Somatotopic dominance in tactile temporal processing
The sense of touch is initiated by stimulation of peripheral mechanoreceptors, and then the spatio-temporal pattern of the receptors' activation is interpreted by central cortical processing. To explore the tactile central processing, we psychophysically studied human judgments of the temporal relationships between two tactile events occurring at different skin locations. We examined four types of two-point temporal judgments-simultaneity, temporal order, apparent motion, and inter-stimulus interval-which differ from one another in time scale and task requirement. To perform any of the four temporal judgment tasks, the brain has to integrate spatially separated inputs. The main focus of the present study is to examine how the spatial separation affects the temporal judgment tasks. Two spatial coordinates can be defined in touch: the somatotopic coordinate, defined by cortical topography, and the spatiotopic coordinate, defined in the environment. In our experiments, the somatotopic distance was manipulated by stimulating the middle and index fingers of the same hand or different hands (ipsilateral vs. bilateral conditions), while the spatiotopic distance was manipulated by increasing the stimulators' separation under bilateral conditions (bilateral-near vs. bilateral-far conditions). Our results clearly demonstrated that all four of the temporal judgments were significantly affected by the somatotopic distance, but only slightly by the spatiotopic distance. The present results, together with the previous findings, suggest that tactile temporal judgments in a wide range of time scale, from several to several 100 ms, primarily reflect processing at the level of somatotopic representation unless the performance is further constrained by spatial processing.