Independent control of human finger‐tip forces at individual digits during precision lifting.

Abstract

SUMMARY1. Subjects lifted an object with two parallel vertical grip surfaces and a low centre of gravity using the precision grip between the tips of the thumb and index finger. The friction between the object and the digits was varied independently at each digit by changing the contact surfaces between lifts.2. With equal frictional conditions at the two grip surfaces, the finger-tip forces were about equal at the two digits, i.e. similar vertical lifting forces and grip forces were used. With different frictions, the digit touching the most slippery surface exerted less vertical lifting force than the digit in contact with the rougher surface. Thus, the safety margins against slips were similar at the two digits whether they made contact with surfaces of similar or different friction.3. During digital nerve block, large and variable safety margins were employed, i.e. the finger-tip forces did not reflect the surface conditions. Slips occurred more frequently than under normal conditions (14 % of all trials with nerve block, < 5 % during normal conditions), and they only occasionally elicited compensatory adjustments of the finger-tip forces and then at prolonged latencies.4. The partitioning of the vertical lifting force between the digits was thus dependent on digital afferent inputs and resulted from active automatic regulation and not just from the mechanics of the task.5. The safety margin employed at a particular digit was mainly determined by the frictional conditions encountered by the digit, and to a lesser degree by the surface condition at the same digit in the previous lift (anticipatory control), but was barely influenced by the surface condition at the other digit.6. It was concluded that the finger-tip forces were independently controlled for each digit according to a 'non-slip strategy'. The findings suggest that the force distribution among the digits represents a digit-specific lower-level neural control establishing a stable grasp. This control relies on digit-specific afferent inputs and somatosensory memory information. It is apparently subordinated to a higher-level control that is related to the total vertical lifting and normal forces required by the lifting task and the relevant physical properties of the manipulated object.

Visual and Somatosensory Information about Object Shape Control Manipulative Fingertip Forces

“…If there was a mismatch between the anticipated force requirements and those actually imposed by the prevailing surface angle, an adjustment of the force output was initiated some 0.1 sec after the object was contacted. This adaptation to a new object shape mediated by somatosensory information is reminiscent of the adaptation of force coordination to frictional changes when force output changes after ϳ0.1-0.2 sec after the contact with the object (Johansson and Westling, 1984a;Johansson and Westling, 1987;Edin et al, 1992). This similarity in adjustments of force coordination to changes in object shape and friction suggests that a similar discrete event, sensory-driven control policy is used in both instances.…”

“…However, in contrast to object shape, available data suggest that vision is of little importance for anticipatory adjustments of the force output to frictional conditions (Edin et al, 1992). Rather, signals in tactile afferents innervating the object-digit interface seem to be used exclusively in frictional adaptation in combination with anticipatory parameter control based on force coordination requirements in previous lifts.…”

“…Anticipatory parameter control is used to specify motor commands parametrically in advance of the movement based on previous experience with the object or estimated from perceptual cues. For example, in lifting, the development of forces immediately after the object is grasped reflects both the weight of the object and the frictional conditions in the preceding lift (Johansson andWestling, 1984a, 1988a;Edin et al, 1992;Forssberg et al, 1995). The discrete event, sensory-driven control policy uses so- Right panels, horizontal force plotted against vertical force for the same data.…”

“…The fingertip forces reflected predictions based on sensorimotor memory related to the shape of an object in a previous lifting trial during initial contact with the object in the blindfolded condition. This also applies to lift series in which the friction between the digits and the test object is varied between trials (Johansson and Westling, 1984a;Edin et al, 1992). If there was a mismatch between the anticipated force requirements and those actually imposed by the prevailing surface angle, an adjustment of the force output was initiated some 0.1 sec after the object was contacted.…”

“…Accordingly, we automatically adjust the ratio between the normal and tangential forces to the frictional status at the digit-object interface such that an adequate safety margin against frictional slips is maintained during different frictional conditions (Johansson and Westling, 1984a;Westling and Johansson, 1984;Edin et al, 1992;Cole and Johansson, 1993;Forssberg et al, 1995;Cadoret and Smith, 1996). A sensorimotor memory related to the frictional experiences in previous interactions with the object determines the force balance used (Johansson and Westling, 1984a;Edin et al, 1992) according to an "anticipatory parameter control" policy (for an overview, see Johansson, 1996). When necessary, however, this memory is updated to frictional changes based on tactile afferent information (Johansson andWestling, 1984a, 1987).…”