Mapping proprioception across a 2D horizontal workspace

Abstract

Relatively few studies have been reported that document how proprioception varies across the workspace of the human arm. Here we examined proprioceptive function across a horizontal planar workspace, using a new method that avoids active movement and interactions with other sensory modalities. We systematically mapped both proprioceptive acuity (sensitivity to hand position change) and bias (perceived location of the hand), across a horizontal-plane 2D workspace. Proprioception of both the left and right arms was tested at nine workspace locations and in 2 orthogonal directions (left-right and forwards-backwards). Subjects made repeated judgments about the position of their hand with respect to a remembered proprioceptive reference position, while grasping the handle of a robotic linkage that passively moved their hand to each judgement location. To rule out the possibility that the memory component of the proprioceptive testing procedure may have influenced our results, we repeated the procedure in a second experiment using a persistent visual reference position. Both methods resulted in qualitatively similar findings. Proprioception is not uniform across the workspace. Acuity was greater for limb configurations in which the hand was closer to the body, and was greater in a forward-backward direction than in a left-right direction. A robust difference in proprioceptive bias was observed across both experiments. At all workspace locations, the left hand was perceived to be to the left of its actual position, and the right hand was perceived to be to the right of its actual position. Finally, bias was smaller for hand positions closer to the body. The results of this study provide a systematic map of proprioceptive acuity and bias across the workspace of the limb that may be used to augment computational models of sensory-motor control, and to inform clinical assessment of sensory function in patients with sensory-motor deficits.

Figure 1. Experimental Apparatus.

The subject grasped a robotic manipulandum that moved the hand along the surface of a desk. (B) Side view of set-up for Experiment 2 (visual reference experiment). (C,D) Overhead view: In Experiments 1 and 2, each participant performed proprioceptive tests at three lateral locations and along two axes (left-right (LR), forwards-backwards (FB)) either in front of the left or right shoulder or at the midline of the body. Three test positions were determined for each individual and were at 80% (far), 50% (middle) and 20% (near) of the participants maximum reach (MR). Proprioception of participants' right (C) and left (D) arms was tested.

Figure 2. Proprioceptive Testing Procedure.

On each trial, the subject's arm was moved by the robot to a reference location followed by a judgment location. Subjects made a two-alternative forced choice judgment about the position of their hand relative to the reference location. To eliminate speed or timing cues and preclude feedback about performance on a trial-to-trial basis, distractor movements were used before and after each judgement location by bringing the hand a random distance away from the test location (14±2 cm), in a random duration (700–1600 ms) and in a random direction along the test axis (left vs right or forward vs back). In Experiment 1 participants made judgments with respect to a proprioceptive reference location. In Experiment 2, participants made judgments with respect to a visual reference.

Figure 3. Sample Psychometric Function.

At each testing location, subjects' responses were fit to a binomial model using a cumulative normal distribution function. Here we show a sample function from one subject tested at a single proprioceptive test location, along a left-right axis. Filled circles represent the proportion of times, at a given judgment location, that the subject responded that their hand was to the right of the reference location.

Figure 4. Proprioceptive Acuity as a function of testing direction.

The width of the uncertainty range is shown as a function of testing direction, left-right (LR) vs forward-backward (FB), for Experiment 1 (A) and Experiment 2 (B). Data shown are averaged over workspace position and limb.

Figure 5. Proprioceptive Acuity as a function of distance from the body.

Uncertainty range is shown as a function of distance from the body (near, middle, far) for left-right test directions (A,B) and forward-backward directions (C,D) in Experiments 1 (A,C) and 2 (B,D). Data shown are averaged over workspace position and limb.

Figure 6. Proprioceptive Acuity as a function of lateral position.

Uncertainty range is shown as a function of lateral position (left, centre, right) for left and right arms, in Experiments 1 (A) and 2 (B). Data shown are averaged over distance from the body and test direction.

Figure 7. Proprioceptive Bias across the workspace.

Proprioceptive bias is plotted as a vector (the vector sum of bias in the left-right and forward-backward directions) for individual subjects (gray) and the group mean (black), for the left and right hands at three lateral positions and three distances from the body, for Experiment 1 (A) and 2 (B). For purposes of visualization, vector length has been increased by a factor of 20.

Figure 8. Proprioceptive Bias as a function of limb and distance from the body.

Proprioceptive bias in the left-right direction is shown as a function of distance from the body (near, middle, far) for the left and right arms, in Experiments 1 (A) and 2 (B). Data shown are averaged over lateral position.

Figure 9. Proprioceptive Bias as a function of lateral position.

Proprioceptive bias along the forward-backward direction is shown as a function of lateral position (left, centre, right) for the left and right hands, in Experiment 1 (A) and 2 (B). Data shown are averaged over distance from body.

Figure 10. Tests of proprioceptive drift of the right arm.

Change in proprioceptive bias (mean ± 1 standard error) over time for early trials (first 7 trials of each block) and late trials (last 7 trials of each block) in the left-right (A,B) and forward-backward (C,D) test directions for Experiment 1 (A,C) and 2 (B,D). Data shown are averaged over workspace position.

Figure 11. Tests of proprioceptive drift of the left arm.

Change in proprioceptive bias (mean ± 1 standard error) over time for early trials (first 7 trials of each block) and late trials (last 7 trials of each block) in the left-right (A,B) and forward-backward (C,D) test directions for Experiment 1 (A,C) and 2 (B,D). Data shown are averaged over workspace position.