Perception of Arm Position in Three-Dimensional Space

Abstract

Proprioception refers to the senses of body position, movement, force and effort. Previous studies have demonstrated workspace and direction-dependent differences in arm proprioceptive sensitivity within the horizontal plane. In addition, studies of reaching in the vertical plane have shown that proprioception plays a key role in anticipating arm configuration dependent effects of gravity. This suggests that proprioceptive sensitivity could vary with the direction of arm displacement relative to the gravitational vector, as well as with arm configuration. To test these hypotheses, and to characterize proprioception more generally, we assessed the direction-dependence and arm postural-dependence of proprioceptive sensitivity in 3D space using a novel robotic paradigm. A subject's right arm was coupled to a 7-df robot through a trough that stabilized the wrist and forearm, allowing for changes in configuration largely at the elbow and shoulder. Sensitivity was evaluated using a "same-different" task, where the subject's hand was moved 1-4 cm away from an initial "test" position to a 2nd "judgment" position. The proportion of trials where subjects responded "different" when the positions were different ("hit rate"), and where they responded "different" when the positions were the same, ("false alarm rate"), were used to calculate d', a measure of sensitivity derived from signal detection theory (SDT). Initially, a single initial arm posture was used and displacements were performed in six directions: upward, downward, forward, backward, leftward and rightward of the test position. In a follow-up experiment, data were obtained for four directions and two initial arm postures. As expected, sensitivity (d') increased monotonically with distance for all six directions. Sensitivity also varied between directions, particularly at position differences of 2 and 3 cm. Overall, sensitivity reached near maximal values in this task at 2 cm for the leftward/rightward directions, 3 cm for upward/forward and 4 cm for the downward/backward directions. In addition, when data were grouped together for opposing directions, sensitivity showed a dependence upon arm posture. These data suggest arm proprioceptive sensitivity is both anisotropic in 3D space and configuration-dependent, which has important implications for sensorimotor control of the arm and human-robot interactions.

Keywords: position sense; proprioception; psychophysics; rehabilitation; robotics.

Figure 1

(A) Human-robot coupling at the test position. Photograph used with permission of subject. (B) Arm postures examined in this study. (C) Locations of endpoints and via points (i.e. distractors) with respect to the reference position. An example path taken by the robot on a single trial is also shown (arrows).

Figure 2

(A) Experimental protocol. Four distances were evaluated for each direction, with distance order randomized across directions. For simplicity, only two distances are shown. “S”: same; “D”: different. (B) Sequence of events on a single trial.

Figure 3

Percent correct (A), hit rate and false alarm rate (B) and d’ (C) for a single subject. Data for the upward and downward directions are shown.

Figure 4

Mean (±SEM) hit rates, false alarm rates and d’ values for all subjects. (A,B) Upward and downward directions. (C,D) Forward and backward directions. (E,F) Rightward and leftward directions.

Figure 5

Boxplots of the sensitivities (d’) at each distance and direction for all subjects. Corresponding mean sensitivities (diamonds) are superimposed on each boxplot.

Figure 6

Mean (±SD) sensitivities for the leftward/rightward and forward/backward axes in both arm postures. Data for all subjects at the 2 and 3 cm distances are shown.