Functional properties of corticotectal neurons in the monkey's frontal eye field

Abstract

1. We examined the activity of identified corticotectal neurons in the frontal eye field of awake behaving rhesus monkeys (Macaca mulatta). Corticotectal neurons were antidromically excited using biphasic current pulses passed through monopolar microelectrodes within the superior colliculus. The activity of single corticotectal neurons was studied while the monkey performed behavioral tasks designed to test the relation of the neuron's discharge to visual and oculomotor events. 2. Fifty-one frontal eye field corticotectal neurons were examined in two monkeys. Current thresholds for antidromic excitation ranged from 6 to 1,200 microA, with a mean of 330 microA. Antidromic latencies ranged from 1.2 to 6.0 ms, with a mean of 2.25 ms. 3. Fifty-three percent of the identified corticotectal neurons were classified as having movement-related activity. They had little or no response to visual stimuli, but very strong activity before both visually guided and memory-guided saccades. An additional 6% of corticotectal neurons had visuomovement activity, combining both a visual- and a saccade-related response. In each case, visuomovement neurons antidromically excited from the superior colliculus had movement-related activity, which was much stronger than the visual component of their response. 4. Twenty-two percent of the corticotectal neurons were primarily responsive to visual stimulation of the fovea. These included both neurons responding to the onset and neurons responding to the disappearance of a light flashed on the fovea. 5. The remaining 20% of the corticotectal neurons were a heterogeneous group whose activity could not be classified as movement, visuomovement or foveal. Their responses included postsaccadic, anticipatory, and reward-related activity, as well as activity modulated during certain directions of smooth-pursuit eye movements. One neuron was unresponsive during all of the behavioral tasks used. There were no corticotectal neurons that could be classified as primarily responsive to peripheral visual stimuli. 6. Histological reconstructions of electrode penetrations localized corticotectal neurons to layer V of the frontal eye field. For 22 corticotectal neurons tested, each had its minimum threshold for antidromic excitation within the superior colliculus, as judged by either histological confirmation, or surrounding neuronal responses recorded through the stimulation microelectrode. The majority of these neurons had minimum threshold sites within the intermediate layers, a few minimum threshold sites were located within the superficial or deep collicular layers.(ABSTRACT TRUNCATED AT 400 WORDS)