TASK CONSTRAINTS IN VISUAL WORKING-MEMORY

This paper examines the nature of visual representations that direct o ngoing performance in sensorimotor tasks. Performance of such natural tasks requires relating visual information from different gaze positio ns. To explore this we used the technique of making task relevant disp lay changes during saccadic eye movements. Subjects copied a pattern o f colored blocks on a computer monitor, using the mouse to drag the bl ocks across the screen. Eye position was monitored using a dual-purkin je eye tracker, and the color of blocks in the pattern was changed at different points in task performance. When the target of the saccade c hanged color during the saccade, the duration of fixations on the mode l pattern increased, depending on the point in the task that the chang e was made. Thus different fixations on the same visual stimulus serve d a different purpose. The results also indicated that the visual info rmation that is retained across successive fixations depends on moment by moment task demands. This is consistent with previous suggestions that visual representations are limited and task dependent. Changes in blocks in addition to the saccade target led to greater increases in fixation duration. This indicated that some global aspect of the patte rn was retained across different fixations. Fixation durations reveale d effects of the display changes that were not revealed in perceptual report. This can be understood by distinguishing between processes tha t operate at different levels of description and different time scales . Our conscious experience of the world may reflect events over a long er time scale than those underlying the substructure of the perceptuo- motor machinery. (C) 1997 Elsevier Science Ltd.