The control and speed of shifts of attention.

We measured the detectability of a target pattern in a display consisting of 12 elements in a circle around the central fixation point. The display was presented briefly and followed after a variable amount of time by a mask. We found that presenting a pre-cue, designating the target position, facilitated target detectability. Attention is directed to the cued location. When the observer has to detect a (second) target among the non cued elements, performance for locations close to the cue is not significantly different from performance for locations further away. This suggests that there is no "scan-path" or proximity effect. We also found that the identification of the cued element delayed the detectability of the subsequent target by more than 160 msec. In another series of experiments we studied the control of attentional shifts. We found that, for short mask delays (100, 160, and 260 msec) the observer is unable to selectively process elements which are not physically cued but only verbally defined by their position relative to the cue. When we increase the positional uncertainty of the target by increasing the number of physical cues, performance drops until it reaches an asymptote with 5 elements. We infer that, even though the target is very similar to the background, a parallel mechanism, used for the extraction of stimulus features, designates prospective target locations which may be subsequently checked by a (slow) attentional process.