Jamie R. Lukos

Choice of Contact Points during Multidigit Grasping: Effect of Predictability of Object Center of Mass Location

It has been shown that when subjects can predict object properties [e.g., weight or center of mass (CM)], fingertip forces are appropriately scaled before the object is lifted, i.e., before somatosensory feedback can be processed. However, it is not known whether subjects, in addition to these anticipatory force mechanisms, exploit the ability to choose where digits can be placed to facilitate object manipulation. We addressed this question by asking subjects to reach and grasp an object whose CM was changed to the left, center, or right of the object in either a predictable or unpredictable manner. The only task requirement was to minimize object roll during lift. We hypothesized that subjects would modulate contact points but only when object CM location could be predicted. As expected, object roll was significantly smaller in the predictable condition. This experimental condition was also associated with statistically distinct spatial distributions of contact points as a function of object CM location but primarily when large torques had to be counteracted, i.e., for right and left CM locations. In contrast, when subjects could not anticipate CM location, a “default” distribution of contact points was used, this being statistically indistinguishable from that adopted for the center CM location in the predictable condition. We conclude that choice of contact points is integrated with anticipatory force control mechanisms to facilitate object manipulation. These results demonstrate that planning of digit placement is an important component of grasp control.

Anticipatory Control of Grasping: Independence of Sensorimotor Memories for Kinematics and Kinetics

We have recently provided evidence for anticipatory grasp control mechanisms in the kinematic domain by showing that subjects modulate digit placement on an object based on its center of mass (CM) when it can be anticipated (Lukos et al., 2007). This behavior relied on sensorimotor memories about digit contact points and forces required for optimal manipulation. We found that accurate sensorimotor memories depended on the acquisition of implicit knowledge about object properties associated with repeated manipulations of the same object. Whereas implicit knowledge of object properties is essential for anticipatory grasp control, the extent to which subjects can use explicit knowledge to accurately scale digit forces in an anticipatory manner is controversial. Additionally, it is not known whether subjects are able to use explicit knowledge of object properties for anticipatory control of contact points. We addressed this question by asking subjects to grasp and lift an object while providing explicit knowledge of object CM location as visual or verbal cues. Contact point modulation and object roll, a measure of anticipatory force control, were assessed using blocked and random CM presentations. We found that explicit knowledge of object CM enabled subjects to modulate contact points. In contrast, subjects could not minimize object roll in the random condition to the same extent as in the blocked when provided with a verbal or visual cue. These findings point to a dissociation in the effect of explicit knowledge of object properties on grasp kinematics versus kinetics, thus suggesting independent anticipatory processes for grasping.

The effects of task and content on digit placement on a bottle

In addition to hand shaping, previous studies have shown that subjects adapt placement of individual digits to object properties such as its weight and center of mass. However, the extent to which digit placement varies based on task context is unknown. In the present study, we investigated where subjects place their digits on a bottle when the upcoming task (lift versus pour) and object content (i.e., amount of liquid: empty, half, and full) were manipulated. Our results showed that subjects anticipated both the upcoming task and content by varying digit placement when grasping the bottle prior to the onset of manipulation. Specifically, subjects increased the vertical distance between the thumb and index finger for pouring but not for lifting. This larger moment arm might have been established to decrease the amount of force required to tilt the bottle. Content also affected digit placement: the digits were placed higher and were wrapped more around the bottle with increasing content. This strategy may maximize grip surface contact, and hence grasp stability. These findings extend previous research showing that grasp planning not only takes place at a macroscopic level (whole-hand position relative to an object), but also at the level of individual digit placement. This finer level of control appears to be sensitive to the expected mechanical properties of the object and how these may affect grasp stability throughout the upcoming manipulation.

Electrocortical activity distinguishes between uphill and level walking in humans.

The objective of this study was to determine if electrocortical activity is different between walking on an incline compared with level surface. Subjects walked on a treadmill at 0% and 15% grades for 30 min while we recorded electroencephalography (EEG). We used independent component (IC) analysis to parse EEG signals into maximally independent sources and then computed dipole estimations for each IC. We clustered cortical source ICs and analyzed event-related spectral perturbations synchronized to gait events. Theta power fluctuated across the gait cycle for both conditions, but was greater during incline walking in the anterior cingulate, sensorimotor and posterior parietal clusters. We found greater gamma power during level walking in the left sensorimotor and anterior cingulate clusters. We also found distinct alpha and beta fluctuations, depending on the phase of the gait cycle for the left and right sensorimotor cortices, indicating cortical lateralization for both walking conditions. We validated the results by isolating movement artifact. We found that the frequency activation patterns of the artifact were different than the actual EEG data, providing evidence that the differences between walking conditions were cortically driven rather than a residual artifact of the experiment. These findings suggest that the locomotor pattern adjustments necessary to walk on an incline compared with level surface may require supraspinal input, especially from the left sensorimotor cortex, anterior cingulate, and posterior parietal areas. These results are a promising step toward the use of EEG as a feed-forward control signal for ambulatory brain-computer interface technologies.

Grasping uncertainty: effects of sensorimotor memories on high-level planning of dexterous manipulation

For successful object manipulation, the central nervous system must appropriately coordinate digit placement and force distribution. It is known that digit force planning is significantly influenced by previous manipulations even when object properties cannot be predicted on a trial-to-trial basis. We sought to determine whether this effect extends beyond force control to the coordination of digit placement and force. Subjects grasped and lifted an object whose center of mass (CM) was changed unpredictably across trials. Grasp planning was quantified by measuring the torque generated on the object at lift onset. We found that both digit placement and force were systematically affected by the CM experienced on the previous trial. Additionally, the negative covariation between digit forces and positions typically found for predictable CM presentations was also found for unpredictable CM trials. A follow-up experiment revealed that these effects were not dependent on visual feedback of object roll during object lift on the previous trial. We conclude that somatosensory feedback from previous grasp experience alone can affect high-level grasp planning by constraining the relation between digit force and position even when the task behavioral consequences cannot be reliably predicted. As learning of manipulations often involves interactions with objects in novel environments, the present findings are an important step to understanding the control strategies associated with the integration of sensorimotor memories and motor planning.

Parkinson’s disease patients show impaired corrective grasp control and eye–hand coupling when reaching to grasp virtual objects

The effect of Parkinsons disease (PD) on hand-eye coordination and corrective response control during reach-to-grasp tasks remains unclear. Moderately impaired PD patients (n=9) and age-matched controls (n=12) reached to and grasped a virtual rectangular object, with haptic feedback provided to the thumb and index fingertip by two 3-degree of freedom manipulanda. The object rotated unexpectedly on a minority of trials, requiring subjects to adjust their grasp aperture. On half the trials, visual feedback of finger positions disappeared during the initial phase of the reach, when feedforward mechanisms are known to guide movement. PD patients were tested without (OFF) and with (ON) medication to investigate the effects of dopamine depletion and repletion on eye-hand coordination online corrective response control. We quantified eye-hand coordination by monitoring hand kinematics and eye position during the reach. We hypothesized that if the basal ganglia are important for eye-hand coordination and online corrections to object perturbations, then PD patients tested OFF medication would show reduced eye-hand spans and impoverished arm-hand coordination responses to the perturbation, which would be further exasperated when visual feedback of the hand was removed. Strikingly, PD patients tracked their hands with their gaze, and their movements became destabilized when having to make online corrective responses to object perturbations exhibiting pauses and changes in movement direction. These impairments largely remained even when tested in the ON state, despite significant improvement on the Unified Parkinsons Disease Rating Scale. Our findings suggest that basal ganglia-cortical loops are essential for mediating eye-hand coordination and adaptive online responses for reach-to-grasp movements, and that restoration of tonic levels of dopamine may not be adequate to remediate this coordinative nature of basal ganglia-modulated function.

Anticipatory modulation of digit placement for grasp control is affected by Parkinson's disease.

Background Successful object manipulation relies on the ability to form and retrieve sensorimotor memories of digit forces and positions used in previous object lifts. Past studies of patients affected by Parkinsons disease (PD) have revealed that the basal ganglia play a crucial role in the acquisition and/or retrieval of sensorimotor memories for grasp control. Whereas it is known that PD impairs anticipatory control of digit forces during grasp, learning deficits associated with the planning of digit placement have yet to be explored. This question is motivated by recent work in healthy subjects revealing that anticipatory control of digit placement plays a crucial role for successful manipulation. Methodology/Principal Findings We asked ten PD patients off medication and ten age-matched controls to reach, grasp and lift an object whose center of mass (CM) was on the left, right or center. The only task requirement was to minimize object roll during lift. The CM remained the same across consecutive trials (blocked condition) or was altered from trial to trial (random condition). We hypothesized that impairment of the basal ganglia-thalamo-cortical circuits in PD patients would reduce their ability to anticipate digit placement appropriate to the CM location. Consequently, we predicted that PD patients would exhibit similar digit placement in the blocked vs. random conditions and produce larger peak object rolls than that of control subjects. In the blocked condition, PD patients exhibited significantly weaker modulation of fingertip contact points to CM location and larger object roll than controls (p<0.05 and p<0.01, respectively). Nevertheless, both controls and PD patients minimized object roll more in the blocked than in the random condition (p<0.01). Conclusions/Significance Our findings indicate that, even though PD patients may have a residual ability of anticipatory control of digit contact points and forces, they fail to implement a motor plan with the same degree of effectiveness as controls. We conclude that intact basal ganglia-thalamo-cortical circuits are necessary for successful sensorimotor learning of both grasp kinematics and kinetics required for dexterous hand-object interactions.

Hand Function in Parkinson’s Disease

Parkinson’s disease (PD) patients have a number of functional hand impairments. The latency and rate of isometric force generation is impaired in PD. Motor dysfunction is also related to impaired integration of sensory feedback and motor output. Moreover, PD patients exhibit sensory deficits such as decreased spatial and temporal tactile discrimination thresholds of the fingertips. Impairments of reaching and grasping are seen as patients tend to exhibit difficulty in movement initiation to a target. There are deficits in hand preshaping to object geometry. There is a lack of coordination between the timing of the reach and grasp components. Patients have an overall dependence on visual cues to control movement. They exhibit impairments in the planning of where to place their digits, resulting in suboptimal performance of object manipulation. It is hypothesized that predictive force control deficits are a result of central impairments associated with the generation and/or retrieval of sensorimotor memories for movement planning.