Interspike intervals as a correlate of periodicity pitch in cat cochlear nucleus

Amplitude modulated (AM) signals have often been used as precisely defined partial analogs of speech sounds. This study considers the response to an AM complex with 200% sinusoidal modulation, that is, the amplitudes of the three AM components are equal. By varying the carrier frequency across the entire frequency range of unit response, it is shown that units in the cochlear nucleus of cat are relatively insensitive to variation in the carrier frequency, which is to say that population response to an AM signal at a fixed locus will be widespread. These stimuli and procedures result in the presentation of both harmonic and inharmonic complexes, and thus permit assessment of neural responses for the information needed to make spectral or time-domain pitch matches. It is shown that the reciprocals of the modes (favored intervals) in the interspike interval histogram reflect the first effect of pitch shift, which is defined psychophysically as a proportional shift in pitch to the change in carrier frequency. In particular, interspike intervals of units with a widespread spectral response provide a basis to explain phase and dominant component pitch behavior that early narrow-band pitch theories found problematical. The amplitude of phase locking to individual AM components varies systematically though there are some unexplained variations across the frequency-intensity plane that could be due to combination tones. The unit response to a quasifrequency modulated (QFM) stimulus shows that if pitch is based on interspike intervals, it would remain the smae as pitch for an AM signal. The magnitude of the synchrony response to QFM stimuli is less than to AM stimuli for the majority of cochlear nucleus units; however, there are exceptions.