Spontaneous Gaze Shifts Reveal the Spatial Coding of Touch

An unexpected touch on the shoulder immediately triggers an orienting response towards the touched location. Although this reaction seems automatic, localizing a touch in space is more complex than we might think. In the somatosensory cortex, tactile stimuli are initially coded independently of body posture. They are further reported on a cognitive map used to match tactile coordinates with postural information, under the control of the posterior parietal cortex. However, the mechanism underlying such tactile remapping remains unclear. To uncover this mechanism, we recorded the gaze behavior of participants receiving tactile stimuli on the fingers of their right hand in a palm-down or palm-up posture. Tactile stimuli induced horizontal gaze shifts whose direction was determined by the external position of the stimulated finger relative to the middle finger rather than by the external position of the stimulated hand. Comparison of the palm-up and palm down conditions showed that hand rotation reversed the direction of gaze shifts induced by each finger, except for the middle finger for which gaze remained central in both conditions, despite the rightward location of the hand. Eyetracking data thus indicate that spatial attention moves on either side of the axis extending along the middle finger to match the tactile coordinates of fingers with the current hand posture. We conclude that tactile remapping proceeds by shifting attention around a reference defined by the main axis of body parts, and that these shifts occur in an internal representation of the body that keeps track of postural information.