Physiological correlates of auditory stimulus periodicity

Abstract

Microelectrode studies of single auditory nerve fibers provide information on physiological mechanisms postulated in the two classical theories of pitch perception. The place principle is exemplified by the threshold vs. frequency or tuning curve for a fiber which typically exhibits maximum sensitivity to a narrow band of frequencies. Computer-controlled stimulus presentation has permitted systematic exploration of the supra – threshold response area throughout the entire domain of effective frequencies and SPL's. These studies demonstrate that the assignment of a single characteristic frequency constitutes an inadequate description of the frequency selectivity of a fiber since the frequency of lowest threshold may be appreciably different from the frequency which causes maximal discharge at higher SPL's. In general, the complexity of the response areas is difficult to reconcile with the simplistic concept that each fiber can be associated with a specific pitch.

The Volley theory predicts that stimulus periodicities will be reflected in the temporal rhythms of neural discharge. Precisely this result is seen in the phase-locked response of auditory nerve fibers to low frequency pure tones for which the discharges occur preferentially during the restricted segment of the stimulus cycle. Under these circumstances the discharges are spaced at intervals which are grouped around the integral multiples of the period of the tone, regardless of the frequency or intensity of an effective stimulus and independent of the characteristic frequency of the fiber. When 2 harmonically related pure tones are combined, the temporal distribution of spikes throughout the period of the resulting complex weve resembles the stimulus waveform. Domination of the excitatory process by 1 tone as its SPL is raised relative to the other leads to results which may relate to psychoacoustic masking. These observations argue that hearing theorists should give greater consideration to the temporal aspects of neural discharge.

Key Words:

• Auditory nerve fibers
• Temporal discharge patterns
• Pitch perception
• Two-tone interaction
• Auditory masking