Arousal effects on unit responsiveness to vocalizations in squirrel monkey auditory cortex

Single neurons in the auditory cortex of awake squirrel monkeys were tested with species-specific vocalizations with and without concurrent stimulation of the midbrain reticular formation (RF). Fourteen of 55 cells (25%) tested with RF stimulation alone gave time-locked responses to successive trains of RF stimulation. Fifteen of 39 neurons (36%) showed changes in response strength or pattern to vocalizations presented with concurrent RF stimulation. However, only 4 (10%) changed their selectivity to specific vocalizations. These included cells driven by trains of RF stimulation, and those which were not. Non-stationarity in spontaneous discharge rate occurred in many of the neurons studied, but was not correlated with changes in responsiveness to vocalizations. Analysis of responsiveness to vocalizations and spontaneous and RF-induced changes in cortical EEG activity likewise revealed no correlation. These results suggest that the response selectivity to specific vocalizations of auditory cortex neurons is relatively stable, and is not greatly influenced by RF-mediated changes in cortical excitability.