Unitary afterdischarges from the auditory cortex of the squirrel monkey

The origin, characteristics, and functional significance of cortical-evoked after-discharge (AD) potentials after a primary response due to some form of sensory stimulation have not been determined. The purpose of this investigation was to search the primary auditory cortices for single neuron responses which contain late-appearing AD potentials, describe their characteristics, and determine their cytoarchitectonic origin. Burst AD appeared in the lower layers of Golgi lamina III; true reverberating AD units (RAD) appeared in laminae IV and V, were frequency sensitive and contained three to nine responses. These RAD units were found only in the superior bank of the superior temporal gyrus and auditory operculum. RAD units fired better at stimuli frequencies of 0.5 and 1.0 kHz in the superficial layers of lamina IV, whereas most of the lamina V showed better responses at 2 and 4 kHz than at 0.5 or 1 kHz. Some units found in laminae IV and V exhibited a shift of firing from the first and second reverberating responses at low frequency stimuli to the third and fourth reverberating potentials at the higher frequencies. These AD units not only had a horizontal organization but also had vertical aggregation since a series of AD units appeared, separated by about 25 μ, along the electrode insertion in laminae IV and V. Comparing the unit firing with the evoked response, AD potentials always appeared at the bottom of the first negative wave. We concluded that the AD potentials arise from certain cells in Golgi laminae IV and V of the primary auditory cortices, and propose that the late-appearing AD responses are a function of intragyral neuronal feedback systems.