Grasping the point

Abstract

When you look for your copy of TICS among the mail in your mailbox, your visual-attentional mechanisms marshal saccadic eye movements to search for specific attributes: a glossy cover, a colored spine, etc. In other words, when you know what you're searching for beforehand, visual selective attention uses information about the searched-for object to guide eye movements to appropriate items in the visual scene. But just as relevant object features vary from situation to situation, so do our reasons for searching in the first place. Sometimes we look for an object because we merely want to locate it; other times we might be looking for that object with different intentions in mind – picking it up for instance. When we search for an object with the aim of making a move towards it, how much pull do our intentions about the action have in the economies of visual attentional processing?Bekkering and Neggers asked this question in a recent experiment designed to see whether action intentions modulate the processes of visual attention [1xSee all References][1]. They exploited a difference in the processing constraints of two sorts of goal-directed movements: pointing and grasping. Both types of movement are preceded by saccadic eye movements, but they each engage visuomotor processing in different ways. For example, the orientation of an object doesn't matter much for the way you point at it, but is crucial for the way you will grasp it to pick it up. Color, on the other hand, is not so important in the visual guidance of either pointing or grasping movements. If action intention does modulate visual attention, object orientation should bias visual attention in grasping movements but not in pointing movements, whereas an object's color should not produce a bias in either case.Bekkering and Neggers tracked subjects’ eye, head and hand movements as they located, and either grasped or pointed to, a target in an array of distractors on a specially designed illuminated board. The target object was a three-dimensional rectangle specified by two relevant features, color (either green or orange) and orientation (45 deg left or 45 deg right). The distractor rectangles surrounding the target could also be either green or orange and angled either to the left or right, but only the target object embodied the relevant conjunction of color and orientation. The experimenters found that when subjects made erroneous saccades, it was either to a rectangle of the wrong color, or to one of the wrong orientation. Moreover, the errors did indeed reflect an object orientation bias related to an intention to grasp, but not to an intention to point. Thus, in the grasping condition, subjects made significantly fewer saccade errors with respect to orientation than in the pointing condition. By contrast, there were no significant difference in color errors between the pointing and grasping conditions. These results, then, point to an easily-grasped conclusion: that action intention works together with task-relevant object features to modulate visual attention.