Saccadic reaction times and activation of the prelunate cortex: parallel observations in trained rhesus monkeys

Rhesus monkeys were trained to fixate a small spot and saccade to a second stimulus in the near periphery if the fixation spot went off. In different tests the target stimulus could occur at various delay times before or after the offset of the fixation spot. During periods of single unit recording from the prelunate cortex neural events were measured together with saccadic reaction times (SRT): If the stimulus was visible for a period of time (1 or 0.5 s) before the fixation spot disappeared (positive "delayed saccade" task) the SRT reached values of more than 300 ms. The SRTs were shorter when the target stimulus occurred simultaneously with the offset of the fixation spot ("saccade" task). SRT were shortest (approximately 150 ms) if the target stimulus appeared 100-250 ms after the offset of the fixation spot (negative "delayed saccade" task). Moreover, they decreased with the time of daily training. The different behavioural conditions resulted in different types of cortical activity with different latencies: In "saccade" and negative "delayed saccade" tasks the neurons on-responses could be enhanced in comparison to the passive visual on-responses during stationary fixation. The latencies of the on-response and the enhanced on-response were equal with approximately 80 ms. In striking contrast the latencies of the presaccadic activation (PSA) in the positive "delayed saccade" tasks were more than twice as long with about 200 ms. Daily training influences both the SRTs and the PSA: The SRTs become shorter by more than 150 ms in positive "delayed saccade" tasks (delay: 300-500 ms) and the percentage of PSA-neurons decrease from more than 70% to less than approximately 20% after 3 weeks of daily training and recording. The temporal aspects of events preceding visually guided eye movements are important to understand the serial and parallel processing in cortical and subcortical structures that are involved in the learning, initiation, and execution of goal directed movements.