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Attention and the visuomotor system : can the dorsal stream process two action targets simultaneously? Cameron, Brendan D.

Abstract

The processing of visual information for the purpose of executing goal-directed actions is, according to dual-stream theory, carried out by the 'dorsal stream', an occipito-parietal route of visual processing in the primate brain. The current study investigated the attentional capacity of the dorsal stream: Can it attend to more than one action target at a time? Two experiments were carried out to address this question. In the first experiment, 11 healthy participants executed fast sequential aiming movements in which they tapped two horizontally aligned targets. On a given trial, either, or both, of the targets could jump to a new location. This perturbation was tied to the participant's saccade so that the target movement would not be consciously perceived, and the targets were extinguished 20ms after the perturbation. The targets were, therefore, no longer visible when the eyes landed. The results of Experiment 1 revealed that participants consistently adjusted their aiming movements in accordance with the perturbations, regardless of which target jumped. Experiment 2 employed a similar protocol, but added a 3sec delay between the saccadically-triggered double-step and the onset of the participant's movement. This manipulation was intended to eliminate dorsal-stream involvement in the guidance of the movement, as the dorsal-stream is thought to retain visual information for, at most, 2sec. After this 2sec period, movement guidance should rely on a ventrally-stored conscious visual representation. We hypothesized, therefore, that the delay would eliminate the effect observed in Experiment 1. Nine healthy participants took part in Experiment 2, and, contrary to our hypothesis, produced aiming movements very similar to those of Experiment 1. The results of Experiments 1 and 2 demonstrate that location information about both targets in a sequential aiming movement is processed simultaneously and that this information can be used to update the movement. However, the results do not allow us to confidently conclude that the dorsal stream can process two targets simultaneously. Potential problems with the saccadically-triggered double-step design as a means of investigating dorsal stream processing are discussed.

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