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Biomedical subjects

J G Colebatch

Publications and source records attributed to J G Colebatch.

At least 55 records · Page 3Linked to original sources
PubMed1995

Mapping of cortical sites where transcranial magnetic stimulation results in delay of voluntary movement.

Transcranial magnetic stimulation over the motor cortex has been shown to delay voluntary movement in man in a reaction time paradigm. In this study, we used a focal magnetic coil to stimulate 16 sites over the left hemisphere in 8 normal subjects. We measured the delay that occurred in the onset of a ballistic wrist flexion response and the size of the motor potentials evoked in the wrist flexors by stimulation at each site. The optimal site of stimulation to delay voluntary movement coincided with the optimal site to elicit an MEP in the agonist (flexor) muscle. However, MEPs were also obtained from some sites at which no delay in movement was induced.

Adult↗
PubMed1995

Tapping the head activates the vestibular system: a new use for the clinical reflex hammer.

We investigated the use of skull taps with a modified clinical reflex hammer as a method of vestibular activation. Using recently described EMG techniques to measure vestibulocollic reflexes in response to clicks, we were able to show analogous short-latency potentials to taps. The earliest responses were invariably absent on the side of a previous vestibular nerve section but were preserved in profound sensorineural or conductive hearing loss. We propose that the taps activated the vestibular apparatus directly by a bone-conducted vibration wave.

Acoustic Stimulation↗
PubMed1995

Vestibular evoked myogenic potentials in the sternomastoid muscle are not of lateral canal origin.

We studied vestibular evoked myogenic potentials in 6 patients after unilateral vestibular neurectomy and in 22 patients after unilateral vestibular neuritis and unilateral absent caloric responses. We found that the ipsilesional vestibular evoked potentials were abolished in every patient after unilateral vestibular neurectomy. In vestibular neuritis patients we found that the ipsilesional vestibular evoked potentials were absent in some but not in all cases. These findings confirm that the p13-n23 potential is of vestibular origin and also show that it is not of lateral canal origin.

Acoustic Stimulation↗
PubMed1994

Click-evoked vestibular activation in the Tullio phenomenon.

Click-evoked vestibulocollic reflexes were studied in a patient with a unilateral Tullio phenomenon (sound induced vestibular symptoms) and the findings were compared with those of a group of normal subjects. Compared with normal subjects, the reflexes elicited from her symptomatic side were large and had an abnormally low threshold, but retained a normal waveform. The click-evoked responses in this patient show changes consistent with her symptomatology and are indicative of a pathological increase in the normal vestibular sensitivity to sound.

Adult↗
PubMed1994

Myogenic potentials generated by a click-evoked vestibulocollic reflex.

Electromyograms (EMGs) were recorded from surface electrodes over the sternomastoid muscles and averaged in response to brief (0.1 ms) clicks played through headphones. In normal subjects, clicks 85 to 100 dB above our reference (45 dB SPL: close to perceptual threshold for normal subjects for such clicks) evoked reproducible changes in the averaged EMG beginning at a mean latency of 8.2 ms. The earliest potential change, a biphasic positive-negativity (p13-n23), occurred in all subjects and the response recorded from over the muscle on each side was predominantly generated by afferents originating from the ipsilateral ear. Later potentials (n34, p44), present in most but not all subjects, were generated bilaterally after unilateral ear stimulation. The amplitude of the averaged responses increased in direct proportion to the mean level of tonic muscle activation during the recording period. The p13-n23 response was abolished in patients who had undergone selective section of the vestibular nerve but was preserved in subjects with severe sensorineural hearing loss. It is proposed that the p13-n23 response is generated by activation of vestibular afferents, possibly those arising from the saccule, and transmitted via a rapidly conducting oligosynaptic pathway to anterior neck muscles. Conversely, the n34 and p44 potentials do not depend on the integrity of the vestibular nerve and probably originate from cochlear afferents.

Acoustic Stimulation↗
PubMed1994

New tests of vestibular function.

Three new, simple, clinically applicable tests of vestibular function are described. The first is a test of the response of the lateral semicircular canals to high accelerations. The test can even be done at the bedside where it can reveal severe unilateral or bilateral loss of lateral canal function. The test can also be recorded in a laboratory where it might show a less severe deficit of lateral canal function. The second is a simple, laboratory test of utricular function which depends on a subject's ability to align a bar with the subjective visual vertical. Patients with acute unilateral peripheral vestibular lesions invariably set the bar toward the side of the lesion. The third is a laboratory test of saccular function relying on a click-evoked inhibitory vestibulo-collic reflex recorded in the ipsilateral sternomastoid muscle. It can be done with equipment used for auditory evoked potentials.

Functional Laterality↗
PubMed1993

A positron emission tomography study of essential tremor: evidence for overactivity of cerebellar connections.

The origin of essential tremor is unknown. Animal models have suggested that the inferior olivary nucleus may act as a tremor generator. We used positron emission tomography to study changes in regional cerebral blood flow associated with involuntary postural tremor and passive wrist oscillation in patients with essential tremor. Activation due to voluntary wrist oscillation and arm extension without tremor was studied in normal control subjects. The essential tremor group had bilaterally increased cerebellar blood flow at rest (without tremor) compared with the control group. Involuntary postural tremor was associated with further bilateral cerebellar activation, and also contralateral striatal, thalamic, and sensorimotor cortex activation. Voluntary wrist oscillation, maintained arm extension without tremor, and passive wrist oscillation were all associated with significant ipsilateral rather than bilateral cerebellar activation. We conclude that essential tremor is associated with increased bilateral cerebellar activity both at rest and during tremor.

Adult↗
PubMed1992

Voluntary stimulus-sensitive jerks and jumps mimicking myoclonus or pathological startle syndromes.

Five patients who presented with stimulus-induced jerking as part of an apparent myoclonic or pathological startle syndrome are reported. Neurophysiological observations in these patients suggested the jerks were voluntary in origin. These included (a) variable latencies to the onset of stimulus induced jerks, (b) latencies were greater than that seen in reflex myoclonus of cortical or brainstem origin, and were (c) longer than the fastest voluntary reaction times of normal subjects, (d) variable patterns of muscle recruitment within each jerk and, (e) significant habituation with repeated stimulation. It is argued that these features are consistent with a voluntary origin for the jerks and enable them to be distinguished from the stereotyped electrophysiological characteristics of myoclonus of cortical and brainstem origin. Electrophysiological recordings may help identify patients with this form of psychogenic movement disorder.

Adult↗
PubMed1992

Interhemispheric inhibition of the human motor cortex.

1. Using two magnetic stimulators, we investigated the effect of a conditioning magnetic stimulus over the motor cortex of one hemisphere on the size of EMG responses evoked in the first dorsal interosseous (FDI) muscle by a magnetic test stimulus given over the opposite hemisphere. 2. A single conditioning shock to one hemisphere produced inhibition of the test response evoked from the opposite hemisphere when the conditioning-test interval was 5-6 ms or longer. We shall refer to this as interhemispheric inhibition. However, the minimum latency of inhibition observed using surface EMG responses may have underestimated the true interhemispheric conduction time. Single motor unit studies suggested values 4-7 ms longer than the minimum interval observed with surface EMG. 3. Interhemispheric inhibition was seen when the test muscle was active or relaxed. Increasing the intensity of the conditioning stimulus increased the duration of inhibition: increasing the intensity of the test stimulus reduced the depth of inhibition. 4. The conditioning coil had to be placed on the appropriate area of scalp for inhibition to occur. The effect of the conditioning stimulus was maximal when it was applied over the hand area of motor cortex, and decreased when the stimulus was moved medial or lateral to that point. 5. The inhibitory effect on the test stimulus probably occurred at the level of the cerebral cortex. In contrast to the inhibition of test responses evoked by magnetic test stimuli, test responses evoked in active FDI by a small anodal electric shock were not significantly inhibited by a contralateral magnetic conditioning stimulus. Similarly, H reflexes in relaxed forearm flexor muscles were unaffected by conditioning stimuli to the ipsilateral hemisphere. However, inhibition was observed if the experiment was repeated with the muscles active.

Adult↗
PubMed1991

Cortical areas and the selection of movement: a study with positron emission tomography.

Regional cerebral blood flow was measured in normal subjects with positron emission tomography (PET) while they performed five different motor tasks. In all tasks they had to moved a joystick on hearing a tone. In the control task they always pushed it forwards (fixed condition), and in four other experimental tasks the subjects had to select between four possible directions of movement. These four tasks differed in the basis for movement selection. A comparison was made between the regional blood flow for the four tasks involving movement selection and the fixed condition in which no selection was required. When selection of a movement was made, significant increases in regional cerebral blood flow were found in the premotor cortex, supplementary motor cortex, and superior parietal association cortex. A comparison was also made between the blood flow maps generated when subjects performed tasks based on internal or external cues. In the tasks with internal cues the subjects could prepare their movement before the trigger stimulus, whereas in the tasks with external cues they could not. There was greater activation in the supplementary motor cortex for the tasks with internal cues. Finally a comparison was made between each of the selection conditions and the fixed condition; the greatest and most widespread changes in regional activity were generated by the task on which the subjects themselves made a random selection between the four movements.

Adult↗
PubMed1991

Decreases in regional cerebral blood flow with normal aging.

Positron emission tomographic (PET) images of regional cerebral blood flow (rCBF) from 30 normal, resting volunteers aged 30 to 85 years were analysed to identify areas where rCBF fell with age. Images were anatomically normalised, and a pixel-by-pixel linear regression was performed to remove differences in global CBF between subjects. Pixels at which rCBF then showed a significant (p less than 0.01) negative correlation with age were identified. They were displayed as a statistical parametric map (SPM) of correlations. We demonstrate an age-related decrease in adjusted rCBF in the cingulate, parahippocampal, superior temporal, medial frontal, and posterior parietal cortices bilaterally, and in the left insular and left posterior prefrontal cortices (omnibus significance, chi 2 = 2,291, p less than 0.0001, df = 1). Decreases in rCBF suggest a regionally specific loss of cerebral function with age. The affected areas were all limbic, or association, cortices. Therefore, these decreases may constitute the cerebral substrate of the cognitive changes that occur during normal aging.

Adult↗
PubMed1991

Regional cerebral blood flow during volitional breathing in man.

1. Positron emission tomographic imaging of brain blood flow was used to identify areas of motor activation associated with volitional inspiration in six normal male subjects. 2. Scans were performed using intravenous infusion of H2(15)O during voluntary targeted breathing and positive pressure passive ventilation at the same level. 3. Regional increases in brain blood flow, due to active inspiration, were derived using a pixel by pixel comparison of images obtained during the voluntary and passive ventilation phases. 4. Pooling data from all subjects revealed statistically significant increases in blood flow bilaterally in the primary motor cortex (left, 5.4%; right, 4.3%), in the right pre-motor cortex (7.6%), in the supplementary motor area (SMA; 3.1%) and in the cerebellum (4.9%). 5. The site of increased neural activation in the motor cortex, associated with volitional inspiration, is consistent with an area which when stimulated, either directly during neurosurgery or transcranially with a magnetic stimulus, results in activation of the diaphragm. 6. The presence of additional sites of neural activation in the pre-motor cortex and SMA appears analogous to the results of studies on voluntary limb movement. The site of the increase in the SMA was posterior to that previously reported for arm movements. These areas are believed to have a role 'upstream' of the motor cortex in the planning and organization of movement. 7. This technique provides a means of studying the volitional motor control of respiratory related tasks in man.

Adult↗
PubMed1991

Regional cerebral blood flow during voluntary arm and hand movements in human subjects.

1. Regional cerebral blood flow (rCBF) was measured using positron emission tomography in six normal volunteers while at rest and while performing four different repetitive movements of the right arm. 2. The four movements were performed in random order and consisted of abduction of the index finger, making a fist, sequential thumb to digit opposition, and shoulder flexion. All the movements were done at the same rate, using an auditory cue and involved displacements through similar amounts of the physiological range at each joint. 3. Increases in rCBF were interpreted as evidence of local neural activation and all four movements were associated with significant increases in CBF in the contralateral sensorimotor and premotor areas and in the supplementary motor area (SMA). 4. The average increase in blood flow in the contralateral sensorimotor cortex was significantly greater for the shoulder movement (31%) than for the three other movements. The increases with finger opposition (21%) and fist-making (24%) were not significantly different, and both were significantly greater than with index finger movement (13%). These data indicate that neither "fractionation" nor distal movement per se cause selective activation of sensorimotor cortex. 5. Significantly greater increases in blood flow in both the contralateral premotor cortex and the SMA ("nonprimary motor areas") occurred with shoulder movement than with the other movements. Because this difference may be related to the significantly greater activation occurring concurrently in the sensorimotor cortex, this finding does not prove unequivocally a "selective" role of the nonprimary motor areas in proximal movement. 6. Neither of the two nonprimary motor areas showed selective activation when a simple sequence of finger movements was performed compared with repetitive contractions of the same fingers. 7. Shoulder movement alone was associated with significant increases in rCBF in the ipsilateral sensorimotor cortex (10%), the superior vermis of the cerebellum (19%), and Brodmann areas 5 and 40 in the contralateral hemisphere. 8. The average location of the center of excitation in the sensorimotor cortex and SMA differed for the four movements and was interpreted as evidence of within-limb somatotopy. The shoulder focus lay highest in the sensorimotor cortex and lowest in the SMA.

Adult↗
PubMed1990

Preliminary report: activation of the cerebellum in essential tremor.

Several images of cerebral blood flow were recorded during inhalation of carbon-15-labelled carbon dioxide by positron emission tomography in four patients with essential tremor and four normal controls. Unilateral involuntary postural tremor in essential tremor patients was associated with blood flow significantly greater than that at rest in the contralateral sensorimotor cortex, both lateral premotor regions, and both cerebellar hemispheres. Of these regions, only the cerebellum was not activated in normal controls holding a posture without tremor or in essential tremor patients undergoing passive wrist movement. The increased flow in the cerebellum therefore seems to represent neural activity involved in tremor generation. It is proposed that essential tremor is due to oscillation within cerebello-olivary pathways, relayed by way of the thalamus and motor cortex to the spinal cord.

Administration, Inhalation↗
PubMed1990

Striatal function in normal aging: implications for Parkinson's disease.

Central to several current theories of the etiology of Parkinson's disease is the premise that the nigrostriatal dopaminergic system degenerates with normal aging. Much of the evidence for this assertion has come from postmortem neurochemical studies. We have used L-6-[18F] fluoro-Dopa and positron emission tomography in 26 healthy volunteers (age range, 27-76 years) to examine striatal and frontal cortical tracer uptake. Data have been analyzed by using a graphical approach to calculate an influx constant (Ki) for L-6-[18F]fluoro-Dopa uptake into the caudate, putamen, and medial frontal cortex of each subject. In the population studied, there was no decline in Ki with age for any of these structures. A series of physiological measurements made on the older subjects also showed few significant changes with age. The positron emission tomographic findings demonstrate preservation of nigrostriatal dopaminergic function in normal aging. The pathological process causing Parkinson's disease may operate closer to the time of presentation than has been suggested.

Adult↗
PubMed1990

Exaggerated startle reflexes in an elderly woman.

A 76-year-old woman with a 5-year history of excessive startle is reported. Electrophysiological recordings were made of muscle activation after acoustic and proprioceptive stimulation. The findings are discussed in relation to current knowledge of the startle reflex and classification of startle syndromes. It is concluded that this patient is best regarded as having a pathological exaggeration of the normal startle reflex rather than other causes of stimulus-sensitive muscle jerks.

Aged↗
PubMed1990

Responses of monkey precentral neurones to passive movements and phasic muscle stretch: relevance to man.

Single cell recordings were made from movement-related neurones from the precentral cortex of two monkeys, trained to perform a simple lever-pulling task. They were also trained to remain relaxed while the arm was explored with passive movements at different joints, cutaneous stimuli and during the application of two types of phasic muscle stretch: percutaneous vibration and percussion of muscle tendons. Recordings were made of the responses of cortical neurones both to the 'natural' stimuli and to vibration of specific muscle tendons or percussion of the triceps tendon. Both tendon percussion and vibration excited neurones within area 4 with an average latency for tendon percussion of 21.0 msec. There was a high degree of consistency in the effects on single neurones of tendon percussion and vibration at the same site. Although long-term facilitation was not seen. vibration-induced discharge in the motor cortex should be considered as a potential mechanism of its effects in intact man. In contrast to the similarity of the effect of the two forms of phasic stretch, the relationship between a single neurone's response to either tendon percussion or vibration and to passive movement was complex. The dissociation seen between the effects of phasic muscle stretch and that of passive movement may underlie the failure, in man, to find uniformly increased long-latency stretch reflexes in clinical states of extrapyramidal rigidity.

Action Potentials↗