Perceptual grouping can affect the online control of goal-directed hand movements

If the target of a goal-directed arm movement is suddenly displaced, the movement is quickly adjusted accordingly. There are situations in which grouping gives the impression that many items have been displaced, although the change that determines the direction of the perceived displacement occurs far from the target. When a single item disappears from one end of a row of items just when an item appears at the other end, it looks as if a single item is displaced from one end to the other (element motion). But if the whole row of items briefly disappears when this change occurs, the whole row seems to have been displaced (group motion). We asked participants to move their finger to a purple target item at the centre of a row of equally spaced black items. Hand movements towards the target were adjusted in the direction of both element and group motion. The adjustment of the movement was especially clear for short rows of items. Importantly, the response was much more vigorous under conditions that give rise to group motion than under conditions that give rise to element motion. Thus, a displacement that is propagated to the target through grouping can affect goal-directed movements.