Attention in action: evidence from on-line corrections in left visual neglect.

It is widely accepted that the posterior parietal cortex is critical for the on-line control of action and optic ataxia patients are unable to correct their movements in-flight to changes in target position. The current study investigated on-line correction in patients with left visual neglect, right brain damaged patients without neglect and healthy controls. Participants were asked to reach towards a central target that could jump unexpectedly, at movement onset, to the right or left sides of space. In response to the jump, participants were asked either to follow the target or to stop their movement. Neglect patients were able to correct their ongoing movements smoothly and accurately towards right and left target jumps. They did so even when told to stop their movement, indicating that these corrections occurred automatically (i.e., without instruction). However, the timing of corrections to the left was delayed in neglect patients and this produced a drastic increase in movement time. To our surprise, we also found that neglect patients were impaired at stopping their ongoing reaches, when compared to the control groups, in response to either left or right jump trials. We suggest that the 'automatic pilot' system for the hand is spared in neglect, but its processing speed is unilaterally slowed due to a deficit in orienting of attention to the contralesional side. We relate these findings to the breakdown of a system that combines information for attention, perception and action. Damage to this system may not only slow corrective movements to the contralesional side, but also produce non-lateralized deficits in interrupting an ongoing reach.