Influences of Size of Fixation Point on Visually Guided Reflexive Saccade

Abstract

Visual stimuli were presented on a monitor in complete darkness. The head of each subject was immobilized. The following two tasks were carried out randomly in different trials : the disappearance of the central fixation point (FP, 0 deg) preceded the appearance of the peripheral saccade target by 200 ms (gap task) and the FP remained after the appearance of the target (overlap task). The target was randomly presented 4 and 8 deg to the right or left of the FP (1 : 3=left : right). The diameter of the FP was randomly set at 0.2, 0.8, or 3.2 deg. The subject was asked to make a saccade to the target after the target appeared. Only the saccades elicited to the right targets were analyzed. The results showed that the normalized saccade peak velocity and amplitude for the 3.2 deg FP were less than those for the 0.2 and 0.8 deg FP. These effects may be caused by the suppression of the saccade because the larger FP would stimulate the fixation cells on the superior colliculus more strongly. However, the larger FP decreased the saccadic reaction time. We explained this contradicting phenomenon by modifying Fischer's hypothesis of the attention dynamics during the saccade.