Perceptual-motor expectancies interact with covert visual orienting under conditions of endogenous but not exogenous control.

Central and peripheral cues were used to endogenously or exogenously direct the subject's attention to the location at which an increase or decrease in the size of a peripheral object was most likely to occur. For each subject, one size change was more likely to occur than the other, and the task was choice reaction time (expansion/contraction). Stimulus/response probability (particular size change) interacted with cue condition (valid, neutral and invalid) when the cues were central (endogenous), but the two variables were additive when the cues were peripheral (exogenous). These data, together with previous evidence, strongly suggest that qualitatively different attentional resources are recruited when visual orienting is endogenously and exogenously controlled.