New Strategies For Investigating Higher-Level Motor Disorders
Our ability to create and computergraphically analyze digital records of the spatio-temporal patterns of limb trajectories allows quantitative analysis of motor behavior that previously had been described primarily at a qualitative level. I will present the application of these methods for the analysis of some motor disorders which derive from restricted damage to specific motor, or sensory, systems of the brain. One such disorder we have been studying is Parkinson’s disease (PD). We have been investigating the motor control deficits of PD patients from a number of perspectives: reaching accuracy to remembered targets presented in 3D space, multi-joint coordination in 3D reaches, coordination of reach and grasp movements when PD patients grasp objects of various shapes, and the nature of their learning new sensorimotor mappings. In one experiment, we restricted visual information in order to challenge the ability of PD patients to construct a spatial map from proprioceptive input and coordinate this map with spatial memory of the target. To restrict visual information, both targets and the hand were localized only by points of light presented in an otherwise completely dark room. PD patients were significantly less accurate than controls when pointing to remembered targets without any visual feedback, or when pointing to actual targets without visual feedback of their arm. However, they were as accurate as controls when pointing to remembered targets with visual feedback of their moving fingertip. Thus, PD patients were selectively impaired in those conditions which required integration of proprioceptive signals with concurrent or remembered visual information that is needed to guide movements. We next investigated possible deficits of PD patients in assembling the sensorimotor mappings required for more complex multi-joint actions by asking PD patients and age-matched controls to reach to and grasp three distinctly shaped objects (square, concave and convex) situated at different locations (center, left and right) in front of them. Subjects had full vision throughout. Patterns of joint angle changes of the fingers predicted final grasp configuration much earlier during the reach in control subjects than in PD patients. For the PD patients, major changes in hand preshaping occurred as the hand neared the object and both could be seen simultaneously. The results are consistent with a hypothesis that the PD patients relied less on early prediction and more on a visual comparison of object shape with hand shape in order to effectuate the grasp. Then, using 3D immersive reality, we found that PD patients are impaired in specific phases of learning new sensorimotor mappings. Finally, we compared movement deficits of PD patients with those of patients with complete loss of proprioception beneath the neck (deafferented patients). We asked deafferented patients to reach without visual feedback to remembered 3D targets and to imitate 3D gestures. The deafferented patients showed markedly impaired absolute 3D errors. These spatial errors appeared to be due to severely impaired interjoint coordination of the shoulder and elbow. We conclude that PD patients have deficits in the higher-order use of proprioception and the internal guidance of movement, in contrast to patients with peripheral loss of proprioception who show severe multijoint coordination deficits.