Comparison of saccadic eye movements in humans and macaques to single-step and double-step target movements

Human and monkey saccade amplitude and latency, in response to 12-36 degrees target steps, differed substantially despite nearly identical experimental conditions. On single-step trials, monkeys did not undershoot targets, and latencies were insensitive to stimulus and contextual factors. Human saccades did undershoot, their latency was longer, and both undershoot and latency were affected by stimulus variables and experimental context. On double-step trials, the second target step altered primary saccade amplitude when the step occurred as little as 40 msec prior to saccade onset for both humans and monkeys. However, humans and monkeys showed somewhat different amplitude transition functions, and monkeys showed little evidence of parallel programming.