Identifying stimuli of different perceptual categories in pure and mixed blocks of trials: evidence for stimulus-driven switch costs.

Responding to stimuli of different perceptual categories is usually faster when the categories are presented isolated from each other, in pure blocks, than when they are presented randomly intermixed, in mixed blocks; a difference denoted as perceptual mixing costs. The present study examined the contribution of strategic and stimulus-driven factors to these costs. The first two experiments showed that perceptual mixing costs were not reduced when participants were informed at the start of each trial in mixed blocks about the impending category. Furthermore, Experiments 1 and 3 showed that mixing costs were concentrated on those trials of mixed blocks where the perceptual category was different from that of the preceding trial. These results support the view that perceptual mixing costs derive from stimulus-driven trial-by-trial adjustments in processing. Some general implications for processing models are discussed.