Abstract
Adaptation studies [Johnston et al. 2006, Burr et al. 2007] suggest that visual events are timed by multiple, spatially selective mechanisms anchored in real-world rather than retinal coordinates [Burr et al. 2007]. To test whether this was a general property of event timing we investigated timing mechanisms for touch, using a paradigm similar to that used in vision [Burr et al. 2007]. Subjects adapted to tactile movement by resting their right index finger on a corrugated grating etched on a wheel moving at 15 cm/sec (45 Hz). After adaptation, subjects compared the duration of a test stimulus (22 Hz moving grating of variable duration) presented to the adapted hand to a probe presented to the index finger of the left hand for 600 ms after a 500 ms pause. Three different conditions were examined: full adaptation, where the test stimulus was presented to the same index finger in the same position as the adaptor; dermotopic adaptation, where the test was presented to the index finger in a different position in space; and spatiotopic adaptation, where the test was presented to the middle finger moved to the same spatial position as the adaptor. Both perceived speed and perceived duration of the tactile stimulus were affected by adaptation. When the speed of the test was adjusted to compensate for the effects of adaptation, the effects of event time in the dermotopic condition were minimal, while the full and spatiotopic adaptation conditions showed large reductions in perceived duration, up to 40%. These results suggest that like visual events, tactile events are timed by neural mechanisms that are spatially selective, not in receptor coordinates but in external or body-centered coordinates. This may be important in constructing and updating a body representation within which tactile events are timed.