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Periodicity pitch is the most salient and important of all pitch percepts.Psycho-acoustical models of this percept have long postulated the existenceof internalized harmonic templates against which incoming resolved spectracan be compared, and pitch determined according to the best matchingtemplates cite{goldstein:pitch}. However, it has been a mystery where andhow such harmonic templates can come about. Here we present a biologicallyplausible model for how such templates can form in the early stages of theauditory system. The model demonstrates that {it any} broadband stimulussuch as noise or random click trains, suffices for generating thetemplates, and that there is no need for any delay-lines, oscillators, orother neural temporal structures. The model consists of two key stages:cochlear filtering followed by coincidence detection. The cochlear stageprovides responses analogous to those seen on the auditory-nerve andcochlear nucleus. Specifically, it performs moderately sharp frequencyanalysis via a filter-bank with tonotopically ordered center frequencies(CFs); the rectified and phase-locked filter responses are further enhancedtemporally to resemble the synchronized responses of cells in the cochlearnucleus. The second stage is a matrix of coincidence detectors thatcompute the average pair-wise instantaneous correlation (or product)between responses from all CFs across the channels. Model simulations showthat for any broadband stimulus, high coincidences occur between cochlearchannels that are exactly harmonic distances apart. Accumulatingcoincidences over time results in the formation of harmonic templates forall fundamental frequencies in the phase-locking frequency range. Themodel explains the critical role played by three subtle but importantfactors in cochlear function: the nonlinear transformations following thefiltering stage; the rapid phase-shifts of the traveling wave near itsresonance; and the spectral resolution of the cochlear filters.

We compared the response properties of single units to tones and sinusoidally, rippled spectral stimuli in the primary auditory cortex (AI) of the, barbiturate-anesthetized ferret. Using two- tone stimuli, we determined the response area of each cell and parameterized it in terms of best frequency (BF), the bandwidth of the excitatory responses at 20dB above threshold (BW20), and an asymmetry index measuring the balance of inhibition and excitation around the BF.Using frequency-modulated (FM) tones, we also determined a directional sensitivity index for the cell. Using broadband stimuli (1-20 kHz) with sinusoidally modulated spectral envelopes (ripples), we measured the response magnitude of each cell as a function of ripple frequency (W) and ripple phase (F), and then reconstructed the magnitude and phase of a ripple transfer function. Most cells (approximately 90 %) were tuned to a specific ripple frequency, denoted as a characteristic ripple frequency (Wo) . Most cells also exhibited a linear ripple phase as a function of W. The intercept of the phase function defined as the characteristic ripple phase (Fo), and is interpreted as the best ripple phase to drive the cell; the slope of the phase function reflects the location of the response area of the cell along the tonotopic axis. By inverse Fourier transforming the transfer function, we obtain the response field (RF) of the cell, an analogue of the response area measured with tonal stimuli. Like the response area, the RF was parametrized by the following measures: BFRF, which is the location of the maximum of the RF along the tonotopic axis, Wo , which is roughly inversely proportional to the width of the RF, and Fo which reflects the asymmetry of the RF. In the ferret Wo , ranges from 0.2 to 3 cycles/octave, with the average of the distribution around 1.0. Fo , ranges over the full cycle in a Gaussian-like distribution around 0o. For a subgroup of cells the sinusoidal modulations of the spectrum were present

We studied the topographic organization of the response areas obtained from single - and multi-unit recordings along the isofrequency planes of the primary auditory cortex (AI) in the barbiturate-anesthertized ferret. Using a two-tone stimulus, the excitatory and inhibitory portions of the response areas were determined and then parameterized in terms of an asymmetry index. The index measures the balance of excitatory and inhibitory influences around the best frequency (BF). The sensitivity of responses to the direction of a frequency-modulated (FM) tone was tested and found to correlate strongly with the asymmetry index of the response areas. Specifically, cells with strong inhibition from frequencies above (below) the BF preferred upward (downward) sweeps. Responses to spectrally shaped noise were also consistent with the asymmetry of the response areas: cells that were strongly inhibited by frequencies higher than the BF responded best to stimuli that contained least spectral energy above the BF. In a local region, most cells exhibited similar response area types and other response features. the distribution of the asymmetry index values along the isofrequency planes revealed systematic changes in the symmetry of the response areas. At the center, response areas with narrow and symmetric inhibitory sidebands predominated. These gave way to asymmetric inhibition, with high frequency inhibition (relative to the BF ) becoming more effective caudally, and low frequency inhibition more effective rostrally. These response types tended to cluster along repeated bands that paralleled the tonotopic asix. One functionaly implication of the response area organization is that cortical responses encode the locally averaged gradient of the acoustic spectrum by their differential distribution along the isofrequency planes. This enhances the representation of such features as the symmetry of spectral peaks, edges, and the spectral envelope. This scheme can be viewed as the one

A functional model of the primary auditory cortex is proposed based on physiological maps of the receptive field organization in ferret AI (Shamma, Fleshman and Wiser [1990]). Systematic changes in the excitatory and inhibitory portions of the receptive fields along the isofrequency planes are approximated by a difference of gaussians function with spatially changing parameters. We consider here only the response properties to stationary stimuli, i.e. those with non-varying spectra. The fundamental functional principle that emerges from the analysis of the model is that the primary auditory cortex encodes the shape of the acoustic spectrum in the distribution of its responses along the isofrequency planes. Specifically, it maps to each isofrequency plane a normalized measure of the locally averaged gradient of the input spectrum at that frequency. Physiological and psychoacoustical correlates and implications of these findings are explained. Parallels to the functional organization of the visual cortex are also discussed.

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