Dual-target interference for the 'automatic pilot' in the dorsal stream.

When a target moves to a new location during a rapid aiming movement, the hand follows it, even when the participant intends not to. Pisella et al. (Nat Neurosci 3:729-736, 2000) claim that the posterior parietal cortex, in the dorsal visual stream, is responsible for this 'automatic pilot'. Here we study the limits of automaticity in the dorsal stream through analysis of aiming movements to two targets in sequence. Participants were given a goal of moving rapidly to two targets, with the first movement being completed within approximately 200 ms. On 30% of trials, the first or the second target jumped unpredictably to a new location at movement onset, allowing us to measure the automatic capture of the hand. The results showed that hand movements were less responsive to target jumps in a 2-target condition than in a 1-target control condition. This indicates that the 'automatic pilot' is susceptible to interference from multiple visual inputs, implying that the dorsal stream is less effective at guiding actions online when multiple targets are attended.