Myo-Hee Chang¹ (), Suk Won Han¹; ¹Chungnam National University

In the environment, multiple stimuli compete to be represented in our resource-limited perceptual/cognitive system. Such competition intensified when a set of heterogeneous stimuli are presented than when the stimuli are homogenous. The increased competition resulting from increased stimulus heterogeneity was found to affect the efficiency of visual search. According to a seminal study (Duncan & Humphreys, 1989), searching for a specific shape suffered to a greater extent when the target was surrounded by a set of heterogeneous shapes than when it was accompanied by homogeneous stimuli. Expanding this study, we investigated whether stimulus heterogeneity in a task-irrelevant dimension also affects visual search efficiency. Specifically, in the present search task, the task was to look for a 45-degree tilted grating either in clockwise or counterclockwise direction, while non-targets were vertical or horizontal gratings. Hence, the task-relevant dimension was orientation. The search stimuli could also vary in task-irrelevant dimensions, such as spatial frequency (Exp 1) or color (Exp 2). Importantly, the search stimuli were either heterogeneous or homogeneous in terms of these task-irrelevant dimensions. The results showed that when the search stimuli were heterogenous in the task-irrelevant dimension (e.g., spatial frequency or color), search efficiency was lower than when the non-targets had homogeneous spatial frequency or color. Furthermore, for a certain feature dimension, this impaired search efficiency was accompanied by diminished attentional capture by a singleton distractor. These findings are remarkable because the spatial frequency or the color of the stimuli did not have to receive attention. Rather, they affected efficiency of the search process. Based upon these, we suggest that the irrelevant heterogeneity disrupts the formation of coherent percept of visual stimuli, increasing perceptual resource demands. Even though our attentional control could be finely tuned, perfect filtering of a certain visual feature from a multi-dimensional stimulus was found to be impossible.