Autocorrelation (Christian Kaernbach )

Subject: Autocorrelation
From:    Christian Kaernbach  <chris(at)PSYCHOLOGIE.UNI-LEIPZIG.DE>
Date:    Fri, 14 Jul 2000 09:19:05 +0200

Peter Cariani wrote: > As far as objections to population-interval representations go, the > conclusions of Kaernbach and Demany should not be taken at face value Dear Peter, I am so sorry I have not the time to follow these interesting but lengthy discussions in detail. Let me point out shortly in answer to your comments on our paper: > 1) The "autocorrelation" model that they knocked down was not neural > model; = It would be difficult to knock down all possible neural autocorrelation models. Our experiment gave a hint that there might be a problem with higher-order regularities which are seen by autocorrelation, but not by perception. I did not see a contribution up to now where somebody showed that neural autocorrelation would produce this asymmetry between first- and higher-order intervals. We supplied some modelers with our stimuli, and they could not make their models produce this asymmetry. > 2) The stimuli were harmonic complexes whose harmonics (F0=3D100 Hz) > were all above 5 kHz In our new submitted paper (you should know of it) we are down to 2 kHz, and the asymmetry is just the same. It is a real problem to go down much lower, as one should exclude frequencies lower than 15 times the fundamental. You did not specify whether in your below-2-kHz click trains resolvable harmonics were excluded (and masked for distortion products). In our understanding, it is not the frequency region but the resolvability that counts. > 3. K & D assumed that each of their clicks would give rise to a spike > in an auditory nerve fiber. Not precisely so. We only assumed that _most_ of the inter-spike intervals on the auditory nerve would correspond to inter-click (inter-stimulus) intervals. But this assumption is not crucial to our argument. Please let me cite from our General Discussion section: It is plausible that the =91=91final=92=92 temporal structure contrib= uting to pitch sensations (either directly or after a conversion into a place code) does not occur in the auditory nerve but at a higher location in the auditory system. We believe that at this stage the ISIs that matter are first-order ISIs. However, the consecutive spikes bounding these ISIs may originate from nonconsecutive spikes at the auditory nerve level. > 5. In short, lower frequency hearing has more autocorrelation-like > qualities (intervening clicks don't mask much; ...), > while high frequency hearing has more modulation-like qualities > (intervening clicks mask, ...). Again, IMHO it is resolvability that counts. When I asked at the ASA meeting in Berlin what could be a generally accepted boundary between low- and high-frequency regions I was pointed to the 4 kHz boundary where neural phase locking ends. With our new publication we are well beyond this limit (i.e. intervening clicks mask in stimuli starting at 2 kHz). On the other hand, it is a very simple demonstration that intervening clicks don't mask in the high-frequency region AS LONG AS THERE ARE RESOLVABLE HARMONICS. I hope this clarifies. Sorry to be lengthy. It will not happen again... Christian

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