Involuntary attentional shifts due to orientation differences.

We tested the ability of orientation differences to cause involuntary shifts of visual attention and found that these attentional shifts can occur in response to an orientation "pop-out" display. Texture-like cue stimuli consisting of discrete oriented bars, with either uniform orientation or containing a noninformative orthogonally oriented bar, were presented for a variable duration. Subsequent to or partially coincident with the cue stimulus was the target display of a localization or two-interval forced-choice task, followed by a mask display. Naive subjects consistently showed greater accuracy in trials with the target at the location of the orthogonal orientation compared with trials with uniformly oriented bars, with only 100 msec between the cue and mask onsets. Discriminating these orientations required a stimulus onset asynchrony (SOA) of 50-70 msec. The attentional facilitation is transient, in most cases absent with a cue-mask SOA of 250 msec [corrected]. These results suggest that the preattentive character of some texture discrimination tasks with SOAs of only 100 msec is vitiated by the involuntary attentional shifts that are caused by orientation differences.