Single-unit activity in the auditory cortex of monkeys selectively attending left vs. right ear stimuli

Single-unit responses were recorded from the auditory cortex of rhesus monkeys that were performing an auditory selective attention task. Acoustic stimuli were presented randomly to either the left or right ear through headphones. In a given block of trials one ear was selected as the ear to be attended, and was indicated by lighting either a left or right response key. The animal's task was to press the lighted key whenever stimuli were presented to the attended ear, but to make no response to stimuli presented to the other ear. The attended ear was alternated on successive blocks of trials. Fourteen of 77 units showed significantly greater rates of evoked discharges for an attended stimulus than for an identical non-attended stimulus. The increase in stimulus-evoked activity was not accompanied by any increase in spontaneous activity or by any changes in the pattern of a unit's response. Changes in firing rate occurred at latencies as early as 20 msec.