Reach Trajectories Characterize Tactile Localization for Sensorimotor Decision Making

Abstract

Spatial target information for movement planning appears to be coded in a gaze-centered reference frame. In touch, however, location is initially coded with reference to the skin. Therefore, the tactile spatial location must be derived by integrating skin location and posture. It has been suggested that this recoding is impaired when the limb is placed in the opposite hemispace, for example, by limb crossing. Here, human participants reached toward visual and tactile targets located at uncrossed and crossed feet in a sensorimotor decision task. We characterized stimulus recoding by analyzing the timing and spatial profile of hand reaches. For tactile targets at crossed feet, skin-based information implicates the incorrect side, and only recoded information points to the correct location. Participants initiated straight reaches and redirected the hand toward a target presented in midflight. Trajectories to visual targets were unaffected by foot crossing. In contrast, trajectories to tactile targets were redirected later with crossed than uncrossed feet. Reaches to crossed feet usually continued straight until they were directed toward the correct tactile target and were not biased toward the skin-based target location. Occasional, far deflections toward the incorrect target were most likely when this target was implicated by trial history. These results are inconsistent with the suggestion that spatial transformations in touch are impaired by limb crossing, but are consistent with tactile location being recoded rapidly and efficiently, followed by integration of skin-based and external information to specify the reach target. This process may be implemented in a bounded integrator framework.

Significance statement: How do you touch yourself, for instance, to scratch an itch? The place you need to reach is defined by a sensation on the skin, but our bodies are flexible, so this skin location could be anywhere in 3D space. The movement toward the tactile sensation must therefore be specified by merging skin location and body posture. By investigating human hand reach trajectories toward tactile stimuli on the feet, we provide experimental evidence that this transformation process is quick and efficient, and that its output is integrated with the original skin location in a fashion consistent with bounded integrator decision-making frameworks.

Keywords: foot; limb crossing; motor control; remapping; touch.

Figure 1.

Experimental setup of Experiments 1 and 2 and time-related characteristics of reach trajectories of Experiment 1. A, Experimental setup. Position of tactile and visual stimulators was kept identical in space across foot positions. Top row, Experiment 1. Bottom row, Experiment 2. B, Condition estimates from the mixed-model analysis of the time of reach turn points. Turns toward tactile targets at crossed feet were initiated later than those at uncrossed feet. Turn point times toward tactile, but not visual, targets were significantly affected by foot posture. Error bars indicate 95% CI. C, Timeline of tactile remapping depicted in direct comparison with the timing of visual spatial processing derived from the time analysis of Experiment 1.

Figure 2.

Reach trajectories of an example subject in Experiment 1. Turn points to the correct target location are depicted as black dots. Top row, Reaches executed with the left hand toward visual targets. Bottom row, Reaches toward tactile targets. Left, Targets located at uncrossed feet. Right, Targets located at crossed feet.

Figure 3.

Experimental setup and results of Experiment 3. A, Experimental setup. B, Trajectories of reaches to tactile targets were normalized in time and then analyzed point by point with fANOVAs for effects of target side and congruence of fixation and target modality. Movements to left targets differed significantly from movements to right targets during the last third of the movement, but congruence of fixation and target modality did not affect the movement profile significantly.

Figure 4.

Spatial characteristics of reach trajectories. A, Model estimates from the mixed-model analysis of the spatial location of turn points. The average spatial location of turn points toward tactile targets was slightly biased toward the incorrect side when the feet were crossed. Spatial locations of turn points toward visual targets were not significantly affected by foot posture. Error bars indicate 95% CI. B, Single-subject example of mean trajectories; reaches to the left target were flipped to be analyzed together with reaches to the right target. Start and end position of reaches were normalized. Points display single-trial turn points for reaches to visual and tactile targets located at uncrossed (light blue/red) or crossed feet (dark blue/red). Dashed line indicates the mean of turn-around trajectories. Top row includes turn-around reaches, a subset of reaches in which the turn point is located left or right of the mean + 2 SDs of turn points in the uncrossed condition. Bottom row excludes turn-around reaches. C, Sequential choice effects in reach trajectories. Mean trajectories of an exemplar subject's reaches toward uncrossed feet of either target modality are ordered according to the number repetitions of left or right targets in the preceding trials.