Spatial and temporal effects of spatial attention on human saccadic eye movements.

A spatial cueing paradigm was used to (a) investigate the effects of attentional orienting on spatial and temporal parameters of saccadic eye movements and (b) examine hypotheses regarding the hierarchical programming of saccade direction and amplitude. On a given trial, the subjects were presented with one of three peripheral cues: a "valid" cue provided correct information, a "neutral" cue no information and an "invalid" cue incorrect information about the location of the subsequent target (the cue was valid on about 50% of the trials). 100 or 500 msec after the cue onset (stimulus onset asynchrony, SOA), the eye movement target was presented at one of six possible locations (from -11.25 degrees to +11.25 degrees, at 3.75 degrees spacing). The results showed a significant effect of cueing on saccade latencies at the 100 msec, but no effect at the 500 msec SOA. The cueing benefits were restricted to the cued location; no other locations within the cued or uncued hemi-field were facilitated. The invalid trial latencies showed: (a) no advantage for targets on the same side as the cue relative to the opposite side, (b) no advantage for targets more proximal to the cue and (c) no advantage for targets sharing the same eccentricity as the cue. In a second experiment, subjects responded to the target by giving a simple manual response [simple reaction time (RT)] while keeping the eyes stationary. The results showed a significant cueing effect on simple RTs both at the 100 msec and, in contrast to saccadic eye movements, the 500 msec SOA. There were specific benefits for targets at the cued location relative to other targets within the same and opposite hemi-fields. Further, in contrast to saccades, there was a general advantage for targets within the same ("cued") hemi-field over targets in the opposite hemi-field. These findings suggest that, for saccadic eye movements (overt orienting), direction and amplitude are programmed holistically; whereas in covert orienting (with eyes stationary), motor parameters (e.g. for directed hand movements) are programmed in a more hierarchical fashion.