Abstract
Processing of tactile stimuli requires both localising the stimuli on the body surface and combining this information with a representation of the current posture. When tactile stimuli are applied to crossed hands, the system first assumes a prototypical (e.g. uncrossed) positioning of the limbs. Remapping to include the crossed posture occurs within about 300 ms. Since fingers have been suggested to be represented in a mainly somatotopic reference frame we were interested in how the processing of tactile stimuli applied to the fingers would be affected by an unusual posture of the fingers. We asked participants to report the direction of movement of two tactile stimuli, applied successively to the crossed or uncrossed index and middle fingers of one hand at different inter-stimulus intervals (15 to 700 ms). Participants almost consistently reported perceiving the stimulus direction as opposite to what it was in the fingers crossed condition, even with SOAs of 700 ms, suggesting that on average they did not incorporate the unusual relative finger positions. Therefore our results are in agreement with the idea that, by default, the processing of tactile stimuli assumes a prototypical positioning of body parts. However, in contrast to what is generally found with tactile perception with crossed hands, performance did not improve with SOAs as long as 700 ms. This suggests that the localization of stimuli in a somatotopic reference and the integration of this representation with postural information are two separate processes that apply differently to the hands and fingers.
Highlights
When you want to respond to a tactile stimulus you first have to find out where on the skin this stimulus is located
It was shown that when tactile stimuli were subsequently applied to crossed hands, a position in which the external spatial frame is incongruent with the somatotopic frame [6,10], participants tended to misperceive the order in which the tactile information was applied, but only when stimuli were administered in rapid succession
This resulted in confusion of the temporal order of tactile stimuli to crossed hands to such an extent that a quarter of the participants reported a complete reversal of the perceived order of stimuli with stimulus onset asynchronies (SOAs) of up to 300 ms
Summary
When you want to respond to a tactile stimulus you first have to find out where on the skin this stimulus is located. The authors concluded that tactile information was first processed according to a prototypical positioning of the limbs and that the second tactile stimulus was applied before the external reference frame was integrated in the representation of the first stimulus This resulted in confusion of the temporal order of tactile stimuli to crossed hands to such an extent that a quarter of the participants reported a complete reversal of the perceived order of stimuli with stimulus onset asynchronies (SOAs) of up to 300 ms. With 90, 140 and 180 ms intervals, no significant cueing effect was found, reflecting a time interval in which none of the possible reference frames is dominant, probably due to an incomplete remapping process In line with this result, using ERPs Heed & Roder [14] found an indication of the use of both an anatomical and a spatial reference frame 100–140 ms after a tactile stimulus on the hand. Longer stimulus intervals provide the opportunity to incorporate the unusual postures into the spatial representation of the tactile stimulus, albeit not completely (on average 95% correct) [5,6,7,9,12]
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