Neurons in both the lateral intraparietal area (LIP) of the monkey parietal cortex and the intermediate layers of the superior colliculus (SC) are activated well in advance of the initiation of saccadic eye movements. To determine whether there is a progression in the covert processing for saccades from area LIP to SC, we systematically compared the discharge properties of LIP output neurons identified by antidromic activation with those of SC neurons collected from the same monkeys. First, we compared activity patterns during a delayed saccade task and found that LIP and SC neurons showed an extensive overlap in their responses to visual stimuli and in their sustained activity during the delay period. The saccade activity of LIP neurons was, however, remarkably weaker than that of SC neurons and never occurred without any preceding delay activity. Second, we assessed the dependence of LIP and SC activity on the presence of a visual stimulus by contrasting their activity in delayed saccade trials in which the presentation of the visual stimulus was either sustained (visual trials) or brief (memory trials). Both the delay and the presaccadic activity levels of the LIP neuronal sample significantly depended on the sustained presence of the visual stimulus, whereas those of the SC neuronal sample did not. Third, we examined how the LIP and SC delay activity relates to the future production of a saccade using a delayed GO/NOGO saccade task, in which a change in color of the fixation stimulus instructed the monkey either to make a saccade to a peripheral visual stimulus or to withhold its response and maintain fixation. The average delay activity of both LIP and SC neuronal samples significantly increased by the advance instruction to make a saccade, but LIP neurons were significantly less dependent on the response instruction than SC neurons, and only a minority of LIP neurons was significantly modulated. Thus despite some overlap in their discharge properties, the neurons in the SC intermediate layers showed a greater independence from sustained visual stimulation and a tighter relationship to the production of an impending saccade than the LIP neurons supplying inputs to the SC. Rather than representing the transmission of one processing stage in parietal cortex area LIP to a subsequent processing stage in SC, the differences in neuronal activity that we observed suggest instead a progressive evolution in the neuronal processing for saccades.
Computational specialization within the cortical eye movement system
