Gd. Horwitz et Wt. Newsome, Target selection for saccadic eye movements: Prelude activity in the superior colliculus during a direction-discrimination task, J NEUROPHYS, 86(5), 2001, pp. 2543-2558
We investigated the role of the superior colliculus (SC) in saccade target
selection while macaque monkeys performed a direction-discrimination task.
The monkeys selected one of two possible saccade targets based on the direc
tion of motion in a stochastic random-dot display; the difficulty of the ta
sk was varied by adjusting the strength of the motion signal in the display
. One of the two saccade targets was positioned within the movement field o
f the SC neuron under study while the other target was positioned well outs
ide the movement field. Approximately 30% of the neurons in the intermediat
e and deep layers of the SC discharged target-specific preludes of activity
that "predicted" target choices well before execution of the saccadic eye
movement. Across the population of neurons, the strength of the motion sign
al in the display influenced the intensity of this "predictive" prelude act
ivity: SC activity signaled the impending saccade more reliably when the mo
tion signal was strong than when it was weak. The dependence of neural acti
vity on motion strength could not be explained by small variations in the m
etrics of the saccadic eye movements. Predictive activity was particularly
strong in a subpopulation of neurons with directional visual responses that
we have described previously. For a subset of SC neurons, therefore, prelu
de activity reflects the difficulty of the direction discrimination in addi
tion to the target of the impending saccade. These results are consistent w
ith the notion that a restricted network of SC neurons plays a role in the
process of saccade target selection.