Abstract
According to the biased competition theory, multiple stimuli compete to be represented in the visual system. Attention is a way to resolve this competition. This competition is enhanced when stimulus heterogeneity increases. Hence, we hypothesized that the effect of attention increased under the high level of competition between multiple, heterogenous stimuli. To test this, we measured the effect of spatial attentional cue either when a target stimulus (tilted Gabor grating) was presented by itself or when the target was accompanied by multiple distractors (vertical grating). Importantly, we manipulated stimulus homogeneity/heterogeneity by the varying spatial frequency of the stimuli. With the heterogenous trials, the spatial frequencies of each stimulus were different, while all the stimuli had the same spatial frequency in the homogeneous trials. Notably, given that the target was distinguished by its orientation, the spatial frequency of the stimuli was irrelevant to the task. The target and distractors were preceded by a salient, peripheral cue. The results showed that the cuing effect significantly increased under distractor-interference. However, in the presence of multiple stimuli, stimulus heterogeneity did not affect the magnitude of cuing effect. In the follow-up experiment, we further varied the number of stimuli, such that the target stimulus was presented by itself or it was accompanied by a variable number of distractors (1, 3, 7). This experiment showed that the efficiency of target searching was significantly impaired when multiple, heterogeneous stimuli were presented. Still, the magnitude of cuing effect was not affected by the stimulus heterogeneity. Taken together, the stimulus heterogeneity in a task-irrelevant dimension seems to have dissociable effects in attentional cuing effect and search efficiency. Presumably, multiple heterogenous stimuli suppress each other to greater extent than homogeneous stimuli, lowering search efficiency. However, this increased competition does not necessarily affect the effect of attentional cue.