Physiological correlates of forward masking in single nerve‐fiber and compound neural responses recorded from the auditory nerve

Abstract

Masking can be defined as a reduction in the detectability of one sound stimulus, the probe, due to the presence of a second sound stimulus, the masker. Reduction in detectability can be quantified by measuring the increase in threshold for the detection of the probe tone. Most previous studies of the correlates of forward masking in the auditory nerve have measured reductions in averaged responses to the probe produced by the masker. It is not possible to relate reductions in averaged responses to changes in detectability since detectability depends on response magnitude and variance. However, two‐interval forced procedures can be used to measure detection thresholds for spike trains of primary, single nerve fibers [Relkin and Pelli, J. Acoust. Soc. Am. 82, 1679–1691 (1987)] and N1 of the compound action potential (CAP) [Relkin and Smith, J. Acoust. Soc. Am. Suppl. 1 83, S98 (1988)]. Relkin and Turner [J. Acoust. Soc. Am. 84, 584–591 (1988)] showed that there are large discrepancies between behavioral thresholds and thresholds for single nerve fibers for forward masking. Particularly at high masker levels, thresholds for the detection of the probe in the activity of single fibers are more sensitive than those measured behaviorally. Several hypotheses that might explain this discrepancy include the effects of spatial and/or temporal processing of the responses of single neurons. These hypotheses are beginning to be tested using both single fiber and compound neural recordings. Forward masking of the CAP is more similar to behavioral forward masking than is forward masking in single neurons, suggesting a spatial summation effect. Methods for measuring peristimulus compound potentials have also been developed to investigate temporal effects such as the relative importance of onset and steady‐state responses.