Modulation of covert visual attention by hand movement: Evidence from parietal extinction after right-hemisphere damage

Abstract

Several recent studies have reported that left visual inattention following right-hemisphere damage can be ameliorated during active movements of a contralesional limb in contralesional space, even when that limb is unseen. This apparent influence of motor activity on sensory performance is of considerable interest, both from a theoretical perspective, and also practically because of its potential implications for rehabilitation. However, the exact basis of the motoric influence has remained unclear, in part because many of the clinical tests used for visual inattention themselves involved motoric factors (e.g. exploratory eye-movements are required in typical cancellation tasks). It is therefore possible that limb activation has its effects largely through an influence on other motor systems (e.g. attracting saccades towards the active limb) rather than by directly affecting perception. Here we test in a single patient, with left-sided visual extinction following a right-hemisphere lesion, whether activation of the contralesional hand can overcome left-sided inattention in a visual task that has no spatial motor component. We conducted a standard computerized test for visual extinction (the patient verbally detected brief targets on the left, right, or both sides, while fixating centrally), with the innovation that the patient triggered each trial by pressing a key with her unseen hand. This start key was located either on the left or right, and the patient used either her left or right hand to trigger the trial. We found that left-sided visual extinction was substantially reduced when the patient used her left hand in left hemispace to start each trial. This confirms that activation of a contralesional effector in contralesional hemispace can have a direct influence on sensory visual inattention, bringing contralesional visual events back into the patient's awareness.