NEURAL CORRELATES OF THE PITCH OF COMPLEX TONES .1. PITCH AND PITCH SALIENCE

1. The temporal discharge patterns of auditory nerve fibers in Dial-an esthetized cats were studied in response to periodic complex acoustic waveforms that evoke pitches at their fundamental frequencies. Single- formant vowels, amplitude-modulated (AM) and quasi-frequency-modulated tones, AM noise, click trains, and other complex tones were utilized. Distributions of intervals between successive spikes (''1st-order int ervals'') and between both successive and nonsuccessive spikes (''all- order intervals'') were computed from spike trains. Intervals from man y fibers were pooled to estimate interspike interval distributions for the entire auditory nerve. Properties of these ''pooled interspike in terval distributions,'' such as the positions of interval peaks and th eir relative heights, were examined for correspondence to the psychoph ysical data on pitch frequency and pitch salience. 2. For a diverse se t of complex stimuli and levels, the most frequent all-order interspik e interval present in the pooled distribution corresponded to the pitc h heard in psychophysical experiments. Pitch estimates based on pooled interval distributions (30-85 fibers, 100 stimulus presentations per fiber) were highly accurate (within 1%) for harmonic stimuli that prod uce strong pitches at 60 dB SPL. 3. Although the most frequent interva ls in pooled all-order interval distributions were very stable with re spect to sound intensity level (40, 60, and 80 dB total SPL), this was not necessarily the case for first-order interval distributions. Beca use the low pitches of complex tones are largely invariant with respec t to level, pitches estimated from all-order interval distributions co rrespond better to perception. 4. Spectrally diverse stimuli that evok e similar low pitches produce pooled interval distributions with simil ar most frequent intervals. This suggests that the pitch equivalence o f these different stimuli could result from central auditory processin g mechanisms that analyze interspike interval patterns. 5. Complex sti muli that evoke strong or ''salient'' pitches produce pooled interval distributions with high peak-to-mean ratios. Those stimuli that evoke weak pitches produce pooled interval distributions with low peak-to-me an ratios.6. Pooled interspike interval distributions for stimuli cons isting of low-frequency components generally resembled the short-time autocorrelation function of stimulus waveforms. Pooled interval distri butions for stimuli consisting of high-frequency components resembled the short-time autocorrelation function of the waveform envelope. 7. I nterval distributions in populations of neurons constitute a general, distributed means of encoding, transmitting, and representing informat ion. Existence of a central processor capable of analyzing these inter val patterns could provide a unified explanation for many different as pects of pitch perception.