LATERAL INHIBITORY INTERACTIONS IN THE INTERMEDIATE LAYERS OF THE MONKEY
SUPERIOR COLLICULUS.
Douglas P. Munoz, Peter J. Istvan.
MRC Group in Sensory-Motor Neuroscience, Department of Physiology, Queen's
University, Kingston, Ontario Canada K7L 3N6.
APStracts 4:340N, 1997.
ABSTRACT
The intermediate layers of the monkey superior colliculus (SC) contain neurons
whose discharges are modulated by visual fixation and saccadic eye movements.
Fixation neurons, located in the rostral pole of the SC, discharge action
potentials tonically during visual fixation and pause for most saccades.
Saccade neurons, located throughout the remainder of the intermediate layers
of the SC, discharge action potentials for saccades to a restricted region of
the visual field. We defined the fixation zone as that region of the rostral
SC containing fixation neurons and the saccade zone as the remainder of the
SC. It has recently been hypothesized that a network of local inhibitory
interneurons may help shape the reciprocal discharge pattern of fixation and
saccade neurons. To test this hypothesis, we combined extracellular recording
and microstimulation techniques in awake monkeys trained to perform oculomotor
paradigms that enabled us to classify collicular fixation and saccade neurons.
Microstimulation was used to electrically activate the fixation and saccade
zones of the ipsilateral and contralateral SC to test for inhibitory and
excitatory inputs onto fixation and saccade neurons.
Saccade neurons were inhibited at short latencies following electrical
stimulation of either the ipsilateral (1 - 5 ms) or contralateral (2 - 7 ms)
fixation or saccade zones. Fixation neurons were inhibited 1 - 4 ms following
electrical stimulation of the ipsilateral saccade zone. Stimulation of the
contralateral saccade zone led to much weaker inhibition of fixation neurons.
Stimulation of the contralateral fixation zone led to short-latency (1 - 2 ms)
excitation of fixation neurons. Only a small percentage of saccade and
fixation neurons were activated by the electrical stimulation (latency: 0.5 -
2.0 ms). These responses were confirmed as either orthodromic or antidromic
responses using collision testing.
The results suggest that a local network of inhibitory interneurons may help
shape, not only the reciprocal discharge pattern of fixation and saccade
neurons, but also permit lateral interactions between all regions of the
ipsilateral and contralateral SC. These interactions may therefore be critical
for maintaining stable visual fixation, suppressing unwanted saccades, and
initiating saccadic eye movements to targets of interest.
Received 18 September 1997; accepted in final form 25 November 1997.
APS Manuscript Number J773-7.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1997 The American Physiological Society.
Published in APStracts on 12 December 1997