Motor memory and the preselection effect in Huntington's and Parkinson's disease

Is Short-Term Memory for Discrete Arm Movements Impaired in Huntington's Disease?

Explicit memory for arm movements was investigated in patients with Huntington's disease (HD) and normal controls (NC). Participants recalled the end-position of arm movements determined by the examiner following an immediate or a 30-second delay that was either unfilled or filled with a motor distractor task. Movement error was measured in absolute (AE), variable (VE), and constant (CE) error. When performance was measured in AE, HD patients were impaired equally across delays compared to controls, regardless of whether the delay was filled or unfilled. HD patients did not demonstrate a directional bias (i.e., CE), but they were more inconsistent in their reproduction of arm movements (i.e., VE). Both groups were more variable in their reproduction of the movements following the delay, and were most variable when the delay was filled with motor distraction. Results preliminarily suggest that the caudate is specifically involved in the encoding, or temporary storage, of explicit motor information and may be less involved in the short-term retention of newly learned motor information. Results should be viewed somewhat cautiously, however, as the relative contributions of proprioceptive sense and primary motor functioning on task performance were not fully explored.

The contribution of the study of neurodegenerative disorders to the understanding of human memory

Memory impairment is one of the most common complaints affecting patients with neurodegenerative disorders, and its investigation has provided insights into the function and properties of human memory. The study of Alzheimer's disease has indicated the importance of mesial temporal structures and the hippocampus in episodic memory. In progressive supranuclear palsy, frontotemporal dementias, Parkinson's disease and Huntington's disease fronto-striatal networks are involved in working memory and higher level cognition. The study of semantic dementia, where there is lobar atrophy of the temporal lobe, has shown that the temporal neocortex has an important function in semantic memory. The investigation of human memory in neurodegenerative disorders suggests that the interaction of networks subserving episodic memory, semantic memory, and working memory contributes to higher level cognition and results in the fundamental homeostatic processes of recall and learning.

Deficits in sensorimotor control during precise hand movements in Huntington's disease

OBJECTIVES

To investigate the performance of patients with Huntington's disease (HD) while manipulating objects using a precision grip.

METHODS

The grip forces developed by the fingers were studied while subjects lifted an object of unpredictable weight in the hand. The ability to stabilize grip force after externally imposed weight change was also studied.

RESULTS

Patients used higher grip forces than the normal subjects in both the lifting and holding phases, particularly with a lighter weight. Lift timing was slowed in the patients, most markedly with a lighter weight. Increased levels of inter-trial variation were observed only with a light weight. This indicates that the slowing in HD differs from that in Parkinson's disease, which remains constant regardless of object load, and that the slowing in HD is not due to involuntary antagonist muscle activity resulting from an underlying chorea. The grip force response to sudden weight change was normal, but appeared after a delay which increased at lower rates of weight change.

CONCLUSIONS

Disturbances in precision grip timing and magnitude in HD may result from a reduced ability to process relevant tactile afferent input. The delay in the adaptive response suggests an increased threshold for detection of weight change in HD. Alternatively, this delay may arise from mediation of the response over an additional cerebellar pathway to compensate for damage to the basal ganglia.

Planning and executing an action in Parkinson's disease

We evaluated the possible impairment in planning and executing an action in patients with Parkinson's disease (PD). The action considered in the present study was formed by two successive motor acts: reaching-grasping an object (first target) and placing it on a second target of the same shape and size. We examined the effects of extrinsic properties of the second target (that is, distance) on the various kinematic phases of reaching-grasping movements. Distance, position, and size of both stimuli were randomly varied across the experimental session. Movements were executed with and without visual control of both targets and arm. The performance of six patients with PD was compared with an age-matched control group. The kinematics of the initial phase of reaching was influenced by position and size of the first target and by distance of the second target in both patients and control subjects. In particular, peak acceleration was higher for farther position of the second target. However, in the subsequent phase patients, differently from control subjects, removed the effects of the second target distance by modifying their reaching kinematics. This was obtained by varying the duration of the acceleration phase. In summary, the patients reprogrammed the reaching component by taking into account only the properties of the first target. The decreasing influence of second-target distance on reaching kinematics of patients was more evident during movements executed under visual control. Moreover, their movements executed without visual control were slowed down from the beginning. The second target affected the grasping kinematics only of the control subjects. Globally, these results indicate that PD patients are able to compute the general program of an action that takes into account extrinsic properties of the final target. However, the finding that PD patients reprogrammed the movement during its execution suggests a decay of the program during its time course, that is, basal ganglia can be involved in storing the plan of an action and in controlling its correct execution.