The Effect of Amputation on Spatial Visual Representation in Peripersonal Space

In a recent fMRI study (Makin TR. et al., J. Neurosci. 2007), we found extended cortical representation for a visual stimulus provided that it is presented close to the hand. It is therefore interesting to ask whether changes in the brain following limb amputation might induce changes in the amputees' perception of the body and consequently it's surrounding. In this study, we wish to find behavioral evidence for the question whether an amputation of the arm can lead to changes in visual perception or motor responses to objects in peripersonal space. We hypothesize that changes in the motor and somatosensory hand-related cortices following amputation might lead to changes in parietal hand-related areas. Consequently, we expect to find reduced perception/attention/responses to hand-related objects in the space ipsilateral to the amputation. In this experiment we plan to employ a paradigm which is inspired by the affordance effect, originally introduced by Tucker and Elice (1998): we will present subjects with brief images of either manipulable or non manipulable objects in either the left or right visual field. The subjects will be required to determine whether the object contains a metal or not, by moving either their left or their right shoulder. We predict that while the control group of normal subjects would show a congruency effect (that is superior performance (faster and more accurate responses) when the responding shoulder is congruent with the position of the objects, the amputees will show spatial biases towards the non-amputated side. This effect should be more prominent for the manipulable objects.