Abstract
Visual perception of everyday complex environments involves both the individuation and recognition of objects and the extraction of global image properties and summary statistics of the scene. Evidence suggests different visual attentional mechanisms are engaged for completion of these disparate tasks. Does allocation of one or multiple mechanism afford us this perceptual processing? One option is workings of a single mechanism whose activity ranges on a spectrum from narrowly focused analysis of local binding of features, to global registration of image statistics. Other models suggest that it is a result of two mechanisms working either in parallel or serial manner, where one is selective and effortful and other rapid global and non-selective. We used a dual-task paradigm to test these three possible models, by measuring the effect of simultaneously doing a secondary task on the performance on one of the two primary tasks. One primary task required global processing, to find the direction of average dot motion and the other required focused processing to individuate a single dot’s direction of motion within the same display of dots, both equated for difficulty. In two experiments, secondary task requiring global processing (image categorization or average color detection) was performed simultaneously with a primary task either requiring the same or requiring focused processing. Comparing observers’ performance on two concurrent tasks to that on single-task conditions showed greater performance reduction for the focused task during dual-tasks condition than for the global task. We observe a double dissociation when the secondary task demanding focused processing (object localization or bisected disk differentiation) was performed concurrently with primary task requiring the same, or that requiring global processing. Then performance on the global task deteriorated significantly more than on the focused task. Results support the view of two visual mechanisms being deployed serially in complex visual environments.