Tactile Stimuli on the Fingers are coded on Canonical Hand Maps

Abstract

An unexpected touch on the shoulder immediately triggers an orienting response towards the touched location. Although this reaction seems automatic, localizing touch is more complex than we might think. In the somatosensory cortex, tactile stimuli are initially coded independently of body posture. According to a well-established view, stimuli are further coded on an on-line representation of the body in its current posture, using external coordinates, a process called tactile remapping. However, recently, an alternative account emerged, according to which stimuli are rather coded on a stored representation of the body in its default posture. To uncover the representation used to localize stimuli, we recorded the spontaneous gaze behavior of participants receiving tactile stimuli on the fingers of their right hand placed in different configurationspostures. When comparing the gaze pattern in palms-down and palms-up postures, the gaze direction was determined by the external position of stimuli, supporting the tactile remapping account. However, when the hand was in a neutral posture, the gaze pattern revealed a palm-down representation, and crossing the hand to the left of the body shifted the gaze rightward, clearly supporting the default posture account. Furthermore, when the five fingers were stimulated, the representation reflected in gaze behavior was centered on the middle finger, whereas it was centered on the ring finger when only the last three fingers were stimulated. Taken together, our results show that tactile stimuli activate canonical representations of the body in its usual positions, and these representations are centered on the stimulated area