Effects of behavioral performance on single-unit firing patterns in inferior colliculus of the rhesus monkey

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Abstract
Single-unit responses to auditory stimuli were examined in the inferior colliculus (IC). Rhesus monkeys (6) were trained in a reaction-time (RT) task. Unit responses to stimuli during performance were compared with those to the same stimuli delivered without the behavioral task. For most units, rate of evoked discharge and average initial latency were significantly increased during RT performance; spontaneous rate of discharge was unaffected. In a smaller number of units, increases in evoked discharge were confined to specific portions of the unit''s response pattern, and might be accompanied by suppression of a 2nd component of the unit''s response. Increases in evoked discharge frequently occurred without appreciable latency. Greater and more consistent increases in evoked discharge were seen from 75-200 ms after stimulus onset than from 0-75 ms. Performance-associated increases in evoked discharge were more prominent in suppressed unit responses than in excitatory responses. Offset responses were consistently decreased during performance for stimuli longer than 150-200 msec in duration. Because different response components were affected differently by performance, spontaneous discharge was unaffected and initial latency was increased during performance, a tonic increase in excitatory input to the IC is probably not responsible for the effects described. Changes in initial latency also rule out a stimulus-evoked, descending influence on the IC as the sole mediator of the effects of RT performance. The delayed appearance of maximal discharge rate increase and offset response suppression indicated that stimulus-evoked events may be involved.