Visual and somatosensory information about object shape control manipulative fingertip forces

Abstract

We investigated the importance of visual versus somatosensory information for the adaptation of the fingertip forces to object shape when humans used the tips of the right index finger and thumb to lift a test object. The angle of the two flat grip surfaces in relation to the vertical plane was changed between trials from -40 to 30 degrees. At 0 degrees the two surfaces were parallel, and at positive and negative angles the object tapered upward and downward, respectively. Subjects automatically adapted the balance between the horizontal grip force and the vertical lift force to the object shape and thereby maintained a rather constant safety margin against frictional slips, despite the huge variation in finger force requirements. Subjects used visual cues to adapt force to object shape parametrically in anticipation of the force requirements imposed once the object was contacted. In the absence of somatosensory information from the digits, sighted subjects still adapted the force coordination to object shape, but without vision and somatosensory inputs the performance was severely impaired. With normal digital sensibility, subjects adapted the force coordination to object shape even without vision. Shape cues obtained by somatosensory mechanisms were expressed in the motor output about 0. 1 sec after contact. Before this point in time, memory of force coordination used in the previous trial controlled the force output. We conclude that both visual and somatosensory inputs can be used in conjunction with sensorimotor memories to adapt the force output to object shape automatically for grasp stability.

Fig. 1.

Instrumented test object and measurements taken for analyses. A, Orientation of the grip surfaces for three different surface angles: 30, 0, and −30°. B,Vectorial representation of recorded HF and VF and computed NF and TF exemplified at surface angles of 30, 0, and −30°. C,Fingertip forces and vertical movements of an object shown as a function of time for different phases of a lifting trial terminated by the subject slowly decreasing the grip force until slippage (arrow, slip) after a sound signal (arrow, sound). The intervals a andb indicate the preload phase and the load phase, respectively. Interval c shows the period 3–4 sec after the object was initially touched while static phase measurements were taken. Arrows illustrate different points of measurements of horizontal force during the load phase, i.e., horizontal forces at 10, 50, and 90%, of the static VF and maximum HF.D, End of a trial in which the subjects replaced the object on the support table in an ordinary manner after a sound signal (arrow, sound). The vertical lineindicates contact with the support table.

Fig. 2.

Force coordination during the initial part of lifts by a single subject with surface angles of 30° (solid lines), 0° (dashed lines), and −30° (dotted lines). Data are from lift series in which the surface angle was kept constant in blocks of five consecutive trials.A, Left panel, vertical and horizontal forces and vertical position as a function of time for all five consecutive trials (superimposed) with each surface angle. Right panel, coordination between these forces by displaying the horizontal force against the vertical force. B, Averaged vertical and horizontal forces and horizontal force rate for the same trials as inA. C, Averaged normal force and tangential forces for the same trials. Right panel,coordination between normal force and tangential forces by displaying the normal force against the tangential force. The solid line gives the minimum estimated normal force (Slip force) as a function of the tangential force; the vertical distance between this line and the curves represents the normal force safety margin against frictional slips. B, C, Theshaded zones of the curves give ±1 SEM.A–C, All trials were synchronized in time on touch, i.e., when the horizontal force rate exceeded 2 N/s. In addition to the surface angle given in degrees, the object shape is illustrated by theshaded inset figures (compare Fig.1A).

Fig. 3.

Horizontal forces and duration of preload and load phases during lift series in which each surface angle was presented in blocks of five consecutive trials. A, Static horizontal force (solid line) and static vertical force (dotted line) plotted against surface angle. Mean forces ± SD are illustrated for each subject (Subj. 1–7). B, Horizontal force at vertical forces corresponding to 10, 50, and 90% of the static vertical force, static horizontal force (Static), and maximum horizontal force plotted against the angle. Curves represent average values for all seven subjects. C, Mean duration of preload and load phases plotted against surface angle; 1 SD and 1 SEM are unilaterally indicated for data averaged across all seven subjects.

Fig. 5.

Adjustments to changes in surface angle during lift series in which object shape was unpredictably varied between trials. Data are averaged from eight subjects with normal digital sensibility and who showed similar load phase durations; single trials were synchronized in time when the horizontal force rate exceeded 2 N/s. Vertical and horizontal forces and horizontal force rate as a function of time for trials with vision (A, B) and without vision (C, D) are shown. The shaded zones of the curves give ±1 SEM, and the vertical line indicates the start of horizontal force increase. In addition to surface angle given in degree, the object shapes in current and previous trials are illustrated by the shaded andopen inset figures, respectively (compare Fig.1A). A, C, Adjustment to a smaller angle is illustrated by trials with −30° preceded by trials with 30° (30°→−30°, solid line). Trials with −30° (−30°→−30°, dashed line) and 30° (30°→30°, dotted line) not preceded by a change in surface angle are shown for comparison. B, D, adjustment to a larger angle is illustrated by trials with 30° preceded by trials with −30° (−30°→30°, solid line). Again, trials with −30° (−30°→−30°, dotted line) and 30° (30°→30°, dashed line) not preceded by a change in surface angle are shown for comparison. C, D, Short vertical lines indicate points in time at which the new surface angle was expressed in the motor output. Arrowheads indicate the reduced rate force occurring before the horizontal force again increased toward a level adequate for the current surface angle.

Fig. 9.

Adaptation to shapes of objects during digital anesthesia. A, B, Trials with and without vision, respectively. Left panels, vertical force and horizontal forces and horizontal force rate as a function of time for trials with 30° (solid lines), 0° (dashed lines), and −30° (dotted lines) surface angles. Right panels, horizontal force plotted against vertical force for the same data. In addition to surface angle given in degrees, object shapes are illustrated by the shaded inset figures (compare Fig. 1A). Data are averaged across all trials by the four anesthetized subjects; trials were synchronized at the start of a horizontal force increase when the horizontal force rate exceeded 2 N/s. The shaded zones of the curves give ±1 SEM, andvertical lines indicate the start of horizontal force increase.

Fig. 4.

Static normal force (dashed lines), tangential force (solid lines), and normal force safety margin against frictional slip (dotted lines) as a function of surface angle. Mean values ± SD are illustrated for all individual subjects (Subj. 1–7) who performed lift series in which each surface angle was presented in blocks of five consecutive trials.

Fig. 6.

Coordination of horizontal and vertical forces and effects of surface angle in the previous trial for trials with vision (A, C) and without vision (B, D). Subjects had normal digital sensibility in A andB and impaired digital sensibility in Cand D. A–D, The left graph in each panel indicates mean horizontal forces at vertical forces corresponding to 10, 50, and 90% of static vertical force and maximal horizontal force as a function of vertical force.Shaded areas indicate data obtained with a given surface angle (0, 30, or −30°); solid, dashed, anddotted lines refer to the surface angle of the preceding trial, i.e., 0, 30, and −30°, respectively. (For clarity, effects by the previous trials are shown only for trials with a 0° angle inD; i.e., there was a great overlap between data obtained during the dynamic phases of trials by different surface angles in this condition.) The histograms in each panel represent mean static horizontal forces at each surface angle and the influences of the preceding trial: 0° (shaded columns), 30° (filled columns), and −30° (open columns). The pair of vertical bars at thetop of each column gives +1 SD and +1 SEM, respectively. Data are pooled from all subjects who performed series with unpredictable variation in surface angle between trials.

Fig. 7.

Influences on load phase duration by surface angle and various experimental conditions. A, Load phase duration as a function of surface angle in trials with and without vision and with and without digital nerve blocks. Pooled data are from all trials by the subjects who participated in lift series with unpredictable variation in surface angle between trials.B, Influences of surface angle in a previous trial on load phase duration. Pooled data are from all trials without vision but with normal digital sensibility by the subjects participating in lift series with unpredictable variation in surface angle. A, B, Vertical bars represent SEM, and curves give mean values.

Fig. 8.

Adjustment to a change in surface angle, from 0 to −30° and from 0 to 30° by blindfolded single subjects (Subj. 8–10), during normal digital sensibility. Horizontal force and its rate as a function of time are shown for trials carried out with −30° (dotted line), 0° (solid line), or 30° (dashed line) angle. In all cases the surface angle in the previous trial was 0°. Thus, solid lines represent reference trials with a 0° angle, which were preceded by trials with the same angle. Short vertical lines indicate points in time at which the new surface angle (−30 or 30°) was expressed in the motor output, as judged from a comparison with the 0° trials. Each curve represents average data from nine trials synchronized on start of horizontal force increase when the horizontal force rate exceeded 2 N/s.