Abstract
We report a series of experiments in which participants had to judge the direction in which a pair of vibrotactile stimuli presented to two adjacent digits of either the same or different hands were stimulated (left-to-right or vice versa in experiments 1 and 2; near-to-far or vice versa in experiment 3, at stimulus onset asynchronies varying between 100 and 600 ms). When the participant’s hands were placed side-by-side (anatomical posture), with their fingers either pointing away from them or else pointing toward the midline, directional discrimination performance was generally accurate. By contrast, when the fingers of the two hands were interleaved in either of these postures, performance deteriorated significantly for certain specific combinations of digits, with a more pronounced impairment seen when the fingers pointed away from the participant than when they pointed toward the midline. This decline in tactile direction discrimination performance in the interleaved fingers posture appears to reflect a failure to represent the position of tactile stimuli correctly when the fingers of the two hands are interleaved.
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Notes
In the present study, we contrast the consequences of using an egocentric reference frame with a somatotopic reference frame. However, it is important to note that it is possible participants may actually be using an allocentrically (i.e. environmentally) defined frame of reference instead (i.e. rather than an egocentrically defined one) to determine the location of the vibrotactile stimuli.
In this control experiment, participants (n=10) were significantly more accurate when their fingers were placed in the anatomical posture (mean=8%), than in the interleaved posture (mean=30%) [F(1,9)=34.23, P<0.001]. There was also a significant modulation of accuracy as a function of SOA [F(2,18)=17.20, P<0.001], with participants making more errors at the 200 ms SOA (mean=22%), than at either the 400 ms (mean=18%), or 600 ms SOAs (mean=16%; the comparison between these latter two conditions was not significant).
In particular, participants (n=20) were significantly more accurate overall when their fingers were placed in the anatomical posture (mean error rate of 10%), than when they were placed in the interleaved posture (mean=35%) [F(1,19)=240.7, P<0.001]. The participants also made significantly more errors at the shortest SOA (i.e. 200 ms; mean=25%), than at the longer SOAs (i.e. 400 or 600 ms; mean=21 and 20%, respectively) [F(2,38)=18.68, P<0.001].
It should be noted, however, that given our use of spatial separations between adjacent tactile stimuli that were much smaller than those used by Yamamoto and Kitazawa (2001), we cannot rule out the possibility that tactile apparent motion cues may have influenced performance in some way in experiment 2, especially given that the range of SOAs used (i.e. 100–160 ms) falls within the range in which tactile apparent motion is commonly experienced (e.g. Kirman 1974; see also Sherrick 1968a, b; Gardner and Sklar 1994; Craig 2003). However, given that no subjective ratings of apparent motion were taken in our study, this interpretation must remain speculative.
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Zampini, M., Harris, C. & Spence, C. Effect of posture change on tactile perception: impaired direction discrimination performance with interleaved fingers. Exp Brain Res 166, 498–508 (2005). https://doi.org/10.1007/s00221-005-2390-y
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DOI: https://doi.org/10.1007/s00221-005-2390-y