Orienting of attention and eye movements.

According to the premotor theory of attention, the mechanisms responsible for spatial attention and the mechanisms involved in programming ocular saccades are basically the same. The aim of the present experiments was to test this claim. In experiment 1 subjects were presented with a visual display consisting of a fixation point and four boxes arranged horizontally and located above the fixation cross. Two of the boxes were in the left visual hemifield, two in the right. A fifth box was located on the vertical meridian below the fixation cross. Digit cues indicated in which of the upper boxes the imperative stimulus was most likely to appear. Subjects were instructed to direct attention to the cued box and to perform a saccadic eye movement to the lower box on presentation of the imperative stimulus. The trajectory of the saccades deviated contralateral to the hemifield in which the imperative stimulus was presented. This deviation was larger when the hemifield where the imperative stimulus was presented was the cued one. In experiment 2, the visual display consisted of five boxes forming a cross. The central box served as a fixation point. The cue was a small line, linked to the central box, pointing to different directions and indicating where the visual imperative stimulus would appear. In 50% of trials, the imperative stimulus was a visual stimulus presented either in one of the lateral boxes or in the central one. In the remaining 50% of trials, the imperative stimulus was a non-lateralised sound. Half the subjects were instructed to make a saccade to the upper box at the presentation of the visual imperative stimulus and to the lower box at the presentation of the acoustic stimulus. Half the subjects received the opposite instructions. The results confirmed that the saccades deviate contralateral to the hemifield of stimulus presentation in the case of visual imperative stimuli. Most importantly, they showed that the saccades deviate contralateral to the cued hemifield, also in the case of acoustic imperative stimuli. Experiment 3 was similar to experiment 2. It confirmed the results of that experiment and showed that slow ocular drifts, which are observed in the time interval between cue and imperative stimulus presentation, cannot explain the ocular deviations. Taken together, the experiments demonstrate that spatial attention allocation leads to an activation of oculomotor circuits, in spite of eye immobility.