VISUAL RESPONSE PROPERTIES OF DORSOLATERAL PREFRONTAL NEURONS DURING VISUAL FIXATION TASK

Visual response properties of the dorsolateral prefrontal cortex (PF) neurons were examined while 4 [rhesus] monkeys performed a visual fixation task (VFT). A monkey fixated his eyes on a small circular spot at the center of a tangent screen for 1.5-5 s. Moving or stationary slit stimuli were presented during the fixation period. A total of 327 neurons were recorded. Changes in the discharge rates after presentation of the foveal fixation spot and extrafoveal slit stimuli could be grouped into 3 types, based on the decay times from the peak rate changes. Type I neurons (transient, n = 200) had clear visual receptive fields. When a receptive field included or was located near the fovea, they showed rate change both to the fixation spot and the slit stimuli. Most of type II (steady, n = 85) and type III (transient-steady, n = 13) neurons did not show rate changes to the slit stimuli, and in only a few of them did the continuous rate changes to the spot become suppressed by slit stimuli presented in a limited visual area. Receptive fields (RF) were determined for 31 type I and 5 type II neurons. The size of the RF varied from about 10 .times. 10 to 60 .times. 60.degree. and for 25 type I and 3 type II neurons it was larger than 30 .times. 30.degree.. For 18 type I and 3 type II neurons the RF was contralateral to the recording site, while for 2 type I neurons it was ipsilateral. For 12 type I and 2 type II neurons the RF extended from the contralateral to the ipsilateral side. Of these, 6 type I and 1 type II neuron included the fovea in their receptive field. Ten slit-sensitive neurons showed direction selectivity to a moving slit. Orientation-selective neurons were not found in this study. Type I neurons are presumed to be related to the detection of the gross location of a visual stimulus. Type II and type III neurons are presumed to be related to behavioral factors, such as attention or expectation.