Attentional changes in human perceptual learning.

In two experiments, participants were required to identify a target stimulus by means of same/different judgments. Previously, they had received simultaneous or blocked pre-exposures to the target and a similar stimulus. Participants' ability to judge pre-exposed stimuli as different was better after simultaneous than after blocked pre-exposures. However, the benefit of the simultaneous schedule disappeared when, after pre-exposure, the distinctive elements were made common (and some common elements made distinctive) by changing their shape and position within the stimulus (Experiment 1). Similar results were obtained when only one of the aforementioned physical features was modified (Experiment 2). These manipulations did not affect performance when the stimuli had been pre-exposed in separate blocks of trials. These findings support the idea that the effect of simultaneous pre-exposure on stimulus differentiation is based on a selective attention process by which attention is selectively directed towards the distinctive features of the stimuli and away from the common features (Gibson, 1969).