Digit forces bias sensorimotor transformations underlying control of fingertip position

Abstract

Humans are able to modulate digit forces as a function of position despite changes in digit placement that might occur from trial to trial or when changing grip type for object manipulation. Although this phenomenon is likely to rely on sensing the position of the digits relative to each other and the object, the underlying mechanisms remain unclear. To address this question, we asked subjects (n = 30) to match perceived vertical distance between the center of pressure (CoP) of the thumb and index finger pads (dy ) of the right hand ("reference" hand) using the same hand ("test" hand). The digits of reference hand were passively placed collinearly (dy = 0 mm). Subjects were then asked to exert different combinations of normal and tangential digit forces (Fn and Ftan , respectively) using the reference hand and then match the memorized dy using the test hand. The reference hand exerted Ftan of thumb and index finger in either same or opposite direction. We hypothesized that, when the tangential forces of the digits are produced in opposite directions, matching error (1) would be biased toward the directions of the tangential forces; and (2) would be greater when the remembered relative contact points are matched with negligible digit force production. For the test hand, digit forces were either negligible (0.5-1 N, 0 ± 0.25 N; Experiment 1) or the same as those exerted by the reference hand (Experiment 2).Matching error was biased towards the direction of digit tangential forces: thumb CoP was placed higher than the index finger CoP when thumb and index finger Ftan were directed upward and downward, respectively, and vice versa (p < 0.001). However, matching error was not dependent on whether the reference and test hand exerted similar or different forces. We propose that the expected sensory consequence of motor commands for tangential forces in opposite directions overrides estimation of fingertip position through haptic sensory feedback.

Keywords: feedback; grasping; hand; haptics; psychophysics.

Figure 1

Experiment setup. (A) shows frontal and side views of the handle used for the study (“a” denotes force/torque sensors). (B) shows the frontal view of the handle with thumb and index fingertip center of pressure of the reference hand located at the same y-coordinates (vertical height relative to the base of the object) on the graspable surfaces of the handle (collinear d_y). The red dots denote the CoP of each digit. (C) shows a top view of the experimental setup. The subject is shown contacting the handle with thumb and index fingertip, while the left hand was kept flat on the table. When relaxing in between trials, both hands were kept flat and relaxed. Note that the table top (gray) was opaque and prevented subjects from seeing their forearms and hands but is shown as transparent for graphical purposes only. Forearms and wrists were strapped to the table to prevent movements within and across trials while the handle was anchored to the table.

Figure 2

Experimental protocol and conditions (Experiment 1). (A) shows the time course of the experimental protocol. In the “passive d_y adjustment” phase, the subject’s thumb and index finger were passively placed by an experimenter to a collinear d_y (see Figure 1B). Once the desired d_y was reached and digit forces matched the desired target forces, recording of reference hand d_y started for 5 s while subjects were asked to perceive and memorize the reference hand d_y (“perceive and memorize” phase). During the “relax” phase, subjects were asked to relax their reference hand for 10 s, followed by the “match” phase in which they were asked to reproduce the remembered reference hand d_y using the (same) test hand within 10 s. The test hand d_y was then recorded for 5 s while subjects maintained the digit position and digit forces (“hold” phase). (B) shows the experimental conditions for Experiment 1. The thumb and index finger (filled and open ellipse, respectively) of the reference hand exerted tangential forces either in the same or opposite directions (“Same” and “Opposite”, left and middle column, respectively). In the Same condition, thumb and index finger exerted tangential forces that were both upward or downward (T_UP-I_UP or T_DOWN-I_DOWN, respectively). In the Opposite condition, the tangential forces of the thumb and index finger were directed opposite to each other, i.e., either upward and downward (T_UP-I_DOWN) or downward and upward (T_DOWN-I_UP), respectively. In the Control condition (right column), subjects were asked to exert no tangential force while exerting large or negligible normal forces (“F_n only” or “No F_tan/F_n”, respectively). The magnitude of tangential and normal forces was the same across these conditions (F_tan: 2.5–3.5 N, F_n: 4–5 N) with the exception of the “No F_tan/F_n” condition (F_tan: 0 ± 0.25 N, F_n: 0.5–1 N). The test hand in Experiment 1 exerted only negligible tangential and normal forces (F_tan: 0 ± 0.25 N, F_n: 0.5–1 N).

Figure 3

Experimental conditions (Experiment 2). The experimental conditions of Experiment 2 are shown in the same format as those shown in Figure 2 for Experiment 1. The only difference between Experiments 1 and 2 is that for the latter experiment, subjects were required to exert the same thumb and index fingertip normal and tangential forces across reference and test hands (see text for more details).

Figure 4

Fingertip vertical distance: matching performance by individual subjects. Mean matching errors averaged across five trials from each subject are shown as a function of experimental condition from Experiments 1 and 2 (top and bottom plots, respectively). Each subject data is color coded whereas the thick black line denotes the mean matching error averaged across 15 subjects ± standard error of the mean. For both experiments, positive and negative matching errors indicate that subjects reproduced remembered reference hand d_y by placing the thumb CoP higher and lower, respectively, than the index finger CoP.

Figure 5

Fingertip vertical distance: matching errors. Matching errors were compared across experimental conditions and between experiments. (A) shows average matching error for Experiments 1 and 2 (top and bottom plots, respectively) across matching conditions. The mean matching error in the “F_n only” condition was used as a reference to normalize the matching error in the Same and Opposite conditions (left and middle column, respectively; see text for more details). (B) shows average normalized matching error for the Experiments 1 and 2 (top and bottom plots, respectively) across matching conditions. For all panels, matching and normalized errors were averaged across all subjects (vertical bars denote SE). Asterisks denote significant differences (p < 0.05) from zero, whereas “n.s.” denote non-significant differences.