Subject: Pitch orientation-discriminating feature detectors? From: "Dennis P. Phillips, Ph.D." <Dennis.Phillips(at)DAL.CA> Date: Tue, 17 Sep 2002 14:39:22 -0300
Hi Eliot (and List): At the level of coding by single neurons, there is a significant literature on sensitivity to frequency (as opposed to pitch) change and to direction of frequency change. The issue is complicated by the fact that the neural mechanisms engaged by the stimulus, and therefore the direction preference expressed in the neural responses, may depend on the form that the frequency modulation (FM) takes. Some authors have used sinusoidal (i.e., periodic) FMs. Others have used transient (often linear) FMs, either in the form of an FM tone pulse, or in the form of a transient FM of an otherwise continuous tone. As you might imagine, the constellations of adaptive, inhibitory and other mechanisms triggered by these stimuli could be very different. Indeed, my bet is that a neuron could reverse its FM direction preference depending on whether the FM was presented as a tone pulse or as a modulation of an ongoing carrier. Of course, as is often the case, the ambiguity in the response output of any single neuron is likely resolved by the ensemble response. The following is a small sample of cortical single-neuron studies on FM coding. It will give you a sense of the ways in which some single-unit people have thought about it. I hope that this helps. All good wishes, Dennis Phillips Suga, N. (1965) Analysis of frequency-modulated sounds by auditory neurones of echo-locating bats. J. Physiol., 179: 26-53. Phillips, D.P. (1988) The neural coding of simple and complex sounds in the auditory cortex. In J.S. Lund (Ed.), Sensory Processing in the Mammalian Brain: Neural Substrates and Experimental Strategies. NY: Oxford University Press, 172-203. Heil, P., Rajan, R. and Irvine, D.R.F. (1992) Sensitivity of neurons in cat primary auditory cortex to tones and frequency-modulated stimuli. I: Effects of variation of stimulus parameters. Hearing Res., 63: 108-134. Eggermont, J.J. (1994) Temporal modulation transfer functions for AM and FM stimuli in cat auditory cortex. Effects of carrier type, modulating waveform and intensity. Hearing Res., 74: 51-66. Tian, B. and Rauschecker, J.P. (1998) Processing of frequency-modulated sounds in the cat's posterior auditory field. J. Neurophysiol., 79: 2629-2642.