Spatial remapping of touch: confusion of perceived stimulus order across hand and foot

Abstract

The "body schema," a spatial representation of the body in its environment, has been suggested to be an emergent property of a widespread network of effector-specific frontal and parietal areas, many of which integrate sensory input from the different modalities. On a behavioral level, such multimodality has been shown with temporal order judgment tasks, in which participants decide which of the two hands received a tactile stimulus first. The accuracy of these judgments is influenced by body posture, indicating that tactile stimuli are not simply represented in an anatomical reference frame, but are transformed into external spatial coordinates. So far, these studies have only investigated the hands. It is therefore unclear whether a default remapping of touch into external space is a special feature of visual-manual control or whether all body parts are represented in a common nonanatomically anchored reference frame. In the present study, blindfolded participants made temporal order judgments of stimuli presented to both hands, both feet, or one hand and one foot. The stimulated limbs were held in either a parallel or a crossed posture. Judgments were equally impaired by limb crossing in all limb combinations (hands only, feet only, hand and foot). These results suggest a remapping of tactile location of all body parts into a nonanatomically anchored and, importantly, common reference frame rather than a specific remapping for eye-hand coordination only.

Fig. 1.

Response curves for TOJ right first (ordinates) of each participant plotted against stimulus onset asynchronies (abscissae) for the different limb combinations (Insets show body postures and stimulation sites). Negative values denote left-first stimulation. Gray/red small symbols and thin lines represent data from uncrossed/crossed conditions, respectively. Each dot represents the probability of right-first responses based on 12–16 judgments. Large symbols and thick-lined curves represent group average data and a group fit using the flip model as described (8). Participants sat in a chair with an arm rest and foot rest and responded with the effector they judged as having been stimulated first.

Fig. 2.

Comparison of TOJ accuracy (slopes of linear regression lines) for stimulation of the hands, the feet, and each hand with the contralateral foot. White/black bars represent conditions with uncrossed/crossed limbs, respectively. Error bars represent standard errors of the mean. For each participant and condition, judgment probabilities of the part of the curve in which judgments transgressed from left to right (i.e., the nonasymptotic part of the response curves) were transformed by using probit analysis (9). These values were linearly regressed; a steeper slope indicates that participants require a smaller SOA to respond correctly, and therefore greater accuracy. Linear regression slopes were used for statistical analyses using an ANOVA.

Fig. 3.

Stimulation/response devices were attached to the effectors (hands and feet) with elastic bands. The front part of the device was attached with a hinge and could be depressed with the fingers or toes. The front part of the response devices could be adjusted such that the plate just touched the hand or foot at rest. Devices were adjusted such that a minimal movement was required for a response to avoid displacement of the stimulators during the experiment. Stimulators were integrated in these response devices and could flexibly be placed under the distal phalanx or the stimulated finger or toe. Stimulated fingers and toes were kept in place by using an elastic band or medical tape wrapped around the stimulated phalanx and the stimulator. Participants judged the equality of stimulation at hands and feet only after the devices were satisfyingly fit to the effectors.