Target size modulates saccadic eye movements in humans.

The authors investigated mechanisms involved in transformation of spatially extended targets into saccadic eye-movement vectors. Human subjects performed horizontal saccades to targets of varying diameter, which contained no conspicuous elements within the target shape. With increasing target size, express saccades and saccades with fast regular latencies decreased in frequency, whereas frequency of saccades with slow regular latencies increased. For all targets, saccade amplitude distributions showed a peak close to the geometric center of the targets. However, with large targets, increased scatter of saccade amplitudes and increased undershoot of the target center was observed. These effects may reflect distinct subprocesses involved in sensorimotor transformation to spatially extended targets, and may result from modulation of neuronal activity in the superior colliculus.