Can internal models of objects be utilized for different prehension tasks?

We examined if object information obtained during one prehension task is used to produce fingertip forces for handling the same object in a different prehension task. Our observations address the task specificity of the internal models presumed to issue commands for grasping and transporting objects. Two groups participated in a 2-day experiment in which they lifted a novel object (230 g; 1.2 g/cm3). On Day One, the high force group (HFG) lifted the object by applying 10 N of grip force prior to applying vertical lift force. This disrupted the usual coordination of grip and lift forces and represented a higher grip force than necessary. The self-selected force group (SSFG) lifted the object on Day One with no instructions regarding their grip or lift forces. They first generated grip forces of 5.8 N, which decreased to 2.6 N by the 10th lift. Four hours later, they lifted the same object in the manner of the HFG. On Day Two, both groups lifted the same object "naturally and comfortably" with the opposite hand. The SSFG began Day Two using a grip force of 2.5 N, consistent with the acquisition of an accurate object representation during Day One. The HFG began Day Two using accurately scaled lift forces, but produced grip forces that virtually replicated those of the SSFG on Day One. We concur with recent suggestions that separate, independently adapted internal models produce grip and lift commands. The object representation that scaled lift force was not available to scale grip force. Furthermore, the concept of a general-purpose object representation that is available across prehension tasks was not supported.     

Visual and tactile information about object-curvature control fingertip forces and grasp kinematics in human dexterous manipulation

Most objects that we manipulate have curved surfaces. We have analyzed how subjects during a prototypical manipulatory task use visual and tactile sensory information for adapting fingertip actions to changes in object curvature. Subjects grasped an elongated object at one end using a precision grip and lifted it while instructed to keep it level. The principal load of the grasp was tangential torque due to the location of the center of mass of the object in relation to the horizontal grip axis joining the centers of the opposing grasp surfaces. The curvature strongly influenced the grip forces required to prevent rotational slips. Likewise the curvature influenced the rotational yield of the grasp that developed under the tangential torque load due to the viscoelastic properties of the fingertip pulps. Subjects scaled the grip forces parametrically with object curvature for grasp stability. Moreover in a curvature-dependent manner, subjects twisted the grasp around the grip axis by a radial flexion of the wrist to keep the desired object orientation despite the rotational yield. To adapt these fingertip actions to object curvature, subjects could use both vision and tactile sensibility integrated with predictive control. During combined blindfolding and digital anesthesia, however, the motor output failed to predict the consequences of the prevailing curvature. Subjects used vision to identify the curvature for efficient feedforward retrieval of grip force requirements before executing the motor commands. Digital anesthesia caused little impairment of grip force control when subjects had vision available, but the adaptation of the twist became delayed. Visual cues about the form of the grasp surface obtained before contact was used to scale the grip force, whereas the scaling of the twist depended on visual cues related to object movement. Thus subjects apparently relied on different visuomotor mechanisms for adaptation of grip force and grasp kinematics. In contrast, blindfolded subjects used tactile cues about the prevailing curvature obtained after contact with the object for feedforward adaptation of both grip force and twist. We conclude that humans use both vision and tactile sensibility for feedforward parametric adaptation of grip forces and grasp kinematics to object curvature. Normal control of the twist action, however, requires digital afferent input, and different visuomotor mechanisms support the control of the grasp twist and the grip force. This differential use of vision may have a bearing to the two-stream model of human visual processing.     

Distributing vertical forces between the digits during gripping and lifting: the effects of rotating the hand versus rotating the object

During pinch grip we partition the vertical tangential forces at the digits according to the friction at the grip surfaces, and the mass distribution of the object. However, we cannot predictively partition the vertical forces to adjust to new frictional conditions after viewing a 180-deg rotation of an object with different textures at each grip surface. Hence, the processes that lead to predictive force partitioning may not access object representations, thereby suggesting that these processes are digit-specific. If this is true, then we should fail to predictively partition our fingertip forces when we rotate our hand. We tested this prediction by comparing the effects of object rotation with hand rotation for repeated lifts of an object that had one slippery grip surface and one rough grip surface. Subjects did not predictively redistribute the vertical tangential forces upon grasping the rotated object. Following object rotation, the vertical tangential force trajectories during the first 100 ms after contact indicated that 12/15 subjects failed to anticipate the reversed digit-friction relationships. All subjects appropriately partitioned the vertical tangential forces between the digits by the second lift after object rotation, confirming previous reports that sensory signals update the memory associated with lifting the object. In contrast, after hand rotation, 13/15 subjects anticipated the new digit-friction relationships and upon grasping the object immediately generated a steep rise in the vertical force trajectory at the rough surface. They also delayed the initial rise in vertical tangential force at the digit encountering the low-friction surface by approximately 65 ms. Thus, anticipatory partitioning of vertical fingertip forces is not strictly digit-specific. Internally driven motor plans can access the relevant memories or internal models for predictively partitioning the vertical tangential forces. It is not clear if this process involves rotating internal representations of fingertip force directly, or if the forces are derived after internally rotating a representation of the object. In contrast to the robust effects of vision on reach kinematics, or on wrist and finger configuration, visual signals about object rotation and orientation apparently do not influence vertical tangential fingertip forces.     

Aging reduces the ability to change grip force and balance control simultaneously

The purpose of this study was to investigate the change in the fingertip forces and balance control of young adults and older adults. The subjects lifted an object of constant weight (i.e., 1500 g) using their right hand, first in a seated position and then in a standing position. We quantified the ability of the participants to adjust their fingertip forces across trials by comparing the percentage of change in the peak grip force, peak load force and the ratio between peak grip force and peak load force. Moreover, we quantified their ability to stabilize their balance following the lifting of the object in the standing condition. The results showed that in both conditions young adults reduced their peak grip force much more than older adults across trials. In the seated condition, young adults increased slightly their peak load force, across trials, while older adults reduced it. In the standing condition, both groups showed similar change in peak load force across trials. Remarkably, older adults improved their balance stability similarly to young adults in the standing condition. This observation suggests that the ability of the older adults to modulate grip force applied to an object while standing is diminished probably to dedicate more attention to the balance control task rather than fine-tuning the grip force. Reducing balance instability following repetitive lifting is certainly more beneficial as the consequences of a fall could be more dramatic than dropping a cup of coffee.     

Development of human precision grip

The adaptation of the grip forces to the frictional condition between the digits and an object relies on feedforward sensorimotor mechanisms that use tactile afferent input to intermittently update a sensorimotor memory that controls the force coordination, i.e., the ratio between grip force (normal to the grip surface) and load force (tangential to the grip surface). The present study addressed the development of these mechanisms. Eighty-nine children and 15 adults lifted an instrumented object with exchangeable grip surfaces measuring the grip and load forces. Particularly in trials with high friction (sandpaper), the youngest children used a high grip force to load force ratio. Although this large safety margin against slips indicated an immature capacity to adapt to the frictional condition, higher grip forces were produced for more slippery material (silk versus sandpaper). The safety margin decreased during the first 5 years of age, in parallel with a lower variability in the grip force and a better adaptation to the current frictional condition. The youngest children (18 months) could adapt the grip force to load force ratio to the frictional condition in a series of lifts when the same surface structure was presented in blocks of trials, but failed when the surface structure was unpredictably changed between subsequent lifts. The need for repetitive presentation suggests a poor capacity to form a sensorimotor memory representation of the friction or an immature capacity to control the employed ratio from this representation. The memory effects, reflected by the influences of the frictional condition in the previous trial, gradually increased with age. Older children required a few lifts and adults only one lift to update their force coordination to a new friction. Hence, the present finding suggests that young children use excessive grip force, a strategy to avoid frictional slips, to compensate for an immature tactile control of the precision grip.     

Initiation and development of fingertip forces during whole-hand grasping

The present study examined the initiation of digit contact and fingertip force development during whole-hand grasping. Sixteen healthy subjects grasped an object instrumented with force transducers at each digit and lifted it 10 cm. The grip (normal) and load (tangential) forces and the position of the object were recorded. Twenty-five lifts were performed with various object weights (300 g, 600 g, 900 g) and surface textures (sandpaper and rayon). Despite the large number of degrees of freedom, grip initiation with an object using the whole hand was characterized by stereotypical contact patterns, which are idiosyncratic to each subject across all object weights and textures. However, in spite of the initial asymmetric control, the forces were mainly synchronized by the occurrence of the peak grip and load force rates. The contribution of each digit to the total grip force decreased from radial to ulnar digits. The final force distribution was generally established already at the onset of load forces. Only subtle adjustments were seen thereafter, suggesting a fairly fixed force distribution pattern throughout the grasp. The findings suggest that, despite the large number of degrees of freedom in terms of contact initiation and force distribution in whole-hand grasping: (1) subjects employ preferred movement patterns to establish object contact with their digits, and (2) synchronize the subsequent force development and temporal coordination of the task. Thus while the complexity of the task requires control mechanisms beyond those seen in two-finger precision grasping, there are strategies to simplify the complex task of the initiation and development of fingertip forces in whole-hand grasping.     

Old age affects fingertip forces when restraining an unpredictably loaded object

We investigated the effects of old age on the fingertip force responses that occurred when a grasped handle was pulled unexpectedly to increase the tangential load at the fingertip. These automatic responses, directed normal to the handle surface, help prevent slips between the handle and finger. Old adults (average age 78 years) responded with large peak fingertip forces compared to young adults (average age 30 years), even though the two subject groups showed similar skin slipperiness. For step-shaped loads the average response latency was the same for young and old subjects (about 80 ms). Thus, these automatic responses are not susceptible to the age-related central delays known for simple reaction-time tasks. For ramp-shaped loads the average response latency was inversely related to load rate. Response latency was 25 ms longer for the Old group versus the Young group for loads of 8 N/s, and this difference increased exponentially to a 110-ms difference for 2-N/s loads. A twofold difference in the tangential force required to evoke a response was predicted from linear regressions and can account for the latency difference (0.2 N vs 0.4 N threshold for young and old, respectively, r=0.93 for both groups). This theoretical elevation in load force threshold is consistent with degraded central information processing in old age, and the deterioration of cutaneous mechanoreceptors.     

Selective use of visual information signaling objects' center of mass for anticipatory control of manipulative fingertip forces

The present study examines whether visual information indicating the center of mass (CM) of an object can be used for the appropriate scaling of fingertip forces at each digit during precision grip. In separate experiments subjects lifted an object with various types of visual cues concerning the CM location several times and then rotated and lifted it again to determine whether the visual cues signaling the new location of the CM could be used to appropriately scale the fingertip forces. Specifically, subjects had either no visual cues, visual instructional cues (i.e., an indicator) or visual geometric cues where the longer axis of the object indicated the CM. When no visual cues were provided, subjects were unable to appropriately scale the load forces at each digit following rotation despite their knowledge of the new weight distribution. When visual cues regarding the CM location were provided, the nature of the visual cues determined their effectiveness in retrieval of internal representations underlying the anticipatory scaling of fingertip forces. Specifically, when subjects were provided with visual instructional information, they were unable to appropriately scale the forces. More appropriate scaling of the load forces occurred when the visual cues were ecologically meaningful, i.e., when the shape of the object indicated the CM location. We suggest that visual instructional cues do not have access to the implicit processes underlying dynamic force control, whereas visual geometric cues can be used for the retrieval of the internal representation related to CM for appropriate partitioning of the forces in each digit. Electronic Publication     

Development of human precision grip

When an object held by a precision grip is subjected to an abrupt vertical load perturbation, somatosensory input from the digits triggers an increase in grip force to restore an adequate safety margin, preventing frictional slips. In adults the response occurs after a latency of 60–80 ms. In the present study, children from 2 years old upward and adults grasped and lifted an object using a precision grip. Sudden, unpredicted increases in load force (tangential to the grip surfaces) were induced by the experimenter by dropping a small disc on to a receptacle attached to the object. The impact elicited a grip force response which in young children had a longer latency and a smaller amplitude than was seen in adults. The grip response latency gradually become shorter and its amplitude increased with increasing age, reaching adult values at 6–10 years. The muscle activity underlying the response could have several bursts. The adults showed one brisk response, appearing 40–50 ms after impact, in extrinsic and intrinsic hand muscles, while younger children also exhibited a short-latency burst, appearing about 20 ms after impact. It is suggested that the short-latency response was mediated via spinal pathways, and that these pathways are disengaged by supraspinal centers during development. In a predictable loading situation, when subjects dropped the disc themselves into the receptacle using the contralateral hand, they changed strategy. Adults induced a well-timed anticipatory grip force increase prior to the impact that was scaled to the weight of the object. The youngest children did not time the force increase properly in relation to the impact. Yet, they could scale their anticipatory grip force increase with respect to the weight of the dropped disc. This suggests a well-developed capacity to use information about the weight of objects held by one hand to parameterize a programmed force output to the other hand.     

Control of fingertip forces in multidigit manipulation

Previous studies of control of fingertip forces in skilled manipulation have focused on tasks involving two digits, typically the thumb and index finger. Here we examine control of fingertip actions in a multidigit task in which subjects lifted an object using unimanual and bimanual grasps engaging the tips of the thumb and two fingers. The grasps resembled those used when lifting a cylindrical object from above; the two fingers were some 4.25 cm apart and the thumb was approximately 5.54 cm from either finger. The three-dimensional forces and torques applied by each digit and the digit contact positions were measured along with the position and orientation of the object. The vertical forces applied tangential to the grasp surfaces to lift the object were synchronized across the digits, and the contribution by each digit to the total vertical force reflected intrinsic object properties (geometric relationship between the object's center of mass and the grasped surfaces). Subjects often applied small torques tangential to the grasped surfaces even though the object could have been lifted without such torques. The normal forces generated by each digit increased in parallel with the local tangential load (force and torque), providing an adequate safety margin against slips at each digit. In the present task, the orientations of the force vectors applied by the separate digits were not fully constrained and therefore the motor controller had to choose from a number of possible solutions. Our findings suggest that subjects attempt to minimize (or at least reduce) fingertip forces while at the same time ensure that grasp stability is preserved. Subjects also avoid horizontal tangential forces, even at a small cost in total force. Moreover, there were subtle actions exerted by the digits that included changes in the distribution of vertical forces across digits and slight object tilt. It is not clear to what extent the brain explicitly controlled these actions, but they could serve, for instance, to keep tangential torques small and to compensate for variations in digit contact positions. In conclusion, we have shown that when lifting an object with a three-digit grip, the coordination of fingertip forces, in many respects, matches what has been documented previously for two-digit grasping. At the same time, our study reveals novel aspects of force control that emerge only in multidigit manipulative tasks.     

Altered digit force direction during pinch grip following stroke

This study examined grip force development in individuals with hemiparesis following unilateral stroke. Eleven patients with chronic stroke with severe hand impairment and five age-matched neurologically intact subjects grasped an instrumented object between the index finger and thumb while fingertip forces, digit posture, and muscle electromyographic activity were recorded. We tested a range of different grip conditions with varying grip sizes, object stability, and grip force level. We found that fingertip force direction in the paretic digits deviated from the direction normal to the grip surface by more than twice as much as for asymptomatic digits. Additionally, the paretic thumb had, on average, 18% greater deviation of grip force direction than the paretic index finger. This large deviation of finger force direction for the paretic digits was consistently observed regardless of grip size, grip force level, and object stability. Due to the large deviation of the force direction from the normal direction, the paretic digits slipped and moved more than 1 cm during 55% of all grasping trials. A regression analysis suggests that this altered grip force direction was associated with altered hand muscle activation patterns, but not with the posture at which the digit made contact with the object. Therapies to redirect the force direction at the digits may improve stroke survivors’ ability to stably grip an object.     

Failure to disrupt the ‘sensorimotor’ memory for lifting objects with a precision grip

When repetitively lifting an object with mechanical properties that vary from lift-to-lift, the fingertip forces for gripping and lifting are influenced strongly by the previous lift, revealing a ‘sensorimotor’ memory. Two recent reports indicate that the sensorimotor memory for grip force is easily disrupted by an unrelated task like a strong pinch or vibration, even when the lift was performed with the hand contralateral to the vibration or preceding pinch. These findings indicate that this memory may reflect sensory input or muscle contraction levels, rather than object properties or the specific task of gripping and lifting. Here we report that the predictive scaling of lift force was not disrupted by conditioning tasks that featured exerting a vertical isometric force with the upper extremity. When subjects lifted a 2 N object repetitively the peak lift force rate was 26.4 N/s. The lift force rate increased to 36.1 N/s when the 2 N object was lifted (regardless of hand) after lifting the 8 N object with the right hand, which reveals the expected ‘sensorimotor’ memory. The lift force rate did not increase (24.8 vs. 26.4 N/s for the control condition) when a bout of isometric exertion (9.8 N) in the vertical direction with the distal right forearm preceded lifts of the 2 N object. This finding was confirmed with another isometric task designed to more closely mimic lifting an object with a precision grip. This difference in the sensitivity of grip versus lift force to a preceding isometric contraction indicates that separate sensorimotor memories contribute to the predictive scaling of the commands for gripping and lifting an object.     

Age-related directional bias of fingertip force

We studied the direction of the three-dimensional fingertip force vector in young and old adults during a simple pressing task with the index finger. Ten young and ten old subjects pressed against a force plate with their index finger and maintained target forces that ranged from 2.5 to 15 N. Subjects viewed a display of the force normal to the force plate; forces tangential to the force plate were not displayed. Young adults produced fingertip forces that were nearly perpendicular to the plate at all target forces while old adults produced fingertip forces that deviated in the proximal direction of the horizontal plane, and ulnar direction of the vertical plane. The fingertip force deviated from the onset of pressing in both groups, but in young subjects the force aligned to perpendicular within 200 ms. Additional study is required to determine if these biased force vector directions contribute to clumsiness and slowing that are characteristic of fine manipulation in old age.     

Development of human precision grip I: Basic coordination of force

Summary The coordination of manipulative forces was examined while children and adults repeatedly lifted a small object between the thumb and index finger. Grip force, load force (vertical lifting force), grip force rate and the vertical position of the test object were continuously measured. In adults, the force generation was highly automatized and was nearly invariant between trials. After a preload phase in which the grip was established, the grip and load forces increased in parallel under isometric conditions until the load force overcame the force of gravity and the object started to move. During this loading phase, the force rate profiles were essentially bell shaped and single peaked, suggesting that the force increases were programmed as one coordinated event. Children below the age of two exhibited a prolonged preload phase and a loading phase during which the grip and load forces did not increase in parallel. A major increase in grip force preceded the increase in load force, and at the start of the loading phase, the grip force was usually several Newtons (N). The force rate profiles were multi peaked with step-wise force increases most likely allowing peripheral feedback to play an important role in the control of the forces. After the age of two, the grip force increased less during the preload phase. The loading phase was more regularly characterized by a parallel increase of the grip force and load force and the duration of the various phases decreased. The older children programmed the forces in one force rate pulse indicating the emergence of an anticipatory strategy. Yet, the mature coordination of forces was not fully developed until several years later. It was concluded that the development of the precision grip was based upon the formation of a lift synergy coupling grip and load force generating circuits and that it seems to involve a transition from feedback control to feedforward control.     

Impaired anticipatory control of force sharing patterns during whole-hand grasping in Parkinson’s disease

We examined the coordination of multi-digit grasping forces as they developed during object grasping and lifting. Ten subjects with Parkinson’s disease (PD; OFF and ON medication) and ten healthy age-matched control subjects lifted a manipulandum that measured normal forces at each digit and the manipulandum’s position. The center of mass (CM) was changed from trial to trial in either a predictable (blocked) or unpredictable (random) order. All subjects modulated individual fingertip forces to counterbalance forces exerted by the thumb and minimize object tilt after lift-off. However, subjects with PD OFF exhibited an impaired ability to use anticipatory mechanisms resulting in less differentiated scaling of individual finger forces to the object CM location. Remarkably, these between-group differences in force modulation dissipated as subjects reached peak grip forces during object lift, although these occurred significantly later in subjects with PD OFF than controls and PD ON. Analysis of the tilt of the object during lift revealed all subjects had similar deviations of the object from the vertical, the direction of which depended on CM location. Thus these findings in subjects with PD indicate that: (a) PD-induced impairments in anticipatory force mechanisms appear to be greatly increased in multi-digit grasping as opposed to previous reports from two-digit grasping; (b) inaccurate scaling of fingertip force amplitude and sharing patterns before object lift is recovered during object lift; (c) the implementation of appropriate force amplitude and sharing among the digits during the lift occurs significantly later than for controls; (d) medication improves the temporal recovery of multi-digit force coordination. These results are discussed within the framework of PD-related deficits in sensorimotor integration and control of multi-degrees of freedom movement.     

Factors influencing the force control during precision grip

Summary A small object was gripped between the tips of the index finger and thumb and held stationary in space. Its weight and surface structure could be changed between consecutive lifting trials, without changing its visual appearance. The grip force and the vertical lifting force acting on the object, as well as the vertical position of the object were continuously recorded. Likewise, the minimal grip force necessary to prevent slipping, was measured. The difference between this minimal force and the employed grip force, was defined as the safety margin to prevent slipping.It was found that the applied grip force was critically balanced to optimize the motor behaviour so that slipping between the skin and the gripped object did not occur and the grip force did not reach exeedingly high values. To achieve this motor control, the nervous system relied on a mechanism that measured the frictional condition between the surface structure of the object and the fingers. Experiments with local anaesthesia indicated that this mechanism used information from receptors in the fingers, most likely skin mechanoreceptors. In addition to friction, the control of the grip force was heavily influenced by the weight of the object and by a safety margin factor related to the individual subject. The frictional conditions during the previous trial could also, to some extent, influence the grip force.     

Coordination of fingertip forces during human manipulation can emerge from independent neural networks controlling each engaged digit

We investigated the coordination of fingertip forces in subjects who lifted an object (i) using the index finger and thumb of their right hand, (ii) using their left and right index fingers, and (iii) cooperatively with another subject using the right index finger. The forces applied normal and tangential to the two parallel grip surfaces of the test object and the vertical movement of the object were recorded. The friction between the object and the digits was varied independently at each surface between blocks of trials by changing the materials covering the grip surfaces. The object’s weight and surface materials were held constant across consecutive trials. The performance was remarkably similar whether the task was shared by two subjects or carried out unimanually or bimanually by a single subject. The local friction was the main factor determining the normal:tangential force ratio employed at each digit-object interface. Irrespective of grasp configuration, the subjects adapted the force ratios to the local frictional conditions such that they maintained adequate safety margins against slips at each of the engaged digits during the various phases of the lifting task. Importantly, the observed force adjustments were not obligatory mechanical consequences of the task. In all three grasp configurations an incidental slip at one of the digits elicited a normal force increase at both engaged digits such that the normal:tangential force ratio was restored at the non-slipping digit and increased at the slipping digit. The initial development of the fingertip forces prior to object lift-off revealed that the subjects employed digit-specific anticipatory mechanisms using weight and frictional experiences in the previous trial. Because grasp stability was accomplished in a similar manner whether the task was carried out by one subject or cooperatively by two subjects, it was concluded that anticipatory adjustments of the fingertip forces can emerge from the action of anatomically independent neural networks controlling each engaged digit. In contrast, important aspects of the temporal coordination of the digits was organized by a “higher level” sensory – based control that influenced both digits. In lifts by single subjects this control was mast probably based on tactile and visual input and on communication between neural control mechanisms associated with each digit. In the two-subject grasp configuration this synchronization information was based on auditory and visual cues.     

Prediction of object contact during grasping

The maximum grip aperture (MGA) during prehension is linearly related to the size of objects to be grasped and is adapted to the haptically sensed object size when there is a discrepancy between visual and haptic information. We have investigated what information is used to drive this adaptation process and how the onset of fingertip forces on the object is triggered. Subjects performed a reach-to-grasp task, where the object seen and the object grasped physically never were the same. We measured the movements of the index finger and the thumb and the contact forces between each fingertip and the object. The subjects’ adaptation of the MGA was unrelated both to different fingertip velocities at the moment of object contact, or the fingertip forces. Instead, the ‘timing’ of contact between the fingers and the object was most consistently influenced by introducing a size discrepancy. Specifically, if the object was larger than expected, the moment of contact occurred earlier, and if the object was decreased in size, then the contact occurred later. During adaptation, these timing differences were markedly reduced. Also, the motor command for applying forces on the object seemed to be released in anticipation of the predicted moment of contact. We therefore conclude that the CNS dynamically predicts when contact between the fingertips and objects occur and that aperture adaptation is primarily driven by timing prediction errors.     

The size of the visual size cue used for programming manipulative forces during precision grip

We used a perturbation technique to quantify the contribution of visual size cues to the programming of target force when lifting an object. Our results indicate that the nervous system attaches a reasonable weight to visual size cues when programming the target grip force for a novel object. In a subsequent lift of the same object, however, the confidence attached to the visual size cue fell dramatically. It is not clear whether the decrease in the use of size information was accelerated by the presence of a cue conflict or whether the fall represents the normal shift towards the use of a memory-based representation for programming grip force. In a second experiment, we used the "size-weight illusion" to explore the relationship between the verbal report of an object's weight and the programming of the grip and load force. We found that erroneous motor programming (as indexed by a number of measures) was neither necessary nor sufficient for the size-weight illusion to occur. These findings call for a re-evaluation of a previous explanation for the size-weight illusion. We suggest that the illusion arises because the cognitive system attempts to rationalise the fact that objects of apparently equal density but different size feel as if they have the same weight.     

Evidence for the involvement of the posterior parietal cortex in coordination of fingertip forces for grasp stability in manipulation

Grasp stability during object manipulation is achieved by the grip forces applied normal to the grasped surfaces increasing and decreasing in phase with increases and decreases of destabilizing load forces applied tangential to the grasped surfaces. This force coordination requires that the CNS anticipates the grip forces that match the requirements imposed by the self-generated load forces. Here, we use functional MRI (fMRI) to study neural correlates of the grip-load force coordination in a grip-load force task in which six healthy humans attempted to lift an immovable test object held between the tips of the right index finger and thumb. The recorded brain activity was compared with the brain activity obtained in two control tasks in which the same pair of digits generated forces with similar time courses and magnitudes; i.e., a grip force task where the subjects only pinched the object and did not apply load forces, and a load force task, in which the subjects applied vertical forces to the object without generating grip forces. Thus neither the load force task nor the grip force task involved coordinated grip-load forces, but together they involved the same grip force and load force output. We found that the grip-load force task was specifically associated with activation of a section of the right intraparietal cortex, which is the first evidence for involvement of the posterior parietal cortex in the sensorimotor control of coordinated grip and load forces in manipulation. We suggest that this area might represents a node in the network of cortical and subcortical regions that implement anticipatory control of fingertip forces for grasp stability.