Temporal Position of Discharges in Single Auditory Nerve Fibers within the Cycle of a Sine-Wave Stimulus: Frequency and Intensity Effects

Period histograms, which display the distribution of spikes throughout the period of a periodic stimulus, were computed for discharges recorded from single auditory nerve fibers of the squirrel monkey when low-frequency tones were employed. For frequencies up to about 4 kHz, where phase locking is observed, the average phase angle of the discharges was tracked as frequency was varied in small steps from low to high. To a first approximation, the cumulative shift in average phase angle is a linear function of frequency as would be observed for an ideal delay line. The slopes of these phase-versus-frequency lines were found to be related through a power law to the best frequencies of the fibers. Thus, it seems possible to estimate the travel time of a mechanical disturbance between the oval window and any point on the cochlear partition. A more detailed examination revealed that average phase angle is also sensitive to intensity, depending upon the relation of the stimulating frequency to the best frequency. For stimulating frequencies below best frequency, discharges tend to occur progressively later in the cycle as intensity increases. Above best frequency, an opposite tendency often prevails if a change in phase angle occurs. At or near best frequency, little change in average phase angle is noted as intensity varies.