Motor control abnormalities in Parkinson's disease

Abstract

The primary manifestations of Parkinson's disease are abnormalities of movement, including movement slowness, difficulties with gait and balance, and tremor. We know a considerable amount about the abnormalities of neuronal and muscle activity that correlate with these symptoms. Motor symptoms can also be described in terms of motor control, a level of description that explains how movement variables, such as a limb's position and speed, are controlled and coordinated. Understanding motor symptoms as motor control abnormalities means to identify how the disease disrupts normal control processes. In the case of Parkinson's disease, movement slowness, for example, would be explained by a disruption of the control processes that determine normal movement speed. Two long-term benefits of understanding the motor control basis of motor symptoms include the future design of neural prostheses to replace the function of damaged basal ganglia circuits, and the rational design of rehabilitation strategies. This type of understanding, however, remains limited, partly because of limitations in our knowledge of normal motor control. In this article, we review the concept of motor control and describe a few motor symptoms that illustrate the challenges in understanding such symptoms as motor control abnormalities.

Figure 1.

Long-latency reflexes are larger than normal in Parkinson’s disease. The subject’s hand is linked to a mechanical splint that suddenly displaces the hand so that the wrist is extended. This displacement, shown as a trace of wrist angle versus time (A), causes a sudden stretch of muscles that flexes the wrist. This stretch elicits reflex muscle activity, shown in electromyographic recordings (EMG) from the flexor carpi radialis muscle for a healthy individual (B) and a patient with PD (C). Although the amplitudes of short-latency (<50 msec) responses are similar for both subjects, the long-latency response (>50 msec) is exaggerated for the PD patient. (Adapted from Cody et al. 1986; reprinted, with permission, from Oxford University Press © 1986.)

Figure 2.

Bradykinesia and hypokinesia manifested in finger tapping. Individuals were asked to tap the index finger against the thumb “as big and as fast as possible.” The traces show distance between the tips of the thumb and forefinger, recorded by a motion capture camera, for a healthy individual (A) and a patient with PD of similar age (B). Finger taps for the PD patient were of smaller amplitude (hypokinesia) and lower frequency (bradykinesia) compared with those for the healthy individual. (Data courtesy of Drs. R. McGovern and F. DiBiasio.)

Figure 3.

Parkinson’s disease increases sensitivity to movement effort. Subjects made horizontal reaching movements to targets in a computer setup that provided feedback about movement speed. Movements that satisfied a given speed requirement were “valid” (gray circles), whereas all other movements were “nonvalid” (black circles). Subjects made movements until 20 valid trials accumulated. The total number of trials needed to achieve this criterion, trials to criterion (N_C), was used as a measure of how much a subject was struggling in making movements at the required speed. For the same required speed, age-matched control subjects (A) tended to make fewer nonvalid movements than PD patients (B), and thus required fewer total trials to reach criterion. As the speed requirement increased, subjects from both groups needed more trials to reach criterion. Therefore, trials to criterion (N_C) depended on movement effort (quantified as average acceleration, A_avg) (C). As indicated by the difference in slopes, PD patients showed higher sensitivity to movement effort than control subjects did.

Figure 4.

Healthy individuals prefer certain movement speeds. Subjects performed horizontal reaching movements to a target in a virtual reality environment equipped to record arm motion. Subjects completed alternating blocks of trials in which they reached with either their preferred “comfortable” speed (C, white circles) or with speeds imposed by a computer (I, black squares). Each subject’s naïve speed preference (C naïve) was assessed before experiencing imposed speeds, and speed preference was also retested (C retest) following a day of rest. The graph shows the mean average speed for a single subject. During comfortable speed blocks, the subject was reluctant to move at speeds that were either slower (see example, arrow a) or faster (see example, arrow b) than the naïve preference. The tendency to return to a certain preferred speed is shown by the arrows and occurs despite the fact that movements performed with all speeds resulted in similar accuracies. On the second day of testing, speed preference was similar to the naïve preference (the gray dotted line indicates the average of C naïve and C retest).