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 purl blocks, than when they are presented randomly intermixed, in mixed blocks; a diffe rence denoted as perceptual mixing costs. The present study examined the co ntribution of strategic and stimulus-driven factors to these costs. The fir st two experiments showed that perceptual mixing costs were not reduced whe n participants were informed at the start of each trial in mixed blocks abo ut the impending category. Furthermore, Experiments 1 and 3 showed that mix ing costs were concentrated on those trials of mixed blocks where the perce ptual category was different from that of the preceding trial. These result s support the view that perceptual mixing costs derive from stimulus-driven trial-by-trial adjustments in processing, Some general implications for pr ocessing models are discussed, (C) 1999 Elsevier Science B.V. All rights re served. PsycINFO classification. 2323.