Hand-centered attentional and motor asymmetries in unilateral neglect

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Abstract

On several accounts of ‘selection for action’, acting on a target object among distractors requires that irrelevant inputs and responses to these inputs are inhibited, and relevant inputs and responses selected. In unilateral neglect associated with right-hemisphere lesions, selection processes may be biased toward stimuli on the right, as right is usually defined by head and body hemispace. In normal subjects performing reaching-to-target tasks, selection may be ‘hand-centered’ (Tipper S., Lortie C., Baylis G.C., Journal of Experimental Psychology: Human Perception and Performance 18(4) (1992) 891–905) in that distractor stimuli close to the hand compete strongly with the target for the control of action, causing greater interference than distractors far from the hand. We reasoned that in the context of a reaching task, a left–right asymmetry in unilateral neglect may be defined with respect to the position of the hand. This predicts that target and distractor stimuli to the left of hand (i.e. requiring leftward movements for contact) should compete less strongly for the control of action than stimuli to the right of the hand. We tested this hypothesis by asking eight patients with unilateral neglect (and 12 healthy controls) to reach to central targets presented alone and with surrounding distractors from left or right start positions. Patients with neglect, but not controls, were slower to initiate reaches from right start as compared to left start positions. In this context, patients showed interference from distractors to the right of the hand and facilitation from distractors to the left of the hand. This indicates that a left–right selection asymmetry in neglect may be hand-centered. These data can be explained on a model of damage to the portion of a distributed neuronal population coding movement vectors to stimuli in relatively leftward locations.

Section snippets

Subjects

From a population of MossRehab inpatients and outpatients with unilateral right-hemisphere stroke, we studied eight consecutive patients with evidence of neglect on neurologic examination and in daily activities who met study criteria (arousal, visual acuity, and hearing sufficient to participate; absence of prior neurologic history). Twelve right-handed healthy age-matched adult subjects (nine females and three males) were also enrolled. All subjects provided informed consent to participate.

Errors

No control subject made any errors in any condition. Patients N1, N2, N3, N4, and N6 similarly made no errors. In the left start condition, N5 made four errors and N7 made one error. In the right start condition, N5 made 18 errors and N8 made one error. Sixteen of N5's errors in the right start condition were failures to respond by deadline. The remaining two errors were responses to distractor (rather than target) location.

Total time, no distractor condition

To assess the hypothesis that patients would be slower to reach to the

General discussion

In this study we investigated the hypothesis that a putative asymmetry in selection of targets and responses in neglect may arise with respect to a hand-centered frame of reference in the context of an action task. We assessed the time required for subjects to reach to a central target from right and left start positions, in the presence or absence of distractors. The hypothesis of a hand-centered asymmetry in neglect predicts that patients' performance will have two characteristics. First, it

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