Saccade initiation and accuracy in gaze shifts are affected by visual stimulus significance

The effect of visual stimulus significance on saccade latency and accuracy in gaze shifts was investigated. Stimulus significance refers to the temporal proximity of an imperative stimulus. It was hypothesized that stimulus significance serves as a cue that influences the observer's expectancy for a forthcoming event. Twenty-one undergraduate students participated in two experiments involving monitoring of visual events. Stimuli were a sequence of 1,000 single-digit integers presented one at a time at 9 degrees of visual angle to the left or right of a fixation point in random order. The subject was required to make a manual response after the presentation of a sequence of two odd integers. The probability of the subsequent integer being odd, a "target," after the presentation of an odd integer was 0.75. The integer after a target was always an even integer. When stimulus location was unpredictable (Experiment 1), latency of visually guided saccades to the target was shorter (p < .01) than to the nontarget stimuli. Under the condition of predictable stimulus location (Experiment 2), anticipatory saccades to the target occurred earlier (p < .01), more frequently (p < .01), and more accurately (p < .05). Our results suggest that both timing and accuracy of gaze shift are affected by the observer's expectation of future events.