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UBC Theses and Dissertations

Dual-task performance of visually guided action and perception Liu, Geniva


A currently influential theory of human vision (Milner & Goodale, 1995) posits the existence of two independent streams of visual processing: a ventral system for the conscious perception of objects and a dorsal system for the dynamic guidance of actions toward objects. This theory also proposes that action planning is controlled by the ventral stream, whereas action execution is controlled by the dorsal stream. What is left unspecified by this theory, however, is whether this functional independence also applies to the attentional mechanisms available to each system. To examine this question, a series of experiments was conducted to measure dual-task interference between tasks involving perception and action. One task required the visual identification of one object at the center of gaze, while the other task required concurrently pointing to a second object that appeared in the visual periphery. The online control functions of the dorsal stream were indexed by displacing the pointing target following movement initiation. Results indicated that successful performance of visual identification lead to interference in the planning, but not in the execution of action (Experiments 1, 2, 5, & 6), and this held for movements that were initiated before, during, or after presentation of the identification target (Experiment 3). When a speed stress was placed on pointing, dual-task costs were also observed in the execution and online control of action (Experiment 4). Furthermore, when action execution was forced to rely on ventral stream processes, either by spatially displacing the action from the target (Experiment 7) or by making the action based upon memory (Experiment 8), then visual identification interfered with both the planning and execution of action. These results support some independence in the attentional mechanisms used for ventral and dorsal tasks. Specifically, action planning shares attentional mechanisms with visual identification, but action execution as performed by the dorsal stream does not rely on these mechanisms.

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