A kinematic analysis of distracter interference effects during visually guided action in spatial neglect

Patients with left spatial neglect following right hemisphere damage may sh ow anomalies in ipsilesional-limb movements directed to targets on their af fected side, in addition to their characteristic perceptual deficits. In th is study we examined the extent to which visually guided movements made by neglect patients are susceptible to interference from concurrent visual dis tracters on the contralesional or ipsilesional side of a designated target. Eleven right hemisphere patients with visual neglect, plus 11 matched heal thy controls, performed a double-step movement task upon a digitizing table t, using their ipsilesional hand to respond. On each double-step trial the first component of the movement was cued to a common central target, wherea s the second component was cued unpredictably to a target on either the con tralesional or ipsilesional side. On separate trials lateral targets either appeared alone or together with a concurrent distracter in an homologous l ocation in the opposite hemispace. In addition to being significantly slowe r and more error prone than controls, neglect patients also exhibited a num ber of interference effects from ipsilesional distracters. They often faile d to move to left targets in the presence of a right-sided distracter, or e lse they moved to the distracter itself rather than to a contralesional tar get. The initial accelerative phase of their movements to contralesional ta rgets tended to be interrupted prematurely, and they spent significantly mo re time in the terminal guidance phase of movements to contralesional targe ts in the presence of an ipsilesional distracter. In contrast. contralesion al distracters had little effect on patients' movements to ipsilesional tar gets. We conclude that right hemisphere damage induces a competitive bias t hat favors actions to ipsilesional targets. This bias affects multiple stag es of processing within the visuomotor system, from initial programming thr ough to the final stages of terminal guidance.