Human ability to scale and discriminate forces typical of those occurring during grasp and manipulation

Abstract

When humans manipulate objects, the sensorimotor system coordinates three-dimensional forces to optimize and maintain grasp stability. To do this, the CNS requires precise information about the magnitude and direction of load force (tangential to skin surface) plus feedback about grip force (normal to skin). Previous studies have shown that there is rapid, precise coordination between grip and load forces that deteriorates with digital nerve block. Obviously, mechanoreceptive afferents innervating fingerpad skin contribute essential information. We quantify human capacity to scale tangential and normal forces using only cutaneous information. Our paradigm simulated natural manipulations (a force tangential to the skin superimposed on an indenting force normal to the skin). Precisely controlled forces were applied by a custom-built stimulator to an immobilized fingerpad. Using magnitude estimation, subjects (n = 8) scaled the magnitude of tangential force (0.25-2.8 N) in two experiments (normal force, 2.5 and 4 N, respectively). Performance was unaffected by normal force magnitude and tangential force direction. Moreover, when both normal (2-4 N) and tangential forces were varied in a randomized-block factorial design, the relationship between applied and perceived tangential force remained near linear, with a minor but statistically significant nonlinearity. Our subjects could also discriminate small differences in tangential force, and this was the case for two different reference stimuli. In both cases, the Weber fraction was 0.16. Finally, scaling functions for magnitude estimates of normal force (1-5 N) were also approximately linear. These data show that the cutaneous afferents provide a wealth of precise information about both normal and tangential force.

Figure 1.

Computer-controlled stimulator. Torque motor 2 (m₂), coupled to damper (d), produced a vertical (normal) indenting force on the subject's finger (f), which was immobilized in plasticine (p). Torque motor 1 (m₁), attached to m₂ via beam (b), produced a tangential force on the finger. The direction of the tangential force in the horizontal plane was set by the hub (h), indexed at 15° increments. The flat surface (s), covered with 500 grade sandpaper, was coupled to m₁via a six axis force–torque transducer (t) (an enlargement of this area is shown on the left). The stimulator was mounted on an x–y–z vernier shift to allow accurate positioning in all three dimensions.

Figure 2.

Time sequence of normal and tangential forces. A, For normal force scaling, the plateau force was held for 1.5 sec; the rise time was 0.2 sec, the fall time was 0.1 sec, and successive trials occurred at 5.3 sec intervals. B, For tangential force scaling, normal force (dashed line) was presented first, followed after 1.0 sec by tangential force (solid line). Rise times were 0.2 sec, fall times were 0.1 sec, and successive stimuli occurred at intervals of 6.3 sec. C, For tangential force discrimination, trials comprised a pair of stimuli. The first stimulus in a pair was the standard, and the second stimulus was the comparison; the comparison could be either the same as the standard (S_s) or different (S_d). The interval between standard and comparison was 2.0 sec, and the interval between successive pairs was 3.0 sec.

Figure 3.

Normal force scaling. Dashed lines show perceived magnitude of applied normal force for each of the eight subjects (mean estimates, n = 42 for each level of normal force for each subject). The solid line represents the mean across all eight subjects.

Figure 4.

Tangential force scaling: normal force invariant. A, Perceived magnitude of tangential force applied with a normal force of 2.5 N (dashed line) and 4 N (solid line) for a single subject (S3) (mean ± SE; n = 18). B, Mean estimates of tangential force across the eight subjects for both normal forces (n = 8; mean ± SE shown at one representative point). C, Mean estimates across all subjects (± one representative SE; n = 8). Tangential force is shown as a fraction of normal force; the seven ratios (0.1, 0.2, 0.3, 0.4, 0.5, 0.6, and 0.7) were the same for both normal forces. Tangential force was applied in the radial direction on the fingerpad for all data shown in this figure.

Figure 5.

Effect of tangential force direction on scaling. A, Perceived magnitude of tangential force applied with a constant normal force of 2.5 N in four directions (radial, ulnar, distal, andproximal) for a single subject (S2) (mean ± one representative SE; n = 18). B, Mean performance across all eight subjects for tangential forces applied in all four directions with normal forces of 2.5 and 4N(± one representative SE for each normal force; n = 8). C, Lines show mean estimates across all eight subjects for both normal forces (dashed lines, 2.5 N; solid lines, 4N) in each of the four directions; tangential force is shown as a proportion of normal force.

Figure 6.

Tangential force scaling with concurrently varying normal force. A, Data are mean estimatesforasinglesubject(S8),scalingtangentialforcewithnormalforcevaryingrandomlybetween three magnitudes: 2, 3, and 4 N. Data points are means ± SE; n = 36. B, Data are mean estimates across all eight subjects (±SE; n = 8) at three levels of normal force, which varied randomly throughout blocks of trials. Tangential force was applied in the distal direction for this series.

Figure 7.

Tangential (tang) force discrimination. A, Discrimination functions for a single subject (S6) for both series (standard tangential force of1Nata normal force of 2.5 N and standard tangential force of 1.6 N at a normal force of 4 N). Fine dotted lines show difference limens estimated by linear interpolation. B, Discrimination functions for all subjects (dashed lines) for a standard tangential force of 1.6 N (normal force of 4 N). The solid line shows mean d′ values (n = 8). Filled circles show difference limens (d′= 1.35) for each subject. C, Mean d′ values (averaged across all eight subjects; n = 8) for both standards. Error bars show unidirectional SE. Tangential forces in A–C are shown as the differences from the respective standard tangential forces. Tangential force was applied in the distal direction for discrimination studies.