Hand-centered attentional and motor asymmetries in unilateral neglect
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.