Two‐tone interactions in the auditory cortex of the squirrel monkey

Abstract

Two‐tone interactions are recorded in the responses of single units in the superior temporal gyrus to contralateral acoustic stimulation of the awake squirrel monkey. Four response types are distinguished based primarily on the nature of the two‐tone response, and secondarily on criteria such as the patterns of response to single tones and noise stimuli, thresholds, and spontaneous activity levels. Type A units (24/80) display strong lateral inhibitory influences which may extend up to two octaves on either side, or both sides, of the BF. They are sharply tuned at all intensities, and generally exhibit sustained response to single‐tone stimuli at the BF. The units have nonmonotonic rate‐level functions, and where tested, show little or no response to broadband noise. Most type A units have low spontaneous rates (< 1 spikes/s), and relatively high thresholds (>30 dB SPL). Type B units (22/80) are characterized by relatively high spontaneous rates of activity (>20 spikes/s) and inhibitory responses to single‐tone stimuli. Broadband noise is applied to a few type B units, and in all cases it evokes strong excitatory response. Type C units (17/80) summate the responses to the two‐tone stimulus, and show little or no inhibitory influences. They have V‐shaped tuning curves, monotonic rate‐level functions, low thresholds (<30 dB SPL), moderate spontaneous rates (∼ 10 spikes/s), and a strong and sustained response to noise and single tone stimuli. Type D units (17/80) show “temporal inhibition” to two‐tone stimuli, in that an excitatory response to the first tone suppresses (adapts or inhibits) the response to the second tone. These units generally have moderate to broad frequency tuning and phasic responses to single tone stimuli. Histological examination of electrode tracks suggests that type A units are restricted to A1 (and possibly the rostral field) while other types are distributed over all auditory fields.