Effects of visual stimuli on temporal order judgments of unimanual finger stimuli

Successive tactile stimuli, delivered one to each hand, are referred to spatial representation before they are ordered in time (Yamamoto and Kitazawa in Nat Neurosci 4:759–765 2001a). In the present study, we examined if this applies even when they are delivered unilaterally to fingers of a single hand. Tactile stimuli were delivered left-to-rightward relative to the body (2nd–3rd–4th) or in reverse with stimulus onset asynchrony of 100 ms. Simultaneously with the delivery of tactile stimuli, three of nine small squares arranged in a matrix of 3 × 3 were turned on as if they appeared near the tips of the fingers. Although subjects were instructed to ignore the visual stimuli and make a forced choice between the two orders of tactile stimuli, the correct-judgment probability depended on the direction of visual stimuli. It was greater than 95% when the direction of visual stimuli matched that of the tactile stimuli, but less than 50% when they were opposite to each other. When the right hand was rotated counterclockwise on the horizontal plane (90°) so that the fingers were pointing to the left, the preferred direction of visual stimuli that yielded the peak correct judgment was also rotated, although not to the full extent. These results show that subjects cannot be basing their tactile temporal order judgment solely on a somatotopic map, but rather on a spatial map on which both visual and tactile signals converge.