Event-related potential evidence for a dynamic (re-)weighting of somatotopic and external coordinates of touch during visual-tactile interactions.

The localization of touch in external space requires the remapping of somatotopically represented tactile information into an external frame of reference. Several recent studies have highlighted the role of posterior parietal areas for this remapping process, yet its temporal dynamics are poorly understood. The present study combined cross-modal stimulation with electrophysiological recordings in humans to trace the time course of tactile spatial remapping during visual-tactile interactions. Adopting an uncrossed or crossed hand posture, participants made speeded elevation judgments about rare vibrotactile stimuli within a stream of frequent, task-irrelevant vibrotactile events presented to the left or right hand. Simultaneous but spatially independent visual stimuli had to be ignored. An analysis of the recorded event-related potentials to the task-irrelevant vibrotactile stimuli revealed a somatotopic coding of tactile stimuli within the first 100 ms. Between 180 and 250 ms, neither an external nor a somatotopic representation dominated, suggesting that both coordinates were active in parallel. After 250 ms, tactile stimuli were coded in a somatotopic frame of reference. Our results indicate that cross-modal interactions start before the termination of tactile spatial remapping, that is within the first 100 ms. Thereafter, tactile stimuli are represented simultaneously in both somatotopic and external spatial coordinates, which are dynamically (re-)weighted as a function of processing stage.