Neuronal mechanisms for pitch analysis in the time domain.

Many units in the auditory midbrain nucleus (MLD) of the Guinea fowl are found to be tuned to amplitude modulated tones (AM). For a given response maximum the relationship of the period tau m of the modulation frequency fm and the period tau c of the carrier frequency fc may be given by an empirical equation: m . tau m + n . tau c = 1 . tau l, where m, n and l are small integers typical for a unit. tau l is a time constant of 0.4 ms. The temporal pattern of the neuronal response support these findings. The averages of spike trains oscillate with periods multiple to tau l. These oscillations are elicited by stimulus onsets and zero crossings of fm and may by coupled strongly to fm depending on fc. Variation of fm or fc shifts the mean delay of the phase coupled activity proportional to m . tau m and n . tau c, respectively. These effects may be explained with activity phase coupled to fc which coincides at the level of the recorded units with oscillations coupled to fm. This is expressed by the above given periodicity equation. Psychophysical results with AM-stimuli indicate that the mechanisms described and the periodicity equation are adequate for the explanation of the analysis of periodicity pitch in humans. Hence the period corresponding to pitch is defined by tau p = n . tau c-1 . tau l, where n and 1 are integers and tau l = 0.4 ms. Plots of tau p as a function of tau c reveal steps at 0.4 ms intervals indicating that the neuronal time constant is the same in both species.