Jessie Chen

Lower extremity kinetics for balance control in children with cerebral palsy.

The authors examined and compared the effect of support-surface perturbations of various magnitudes on lower extremity kinetics of 7 children with cerebral palsy (CP) and 8 typically developing (TD) children. Results showed that the highest velocity tolerated without stepping was slower in children with CP than in either age-matched TD or younger TD children. Multimodal torque profiles were more frequent in children with CP than in TD controls. TD groups temporally and spatially organized torque activation, whereas children with CP activated all joints simultaneously and showed altered torque contribution patterns among joints. Those results suggest that impairments in reactive postural control in children with CP result not only from developmental delay but also from pathology. Evidence for pathology included increased numbers of torque bursts required to regain stability and less efficient temporal and spatial organization of torque activation patterns.

Neural Representation of Hand Kinematics During Prehension in Posterior Parietal Cortex of the Macaque Monkey

Studies of hand manipulation neurons in posterior parietal cortex of monkeys suggest that their spike trains represent objects by the hand postures needed for grasping or by the underlying patterns of muscle activation. To analyze the role of hand kinematics and object properties in a trained prehension task, we correlated the firing rates of neurons in anterior area 5 with hand behaviors as monkeys grasped and lifted knobs of different shapes and locations in the workspace. Trials were divided into four classes depending on the approach trajectory: forward, lateral, and local approaches, and regrasps. The task factors controlled by the animal-how and when he used the hand-appeared to play the principal roles in modulating firing rates of area 5 neurons. In all, 77% of neurons studied (58/75) showed significant effects of approach style on firing rates; 80% of the population responded at higher rates and for longer durations on forward or lateral approaches that included reaching, wrist rotation, and hand preshaping prior to contact, but only 13% distinguished the direction of reach. The higher firing rates in reach trials reflected not only the arm movements needed to direct the hand to the target before contact, but persisted through the contact, grasp, and lift stages. Moreover, the approach style exerted a stronger effect on firing rates than object features, such as shape and location, which were distinguished by half of the population. Forty-three percent of the neurons signaled both the object properties and the hand actions used to acquire them. However, the spread in firing rates evoked by each knob on reach and no-reach trials was greater than distinctions between different objects grasped with the same approach style. Our data provide clear evidence for synergies between reaching and grasping that may facilitate smooth, coordinated actions of the arm and hand.

Timings and interactions of skilled musicians

We report here a preliminary study of interactive behavior between two members of a skilled string quartet performing a selected musical passage that required both performers to play several hundred notes in rapid succession at a steady tempo and in synchrony. Bowing movements were recorded using angular velocity sensors attached to their right forearms. The results show a high degree of temporal precision in both players. In addition, both players exhibited embedded rhythmic components in their timekeeping pattern, which arose from the grouping of notes in the musical score: four 16th notes to a beat. Within each group of four notes, we found a consistent timing microstructure: alternate upbows and alternate downbows had different mean durations. Both players’ bowings could be modeled as alternating renewal processes. In addition, we report evidence of interactive coupling between the players as an essential component of their joint performance. The alternating renewal model enables us to propose a note-generation process that has implications for the central generators underlying the observed behavior and their hierarchical organization. We discuss the implications of this model for the organization and execution of more complex motor sequences.

Accuracy and underlying mechanisms of shifting movements in cellists

Accuracy of shifting movements between two notes was examined in nine cellists (intermediate-professional skill levels). Three pairs of notes separated by different distances were tested under the same movement rate. Finger position on the string was measured by a circuit. Angular velocities of the left upper arm and forearm were measured by two angular velocity sensors; thus elbow angular velocity during shifts was estimated. Results showed that with increased elbow velocity and shifting distance endpoint variability stayed constant. The force of gravity assisted elbow extension during shifts toward higher pitched notes compared to flexion towards lower pitched notes, but faster movement velocity did not result in increased landing variability. Performance for note E on the A string was found to be less variable than other notes, suggesting that physical cue from the cello body geometry was used as a landmark for finger position. Cutaneous feedback from the thumb when hitting the body–neck junction enabled faster elbow extension velocity compared to shifts towards other notes. Cellists who showed higher performance accuracy also showed higher perceptual ability and performance proficiency. These results suggest that long-term over-training of fast and accurate movements enables musicians to maintain accuracy and variability across different movement distances and velocities. Higher perceptual ability and performance proficiency are correlated with increased accuracy but not lower variability, indicating although perceptual ability and performance proficiency are important for pitch accuracy, movement variability is still constrained by the capacity of the motor system, which is highly fine-tuned and different than non-musicians.

Stochastic aspects of motor behavior and their dependence on auditory feedback in experienced cellists

This study aimed to investigate movement accuracy of experienced cellists, the statistical properties of their note sequences during a reciprocal task, and the degree to which these movement characteristics depend on auditory feedback. Nine experienced cellists were asked to shift alternately between two notes using only their index finger to make contact with the string and fingerboard. Shifting sequences continued for two minutes at a rate of one note per second. The task was performed under two conditions: with auditory feedback (provided by the bow) or without auditory feedback (i.e., without the use of bow). When the bow was used, subjects had no difficulty in shifting between target notes with precision and stability. Some variability was present, but notes in these sequences were generally uncorrelated. The contact data and correlations in most bowed trials resembled those expected of a renewal process, a process in which successive values are statistically independent and identically distributed. Without the bow, subjects lost their ability to reach the same target positions accurately; contact locations tended to drift and had a random quality, indicating that without the bow subjects were uncertain of the target location in relation to the spatial location of their fingertips. Within these unbowed sequences, finger positions were highly correlated—within and between note sequences. In some trials without the bow, the statistical correlation patterns of the sequence were consistent with the expectations of a discrete Wiener process. Throughout our study, computer simulations of renewal and Wiener processes enabled us to determine the types of correlations to be expected from these theoretical models. The implications of the statistical results in terms of subject behavior are discussed.

Spike trains in posterior parietal and premotor cortex encode trained and natural grasping behaviors.

To investigate the role of somatosensory and motor information during grasping behaviors, we used digital video and burst analysis of simultaneously recorded spike trains to define burst epochs when neuronal firing rates exceeded 1 SD above the mean. We reconstructed the trajectory of hand movements during each burst from successive digital video images as three macaques grasped and manipulated objects in a trained prehension task, and when engaged in natural grasping behaviors to acquire pieces of fruit. In the task, neurons in posterior parietal areas 5 and 7b/AIP and in ventral premotor cortex responded more vigorously during object acquisition than to manipulation. Firing rates rose 250-500 ms before touch, and peaked as the hand was preshaped during reach, or at initial contact with the object. Firing rates declined as grasp was secured, and returned to baseline or were inhibited during subsequent actions. Some neurons responded to grasping actions of the right and left hands (bilateral neurons), suggesting that their firing patterns reflect grasp intentions, or the internal motor commands for execution of these behaviors. Acquisition-sensitive firing patterns were also observed when the animal grasped food morsels at various workspace locations. Firing began as the animal projected the hand towards the food, and continued as the hand tracked it. Figure ​Figure11. Figure 1 Firing peaked as the fingertips contacted the food, and ended when it was secured in the hand. High firing was elicited when food morsels were plucked from a tray, with the fingers preshaped for precision grip, or during tracking actions when the fingers were spread apart to maximize surface area. As in the task, bilateral neurons responded to prehensile actions performed unilaterally by either hand. A second, weaker burst often occurred when food was placed in the mouth. Other neurons responded vigorously to acquisition by the contralateral hand, but fired at highest rates when bilateral actions were coordinated between the left and right hands, as when food morsels were transferred between them. These intrapersonal-coordinated neurons did not just encode equivalent tactile information from either side, but preferentially signaled coincident somatosensory data shared between hemispheres during synergistic hand actions. The two classes of bilateral neurons thus provide somesthetic feedback from both limbs, and encode whether they are acting independently or in concert. Our findings support hypotheses that firing patterns in posterior parietal and premotor cortex reflect the animal’s intentions to accomplish task goals in motor coordinates. They suggest that actions preceding contact reinforce subsequent neural responses, allowing subjects to acquire and manipulate objects in a continuous, smooth sequence.