Arousal effects on cochlear potentials: Investigation of a two-factor hypothesis

Abstract

Cochlear potentials were recorded from the round window in unanaesthetized cats. Cochlear microphonic (CM) and auditory-nerve action potential (AP) responses to clicks were examined in different states of arousal. Middle ear muscles (MEM) were unilaterally tenotomized and acoustic input variability was controlled. High and low click sound pressure levels were selected from regions of maximal and minimal slope, respectively, on the AP intensity function. In MEM-intact ears both CM and AP responses to high-intensity clicks were reduced relative to slow-wave sleep amplitude during periods of ocular quiescence in REM sleep and their abolition by MEM tenotomy establish their dependence on non-reflex MEM activity. Although responses to low-intensity clicks exhibited the same general trends in these states, the effects did not achieve significance. At both stimulus levels, however, there were large and significant reductions in AP during movement in the awake animal. These reductions occurred without associated changes in CM and were only marginally affected by MEM tenotomy. They were attributed to a second factor identified as masking by movement-produced noise. The data support the contention that certain types of rhythmical motor activity involve only minimal activation of the middle ear muscles. However, they provide little support for the proposal that MEM activity might provide protection against masking of neural responses to low-level sounds by movement-produced noise.