Anisotropic distortion in the perceived orientation of stimuli on the arm

Abstract

Mechanoreceptors on the skin are heterogeneously distributed, and the sampling of neural signals in the brain can vary depending on the part of the body. Therefore, it can be challenging for the brain to consistently represent stimuli applied to different body sites. Here, we report an example of a regional perceptual distortion of the tactile space. We used a piezoelectric braille display to examine shape perception on the volar surface of the arm and to compare it to that on the palm. We found that the orientation of perceived stimuli on the arm was distorted in certain areas. In particular, an inwardly-inclined line shape was perceived as being more inwardly-inclined than it actually was. On the other hand, an outwardly-inclined line was perceived accurately. When the same stimuli were applied to the palm, this anisotropic bias was not observed. We also found that changing the posture of the arm changed the angle at which this anisotropic distortion occurred, suggesting the influence of the skin frame of reference on this illusion. This study showed a clear example of how the representation of even simple stimuli is complexly distinct when the stimuli are applied to different body sites.

Anisotropic distortion in the perceived direction of motion on the arm

“…For the sake of brevity, we refrain from considering all these effects in detail. Since it has already been reported that there is a difference between the arm and the hand condition 4 , here we report the partial analyses which highlight the new finding of this study, namely, the difference in performance depending on the direction of the stimuli. To explore the significant interaction of questions and other factors, we conducted separate ANOVAs with the GLMM models on Q1 and Q2.…”

“…The experiment was carried out using the same apparatus (Fig. 1 A) and in the same posture as in the previous study 4 . Participants positioned their left hand or forearm on a desk with the palm down (Fig.…”

“…We have previously reported an error in motion perception at the arm that is difficult to explain solely by the size of the receptive field 4 . When participants touched the braille display with their arms and perceived stimuli presented in a straight path, the perceived angle was found to be distorted only when the stimuli were presented along a specific path.…”

“…Motion detection plays a very basic function even at the arm, as described in the previous study 5 as follows: “Cutaneous mechanoreceptors of the arm and leg are often primarily thought of as sources of warning signals of the site and direction of motion of stimuli (e.g. a crawling insect) upon the skin surface and of guidance cues for reaching movements (to brush away the insect).” Not many studies on the arm have been conducted, but some reported the deviation of the spatial representation of the stimulus on the arm from the actual stimulus 4 , 6 – 9 . Although such deviation from the Euclidean metric of stimulus representation has been also reported for the hand 10 – 14 , they differ in occurrence probability and degree from those on the arm.…”

“…The first question is whether this illusion occurs on the arm in general or requires stimulation of a specific skin area of the arm skin. Since the stimulus was presented to a fixed location on the arm in the previous study 4 , the location on the skin subjected to stimulation was slightly different when a right oblique path was presented and when a left oblique path was presented. Thus, the possibility that the involvement of a specific skin area (e.g., close to the thumb) triggered the observed illusion cannot be excluded.…”