Changing reference frames during the encoding of tactile events.

The mindless act of swatting a mosquito on the hand poses a remarkable challenge for the brain. Given that the primary somatosensory cortex maps skin location independently of arm posture [1, 2], the brain must realign tactile coordinates in order to locate the origin of the stimuli in extrapersonal space. Previous studies have highlighted the behavioral relevance of such an external mapping of touch, which results from combining somatosensory input with proprioceptive and visual cues about body posture [3-7]. However, despite the widely held assumption about the existence of this remapping process from somatotopic to external space and various findings indirectly suggesting its consequences [8-11], a demonstration of its changing time course and nature was lacking. We examined the temporal course of this multisensory interaction and its implications for tactile awareness in humans using a crossmodal cueing paradigm [12, 13]. What we show is that before tactile events are referred to external locations [12-15], a fleeting, unconscious image of the tactile sensation abiding to a somatotopic frame of reference rules performance. We propose that this early somatotopic "glimpse" arises from the initial feed-forward sweep of neural activity to the primary somatosensory cortex, whereas the later externally-based, conscious experience reflects the activity of a somatosensory network involving recurrent connections from association areas.