Brief articleAltered vision near the hands
Introduction
Since the time of Woodworth (1899) researchers have known that visual information can play an important role in guiding movements of the limbs. Only recently have researchers learned that the converse may also be true: the position of the hand can sometimes produce a bias in visual attention: Reed, Grubb, and Steele (2006) had subjects hold one of their hands near either the left or right side of a video display upon which a target was presented. Subjects were faster to detect targets on the side closest to their outstretched hand, even though the target location was random. Proximity to the hand has also been shown to affect vision in patients with visual neglect caused by parietal lesions (diPellegrino and Frassinetti, 2000, Schendel and Robertson, 2004), and may augment the capture of attention by a threatening stimulus (Poliakoff, Miles, Li, & Blanchette, 2007).
The presence of a spatial bias in visual attention toward objects near an extended hand suggests that the evaluation of visual stimuli near the hands may be especially important. The bias might be mediated in part by brain areas that code visual space near the body in a hand-centered coordinate system (e.g., Graziano, 2001, Makin et al., 2007). According to this view, objects near the hands may fall into the receptive fields of hand-centered, visually sensitive neurons – invoking neural mechanisms and processing that would not occur if the hands were far away. Given this possibility, hand position may not merely bias processing toward or away from one hemifield, but instead the hand might alter key aspects of visual processing. Nevertheless, in the work conducted thus far, no one has yet examined whether the processing of stimuli near the hands is fundamentally different from the processing of stimuli far from the hands. We provide initial tests of that possibility here.
In the present study we tested the possibility that proximity to the hands may alter vision. To accomplish that, we examined performance in three classic paradigms that have been used to assess various aspects of visual attention: visual search, inhibition of return, and attentional blink. We had subjects perform each of these tasks under two different postures: one in which they held both of their hands very close to the visual stimuli, and one in which they held their hands on their laps – far from the stimuli. To anticipate the results, we found in each paradigm that subjects were slower to disengage their attention from the visual stimuli when their hands were near the stimuli – revealing a dramatic effect of hand-posture on visual perception.
Section snippets
Experiment 1
We first explored the possibility of altered vision near the hands by having subjects search through visual displays for a target letter while their hands were holding the display (object-proximal posture; Fig. 1a) and when their hands were on their laps – far from the objects being searched (object-distal posture; Fig. 1b). We ran three versions of the visual search experiment. In Experiment 1a subjects used their hands to respond, and in the object-proximal posture they were able to see their
Experiment 2
The changes in vision that we have reported could have been caused by either delayed engagement of attention to items during the search, or delayed disengagement of attention after inspection of each item during the search. It is possible to adjudicate between these alternatives by utilizing an attentional procedure that taps into both engagement and disengagement mechanisms. The technique involves presenting an uninformative peripheral visual cue and subsequently measuring the effects of the
Experiment 3
The experiments reported thus far examined effects of the hand on engagement and disengagement of visual attention in space. In Experiment 3 we studied the effects of hand-posture on the deployment of attention over time. In the experiment subjects were asked to detect two targets in a stream of rapidly presented alphanumeric characters (Fig. 4). A typical finding in such a situation is that people are impaired in identifying the second target when it appears within a few hundred milliseconds
General discussion
In the present article we have shown that people appear to be slow to disengage their attention in both space (Experiments 1a, 1b, 1c, and 2) and time (Experiment 3) when the objects being examined are near their hands. What purpose might be served by such effects of hand proximity on vision? One possibility stems from the fact that objects that are near the hands are likely candidates for physical manipulation, such as when a tool is to be wielded or a food to be eaten. In those circumstances,
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