry McKinnon, `Fork Screech Analysis'