Axial versus distal motor impairment in Parkinson's disease.

We measured axial (head rotation) and distal (wrist flexion and extension) movements in parkinsonian patients with varying stages of disability, before and after administration of L-dopa. Velocities for both movements were proportionately reduced in all stages of disease. L-Dopa administration provided a small but consistent increase in distal movement velocity for most patients, whereas in patients with advanced disease (stages III to V), axial movement velocity actually declined after administration of L-dopa. This decline may be partially responsible for the failure of L-dopa administration to reverse the loss of "righting reflexes" in these patients. Measurement of axial motor control may be a useful tool in evaluating therapeutics for Parkinson's disease patients.

Unloading reflex of a hand muscle.

The unloading reflex of abductor hallucis brevis was elicited by removal of the external load during voluntary contraction. The duration of the unloading pulse was varied experimentally from 1 to 32 msec. With all stimulus durations, the response was a measurable decrease in the level of EMG activity. Expressed as a percentage change from baseline, the mean size of the unloading reflex was a logarithmic function of the unloading pulse duration. The results demonstrate that the hand muscles are sensitive to very brief mechanical stimuli. Although the motoneurons of the intrinsic hand muscles receive strong projections from the cerebral cortex, they exhibit reflex behavior similar to that of the large, postural muscles.

Stretch and unloading reflexes in a human hand muscle.

Stretch and unloading reflexes were demonstrated in the first dorsal interosseous muscle by averaging the electromyographic responses to brief mechanical stimuli. Subjects were not able to abolish these reflexes by altering their preparatory set. When subjects were prepared to contract or relax the muscle in response to a stretching force, the size of the stretch reflex was not affected significantly. When they were prepared to contract or relax in response to unloading, the "silent period" was diminished. In general, these reflexes were not modulated appropriately to improve the performance of an intended voluntary movement. Our results do not support the notion that spinal reflexes of the intrinsic hand muscles are "dominated" by corticospinal inputs.

Recovery of somatosensory evoked potential amplitude after movement.

Sensory transmission is known to be impaired during movement of the stimulated body part. The goal of this study was to determine whether sensory gating persists for a measurable time after completion of a voluntary movement. Tactile stimuli were applied to the right index fingertip at intervals ranging from 63 to 1,000 msec after the completion of rapid thumb movement. Somatosensory evoked potentials (SEPs), recorded over the left cerebral hemisphere, were found to be reduced in size for at least 500 msec after the cessation of movement. The prolonged attenuation of SEPs after movement appears to depend on a central process that persists for a measurable time after the movement has ended.

Central error-correcting behavior in schizophrenia and depression.

A previous study suggested that schizophrenic subjects exhibit an impaired ability to correct their own errors of movement without using exteroceptive signals. However, the performance of schizophrenic subjects was compared to that of only one other psychiatric group (alcoholic subjects), and a relatively small number of subjects was studied. To investigate the specificity of the postulated impairment, 9 schizophrenic, 11 depressed, and 8 normal subjects performed a tracking task designed to prevent the use of exteroceptive cues in correcting errors of movement. The depressed and normal groups did not differ significantly on any performance measure, but the schizophrenic subjects again demonstrated a gross impairment in correcting errors, yet no impairment in initiating correct responses. These findings suggest that the impaired ability to monitor ongoing motor behavior on the basis of internal, self-generated cues may be specific to schizophrenia among major psychiatric disorders.

Velocity-dependent suppression of somatosensory evoked potentials during movement.

Sensory transmission is known to be impaired during movement of the stimulated body part. This phenomenon is manifested by a decrease of perceptual acuity and a correlated decrease in the size of somatosensory evoked potentials (SEPs). The goal of the present study was to define the relationship between SEP amplitude and speed of movement. SEPs were elicited by brief (25 msec) loading of the wrist flexor muscles. Stimuli were applied while the wrist joint was stationary or moving voluntarily at one of several velocities. In all subjects, SEP amplitude was approximately inversely related to speed of movement at the time of stimulation. The findings refine and extend studies suggesting the velocity dependence of sensory suppression during movement.

Gating of somatosensory perception following movement.

Tactile stimuli were applied to the right index finger at selected times following rapid movements of the thumb. The probability of detecting a stimulus was found to be impaired for at least 250 ms after completion of a voluntary movement. This finding complements previous work showing that somatosensory evoked potentials are reduced for a comparable period following movement.

Decrement of somatosensory evoked potentials during repetitive stimulation.

In normal subjects, cerebral potentials were evoked by brief, passive extension of the wrist joint at various interstimulus intervals (ISIs). The resulting somatosensory evoked potentials (SEPs) were found to decrease during repetitive stimulation. The greatest decrement occurred between the first and second responses of each series. After cessation of stimuli, the SEP amplitude returned to control values over a prolonged, exponential time course. The authors postulate that the observed response decrement may be a form of habituation, which provides a model for studying the neuronal substrates of behavior.

Cerebral evoked potentials and somatosensory perception.

In normal subjects, somatosensory evoked potentials (SEPs) were produced by increases or decreases of the load on the biceps muscle during voluntary contraction. The stimuli lasted only 20 msec and caused less than 2 degrees of elbow flexion or extension. When the stimulus was applied during voluntary movement of the elbow, the SEP was attenuated, and the subject was less able to discriminate between loading and unloading pulses. In each of eight subjects, there was a positive correlation between the percentage of correct responses and the size of the SEP. The measurement of both SEPs and perceptual accuracy under various test conditions provides a refined technique for studying the relations between electrical events and sensory processes.

Abnormal unloading reflex in a patient with infarction of the medial lemniscus.

The unloading reflex was measured in the intrinsic muscles of the hand. By removing various fractions of the load on the muscles, we determined the size of the electromyographic response as a function of the change in the external force. This technique was applied to both hands of a patient with a pontine lesion who had impaired position sense in the left hand. The response to fractional unloading was significantly smaller in the left hand, an asymmetry not found in control subjects. The results suggest (1) that the unloading reflex is mediated, at least in part, by suprasegmental pathways, and (2) that the fractional unloading technique can reveal abnormalities not found by conventional methods.

Shortening reaction in patients with cerebellar ataxia.

During passive extension of the elbow, the triceps muscle normally shows a burst of electromyographic activity. This shortening reaction (SR) is known to be exaggerated in extrapyramidal disease states, but the effects of cerebellar disease are unknown. The SR was measured in both arms of a patient with hemiataxia, the unaffected arm serving as a matched control. In the ataxic arm, the SR was significantly larger and did not show the normal increase during "reinforcement." In a patient with bilateral ataxia, the SR was grossly exaggerated as compared with the SRs in a group of normal subjects. The present findings indicate an unexpected point of similarity between cerebellar and extrapyramidal disease states, which is discussed in terms of the anatomical connections of the cerebellum and basal ganglia. The techniques used in this work provide a quantitative approach to disorders of movement and muscle tone.

Shortening reaction in normal and parkinsonian subjects.

During passive extension of the elbow, the triceps shows a shortening reaction (SR), which is manifested by a burst of EMG activity. The SR and the velocity of passive movement (VPM) were measured in parkinsonian patients and control subjects. The mean SR was larger and the mean VPM was smaller in the parkinsonian group. An unexpected finding was that "reinforcement" caused an increase of the SR in normal subjects but not in the patients. The VPM is considered to be a measure of rigidity, and the SR provides a quantitative approach to the pathophysiology of muscle tone.

Impaired central error-correcting behavior in schizophrenia.

Previous work has suggested that normal subjects are able to recognize and correct their own errors of movement without using exteroceptive signals. This ability may be impaired in schizophrenia. Twelve normal subjects, 12 alcoholics, and 14 schizophrenics performed a step-function tracking task designed to prevent the use of exteroceptive signals in correcting errors of movement. The mean probability of correcting an error without external cues was approximately .38 in schizophrenics, .70 in normal subjects, and .75 in hospitalized alcoholic patients. There was no difference between groups in the ability to initiate correct responses. The results suggest that schizophrenics are deficient in the ability to monitor ongoing motor behavior on the basis of internal, self-generated cues.

Myoelectric silence following unopposed passive stretch in normal man.

The response to unopposed passive muscle stretch applied during sustained contraction was studied in normal man. When the subject did not resist the stretching force, the initial response was a brief cessation of EMG activity in the elongated muscle. The myoelectric silence was observed repeatedly in muscles of the upper and lower limbs. The response to passive stretch is discussed in relation to the lengthening reaction and the inverse myotatic reflex. The silent period observed under these experimental conditions is unlikely to be caused by Renshaw inhibition, a pause in spindle afferent discharge, or activity of the group II afferent reflex pathway. Possible mechanisms include autogenetic inhibition and a stretch-evoked decrease of fusimotor activity.

Barognosis in a patient with hemiataxia.

Barognosis was tested in a patient who had a lesion of the right cerebellar hemisphere. While blindfolded, he used each hand to estimate the mass of given test loads, alternating hands between trials. He moved each load sinusoidally, comparing it with the previous one. The period of oscillation was found to be a linear function of the mass. With each load, the period of oscillation was longer when he used the ataxic hand. On 23 of 59 comparisons between loads, the patient reported incorrectly. Ninety-one percent of the errors were in the direction of loads feeling heavier when the ataxic hand was used. To explain these results, it is proposed that (1) the perceived magnitude of a load depends on the corollary discharge, and (2) cerebellar damage may reduce the accuracy of the corollary discharge.

Unloading and shortening reactions in Parkinson's disease.

The unloading reflex was tested on both sides of a patient with asymmetrical Parkinsonism. The motor activity after the silent period was significantly greater on the more affected side. The findings support the hypothesis that the response to unloading and the shortening reaction share a common mechanism which is exaggerated in Parkinsonism.

Understanding and diagnosing senile dementia.

Dementia is one of the most common syndromes afflicting the aged, and it should be ranked as a major cause of death. Senile dementia and Alzheimer's disease should now be viewed as one and the same. Familiarity with the cardinal signs of dementia enables the physician to differentiate it from various "functional" disorders. Before making a diagnosis of senile dementia, it is important to exclude all forms of treatable disease, including toxic, metabolic, infectious, neoplastic, and traumatic disorders. Most older patients with dementia will be found to have Alzheimer's disease, for which no definitive treatment is available.