It turns out that you need more than just your eyes to see things that are right in front of you. Neuroscientists have found that we can't accurately judge the distance of nearby objects without also using our hands.
The area within arm's length - in other words, the area with which we can directly interact - is called the "action space." Previous research had indicated that the brain judges distance within the action space in terms of the hands. However, it had not been confirmed whether the hands merely help our brains better understand spatial positions or if they are actually required to figure out where objects in the action space are.
To test this question, a research team at the Hebrew University of Jerusalem and Hadassah Hospital-Mount Scopus tested a group of people who had had their hands amputated to see how their visual perception of nearby objects had been affected. The volunteers were told to focus on a screen where a cross was projected in the center. On either side were squares of variable distance from the cross. The subjects were asked to judge which of the two squares was farther away.
The results were telling:
The results reveal that hand amputations affect visuospatial perception. When the right square was slightly farther away from the center, participants with right-hand amputations tended to perceive it as being at the same distance from the center as the left square; this suggests that these volunteers underestimated the distance of the right square relative to the left. Conversely, when the left square was farther away, left-hand amputees perceived both squares as being equally far away from the center — these participants underestimated the left side of near space.
Interestingly, when the volunteers were seated farther away from the screen, they were more accurate in judging the distances, indicating that hand amputations may only affect perception of the space close to the body.
This would seem to indicate that the loss of a hand actually decreases the action space on the affected side of the body, with resultant damage to spatial perception. These results may also impact those suffering from spatial hemineglect, a condition often brought on by brain injury in which patients cannot perceive any objects on one side of their bodies. Since those suffering from this condition are often paralyzed on the affected side, the effective loss of their hand may exacerbate the problem because of its crucial role in near space perception.