Re: pitch neurons (1) (Peter Cariani )

Subject: Re: pitch neurons (1)
From:    Peter Cariani  <peter(at)EPL.MEEI.HARVARD.EDU>
Date:    Wed, 9 Oct 2002 18:21:48 -0400

(I sent this out to the list on Monday, but it bounced because I sent it from the wrong email account. I have a second comment that I wrote late this afternoon that I will also send out.) Eli wrote: I would like however to raise another issue: why is there such a pressure to assume low-level representation (i.e. subcortical) of pitch? After all, pitch is a pretty high-level property of sounds, it is invariant to many features of the physical structure of the sounds, and it is affected by all kinds of high-level cognitive effects (e.g. capture of harmonics in streaming conditions). All of these would suggest to me that whatever is responsible for the pure representation of pitch (independent of the physical structure of the sounds) is rather high-level, rather than low-level. ------------------------------------ Hi Eli, everyone I really wonder about the assumption of low-level and high-level properties and their linkage with low-level (early, peripheral) and high-level (later, central) processing. Certainly pitch is affected by how the auditory system forms stable objects/streams, but object formation itself need not be an exclusively cortical operation per se. What evidence leads inescapably to this conclusion? There are a number of demonstrations (of Kubovy and others) that, while low-frequency hearing is largely insensitive to phase spectrum, transient changes in the relative phases of components can lead to perceptual pop-outs/stream separations. Since phase-locked information is most abundant in more peripheral stations, it may be the case that primitive auditory grouping occurs lower down in the system than we might suppose. The cortex might be playing the role of organizing processing in the lower centers rather than a locus for the representations themselves. We understand so little about the detailed workings of the auditory system at and above the midbrain. It's really too early to say that pitch must be processed here or there, especially since no convincing model for the central representation and/or processing of pitch has been proposed. Even the representation of pure tones at these levels has many of the same problematic aspects that are apparent at the level of the auditory nerve (invariance of percepts over a large range of stimulus intensities; the disconnect between neural tuning and frequency discrimination, as a function of frequency). We have yet to find an abundance of units that look like real pitch detectors anywhere in the system (there are reports of a few units here or there, such as Riquimaroux's 16 F0 units or Langer's recently-found units or high-BF units with multipeaked tuning curves at harmonics 1 and 2). I think that this means either 1) that the central representation is "covert" and sparsely distributed across many neurons (either in a spatial or temporal code) or 2) that we are thinking the wrong way about the whole problem, and that the representation is lower down, albeit controlled and accessed by the auditory cortex. The first possibility suggests a mechanism based on "mass action" -- the more units in a given frequency region, the better the coding (Recanzone's study a decade ago), as opposed to a local feature model in which specialist detectors are highly tuned to one freq/periodicity or another. Connectionist theory notwithstanding, we don't really know how a system based on mass-action would work in practice. on the other hand, it is possible that the auditory cortex may be necessary for fine-grained pitch discrimination, but that no fine representation of pitch per se exists there. (I have yet to see any units in the cortical literature with sharp tuning better than 0.3-0.5 octave for 1 kHz tones -- all the really sharp tuning is for BFs > 5 kHz. It makes me wonder. At the lower stations one sees coarse tuning, but then there is also precise and abundant interval information that is available.) I think that it is clear that we need a concerted effort to understand the detailed, mechanistic nature of the central representations and neurocomputations that subserve the basic auditory qualities of pitch and timbre (something like what is now going on for sound localization). Until we have this many aspects of auditory function will remain latent mysteries. To Eckard: There is a common misconception that single ANFs must fire every cycle in order to encode a tone's frequency. Warren Wever (and perhaps LT Troland before him) proposed the "volley principle" to get around this. Even without a volley principle per se, there are ways of processing interval statistics that allow comparison of whole interval distributions (e.g. peaks at 2/f, 3/f, 4/f, ... , n/f). Regarding the issue of coding of high frequencies, we have to remember that phase-locking weakens only for high frequency pure tones. Once multiple simultaneous tones are introduced, then ANFs can potentially follow the time structure of their interactions. In Viemeister's 4000 + 8003 Hz experiment, interaction of the two tones might create a detectable 3 Hz intensity fluctuation at the 8 kHz place. Octave matching and musical interval recognition go to pot above about 4 kHz (a strong argument for an interval basis of pitch for tones < 4 kHz), but this is likely a different sort of perceptual task from the one Viemeister studied. --Peter Cariani

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