Somatosensory saccades reveal the timing of tactile spatial remapping.

Remapping tactile events from skin to external space is an essential process for human behaviour. It allows us to refer tactile sensations to their actual externally based location, by combining anatomically based somatosensory information with proprioceptive information about the current body posture. We examined the time course of tactile remapping by recording speeded saccadic responses to somatosensory stimuli delivered to the hands. We conducted two experiments in which arm posture varied (crossed or uncrossed), so that anatomical and external frames of reference were either put in spatial conflict or were aligned. The data showed that saccade onset latencies in the crossed hands conditions were slower than in the uncrossed hands condition, suggesting that, in the crossed hands condition, remapping had to be completed before a correct saccade could be executed. Saccades to tactile stimuli when the hands were crossed were sometimes initiated to the wrong direction and then corrected in-flight, resulting in a turn-around saccade. These turn-around saccades were more likely to occur in short-latency responses, compared to onset latencies of saccades that went straight to target. The latter suggests that participants were postponing their saccade until the time the tactile event was represented according to the current body posture. We propose that the difference between saccade onset latencies of crossed and uncrossed hand postures, and between the onset of a turn-around saccade and a straight saccade in the crossed hand posture, reveal the timing of tactile spatial remapping.