Showing papers by "Mark Hallett published in 1990"

Motor learning in patients with cerebellar dysfunction.

Abstract: This study examined whether cerebellar dysfunction resulted in deficiencies of motor learning. Patients with cerebellar atrophy only or cerebellar atrophy combined with atrophy of the brainstem and age-matched normal subjects performed two tasks to assess improvements in skilled performance. The first task was repetitive tracing with the hand of an irregular geometric pattern with normal visual guidance, and the second task was repetitive tracing of a different geometric pattern with mirror-reversed vision. Patients with pathology limited to the cerebellum showed impairments in the skilled performance of the movement performed with normal vision that may have been related to a failure to alter movement strategy. Patients with added pathology in the brainstem exhibited impairments in adapting to mirror-reversed vision. Subsidiary experiments indicated that improvements of movements guided by mirror-reversed vision were mediated by vision. These results indicate that the cerebellum and its associated input pathways are involved in motor skill learning.

H-reflex recovery curve and reciprocal inhibition of H-reflex in different kinds of dystonia.

Abstract: We studied the H-reflex recovery curve and reciprocal inhibition of the H-reflex bilaterally in the upper limb of 5 patients with generalized dystonia, 5 patients with blepharospasm, 10 patients with spasmodic torticollis, and 14 patients with writer's cramp. We compared the results with those obtained from a group of healthy volunteers. The recovery curve of the H-reflex was normal in patients with writer's cramp or blepharospasm, but showed an increase of the physiologic recovery at a 200 msec delay in patients with spasmodic torticollis or generalized dystonia. Reciprocal inhibition of the H-reflex showed a decrease in the amount of inhibition in all the patient groups and a facilitation of the H-reflex during the 3rd period of inhibition in the patients with spasmodic torticollis or generalized dystonia.

A theoretical calculation of the electric field induced by magnetic stimulation of a peripheral nerve.

Abstract: A mathematical model is presented that predicts the electric field induced in the arm during magnetic stimulation of a peripheral nerve. The arm is represented as a homogeneous, cylindrical volume conductor. The electric field arises from two sources: the time-varying magnetic field and the accumulation of charge on the arm surface. In magnetic stimulation both of these contributions are significant. The magnitude of the electric field is greatest near the surface of the arm, and is well localized. Various coil orientations are examined; the smallest electric fields are induced when the coil is perpendicular to the arm surface, the largest when the coil is parallel. These results are consistent with many experimental observations in the literature, and aid in the basic understanding of magnetic stimulation of the peripheral nervous system.

An improved method for localizing electric brain dipoles

Abstract: An improved method for noninvasive localization of equivalent dipoles in the brain is presented and evaluated. The method has been tested and evaluated on humans in vivo. The head is represented by a three-layer spherical model. The potential on any point on the scalp due to any source is found by a closed formula, which is not based on matrix rotations. The formulas will accept any surface electrode as the reference electrode. The least-squares procedure is based on optimal dipoles, reducing the number of unknowns in the iterations from six to three. The method was evaluated by localizing five implanted dipolar sources in the human sensorimotor cortex. The distances between the locations of the sources as calculated by the method and the actual locations were between 0.4 and 2.0 cm. The sensitivity of the method to uncertainties encountered whenever a real head has to be modeled by a three-layer model has also been assessed. >

Clinical and electromyographic studies of postpoliomyelitis muscular atrophy

Abstract: Eleven patients with progressive weakness following polio (postpoliomyelitis muscular atrophy syndrome) were compared electromyographically with nine patients who had stable strength following polio. Abnormalities included (1) abnormal motor unit characteristics in many muscles indicating widespread loss of motor neurons and reinnervation in muscles, including many not clinically affected by the polio; (2) prevalent spontaneous denervation potentials; and (3) abnormal single-fiber electromyographic jitter. These electromyographic abnormalities were similar in progressive and stable postpoliomyelitis patients even when muscles were separated by strength, stability, age and duration of the postpoliomyelitis state. Postpoliomyelitis muscular atrophy appears to be the clinically apparent end of the spectrum of abnormalities existing in all postpoliomyelitis patients.

Limb positioning and magnitude of essential tremor and other pathological tremors.

Abstract: We examined the influence of maintaining different postural configurations of the upper extremity on the magnitude of tremor in patients with essential tremor and with postural tremor from a variety of other neurological disorders. Patients maintained postures requiring different angles of forward flexion in the sagittal plane, of horizontal flexion, and of elbow extension. The tremor of patients diagnosed with essential tremor was either unaffected or affected only little by changes in limb position. In contrast, patients with pathological tremors, of the cerebellar postural, parkinsonian, and other types, exhibited positional dependence of their tremor. When there was positional dependence of tremor, it was always largest when the hands were near the nose or chin. These observations suggest a practical method for assistance with the clinical discrimination of essential tremor from other postural tremors.