The Preparation of Goal-Directed Movements Requires Selective Visual Attention: Evidence from the Line-Motion Illusion

Previous research has demonstrated that visual attention is focused on the movement target, both before saccadic eye movements and before manual reaching, allowing for spatially selective object recognition (Deubel and Schneider, 1996 Vision Research 36 1827 – 1837; Deubel, Schneider, and Paprotta, 1996 Perception Supplement, 13 – 19). Here we study the illusory line motion effect (Hikosaka et al, 1993 Vision Research 33 1219 – 1240) in a dual-task paradigm to further investigate the coupling of attention and movement target selection.

Subjects were presented a display with two potential movement targets (small circles). When one of the circles flashed, they performed a reaching movement with the unseen hand to the other stimulus; movements were registered with a Polhemus FastTrack system. At a SOA that was varied between 0 and 1000 ms after the movement cue, a line appeared and connected both stimuli. After the reaching movement, subjects indicated the perceived direction of line motion. In a second experiment, saccadic eye movements instead of reaching movements were studied.

The data show that for short SOAs the subjects reported illusory line motion away from the cue location indicating that attention is automatically drawn to the cue. For longer SOAs but well before movement onset the illusory motion effect inverted—evidence for an attention shift to the movement target. The findings were very similar for manual reaching and for saccadic eye movements. The results confirm the hypothesis that the preparation of a goal-directed movement requires the attentional selection of the movement target. We discuss the assumption of a unitary attention mechanism which selects an object for visual processing, and simultaneously provides the information necessary for goal-directed motor action such as saccades, pointing, and grasping.