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Biomedical subjects

G W Thickbroom

Publications and source records attributed to G W Thickbroom.

At least 19 recordsLinked to original sources
PubMed1995

Comparison of the magnetically mapped corticomotor representation of a muscle at rest and during low-level voluntary contraction.

This study has compared the topography of the corticomotor representation of an intrinsic hand muscle in the relaxed state and during a low-level voluntary contraction using transcranial magnetic stimulation. It was found that the optimum cortical stimulus site and the surrounding area of excitable cortex were shifted approximately 6 mm medially as a result of performing a voluntary contraction. This difference may be due to a combination of factors involving the spatial distribution of excitable intracortical fibres, the effects of muscle afferent activity on motor cortex excitability, and task-dependent modifications of corticomotor output.

Adult↗
PubMed1995

An investigation of the late excitatory potential in the hand following magnetic stimulation of the motor cortex.

Magnetic stimulation of the motor cortex gives rise to a motor evoked potential (MEP) followed by a silent period (SP) during which a late excitatory potential (LEP) may occur in the surface EMG. To elucidate the mechanism of the LEP we investigated the effect of muscle contraction, stimulus intensity and stimulation site on the LEP recorded from the abductor pollicis brevis muscle. The amplitude of the LEP increased with increasing levels of muscle contraction and decreased with increasing stimulus intensity. There was no direct relationship between the amplitude of the LEP and the MEP, but there was an inverse relationship between LEP amplitude and SP duration. The latency of the LEP was unaffected by the level of muscle contraction, but increased with increasing stimulus intensity. Topographic mapping with stimulation at multiple scalp sites yielded a LEP at sites partially encircling but not including the centre of the APB motor area. These results are consistent with the LEP being due to reflex alpha motoneurone firing as a result of gamma motoneurone activation or with a period of disinhibition at cortical level allowing breakthrough of voluntary activity.

Adult↗
PubMed1995

The origin of the soleus late response evoked by magnetic stimulation of human motor cortex.

Transcranial magnetic stimulation (TMS) of the human motor cortex elicits a primary motor evoked potential (MEP) in the soleus muscle at a latency of approximately 30 msec, which may be followed by a late potential with a variable latency of 80-120 msec (soleus late response, SLR). While the MEP is thought to arise from stimulation of the corticospinal tract, the origin of the SLR is uncertain. In the present study we have investigated the properties of the SLR in order to elucidate its origin. An SLR was evoked at a latency of 100-120 msec in 5 out of 10 normal subjects in the relaxed state and at a latency of approximately 100 msec in all subjects when tibialis anterior (TA) was slightly facilitated. The SLR was largest with 5-10% TA contraction, decreased in size with increasing levels of TA contraction and was negligible in all subjects when the foot was immobilised. The latency of the SLR fell by 23 msec when the foot was passively dorsiflexed 20 degrees. A similar response to the SLR, at a latency of 77 msec, was present in all subjects following electrical stimulation of the peroneal nerve. Our findings suggest that the SLR is a soleus stretch reflex resulting from dorsiflexion of the foot due to preferential activation of TA following cortical stimulation.

Adolescent↗
PubMed1991

Saccade onset and offset lambda waves: relation to pattern movement visually evoked potentials.

The lambda (lambda) wave is an occipital EEG potential which occurs when saccadic eye movements are made against an illuminated contrast background. There is some disagreement concerning the presence of sub-components to the lambda-wave, and its relationship to visually evoked potentials. In the present study, lambda-waves were recorded with saccades of different durations (30-110 ms) and compared to VEPs associated with pattern movements of similar durations and velocity. It was found that the lambda-wave consisted of a saccade onset component with positive sub-components at 59 and 100 ms after saccade onset, and a saccade offset component with a positive potential at 74 ms after saccade offset. With small saccades of 30 ms duration or less, these components superimposed to form a single lambda-wave. In the case of pattern movement VEPs, a movement onset component of latency 110 ms following movement onset, and a movement offset component at 89 ms after movement offset, were identified. The similar behaviour of the lambda-wave and VEP under these conditions supports the view that the lambda-wave is a visually evoked potential resulting from movement of the visual field across the retina during a saccadic eye movement.

Adult↗
PubMed1990

Premotor negativity associated with saccadic eye movement and finger movement: a comparative study.

The topography and time course of the premotor negativity (PMN) associated with horizontal saccadic eye movement and with thumb movement has been compared in a group of normal subjects. It was found that the time of onset, slope and distribution of the PMN was essentially the same for both types of movement. This finding suggests that the PMN may reflect primarily a non-movement-specific increase in attention or arousal processes common to both types of movement rather than activity in specific cortical areas concerned with the programming and execution of the movements.

Adult↗
PubMed1990

Computer quantitation of gallium 67 lung uptake in crocidolite (blue asbestos) workers of Western Australia.

Pulmonary inflammation has been evaluated in 43 crocidolite-exposed asbestos (ASB) workers and 12 control subjects, using a quantitative index of gallium uptake (GI). The GI was compared with chest roentgenographs (CXRs) graded by the ILO classification. The ASB workers included 15 with asbestosis (CXR greater than or equal to 1/0), 19 with a normal CXR (CXR 0/0), and 9 with equivocal CXR changes (CXR 0/1). In individuals with asbestosis the GI was 3.6 +/- 0.3 (mean +/- SEM), P less than 0.01 compared with exposed patients without asbestosis. In exposed patients with equivocal CXR changes (0/1) the GI was 3.1 +/- 0.3, and in exposed patients with a normal CXR (0/0) the GI was 2.4 +/- 0.2. The GI for subjects without lung disease was 1.2 +/- 0.2, P less than 0.01 compared with exposed patients without asbestosis. The scans were scored independently by two observers, and the correlation coefficient of the two sets of GI was 0.95. These data demonstrate that subjects with crocidolite-induced asbestosis and exposed patients with equivocal CXR changes or a normal CXR have significantly increased GI.

Asbestos↗
PubMed1987

Presaccadic spike potential: a computer model based upon motor unit recruitment patterns in the extraocular muscles.

Details are presented of a computer model of the presaccadic 'spike' potential based upon the discharge properties of motor units in the extraocular muscles. The model provides support for the hypothesis that the spike potential represents the summated electrical activity from the near-synchronous recruitment of motor units in the extraocular muscles by the presaccadic burst of motoneurone activity.

Action Potentials↗
PubMed1987

Crossed facilitation and post-contraction depression of abductor pollicis brevis motor neurons.

Crossed facilitation of abductor pollicis brevis (APB) motor neurons by voluntary contraction of the contralateral APB was investigated using F response analysis. The integrated F response EMG showed a mean 4-fold increase during contraction. This enhancement was maintained until the onset of subjective fatigue and then declined. After isometric or isotonic contraction of the ipsilateral APB there was a significant depression of F responses with a progressive return to baseline values over a 6 minute period. The significance of the findings is discussed.

Adult↗
PubMed1986

Presaccadic spike potential. Relation to eye movement direction.

The spike potential (SP) which precedes horizontal saccadic eye movement has been shown to arise in the agonist lateral and medial rectus muscles of the two eyes. To determine whether other extra-ocular muscles also generate an SP, recordings were carried out in normal subjects performing vertical and oblique as well as horizontal saccades. An SP was recorded before saccades in all directions. The potential was highest in amplitude with horizontal or oblique abducting saccades and was higher in amplitude with vertical upgoing than downgoing saccades.

Action Potentials↗
PubMed1986

Averaging, spatio-temporal mapping and dipole modelling of focal epileptic spikes.

A computer-based technique is described for the acquisition, spatio-temporal mapping and dipole modelling of epileptic spike events. Spike acquisition is operator-controlled and subsequent data processing is performed off-line. Preliminary averaging and topographical display of wave forms provide complementary information on the spike field. The method promises to be a useful adjunct to conventional EEG in spike analysis.

Brain↗
PubMed1985

Presaccadic 'spike' potential: investigation of topography and source.

Approximately 15-30 ms before a saccade, a large-amplitude, relatively short duration potential whose source has not previously been determined with certainty, can be recorded from the scalp in man. We have measured the surface topography of this potential during horizontal saccades in normal subjects and have found it to be maximal near the eye on the side ipsilateral to the direction of the saccade. Dipole modelling predicts a source near the eye ipsilateral to the direction of gaze, and the application of source derivation supports such an origin. Measurements of the potential close to the orbits in normal subjects and in patients with a unilateral abducens nerve palsy, exenterated orbit or ocular prosthesis suggest that it is a summation potential derived from the ipsilateral lateral rectus and contralateral medial rectus muscles. It is hypothesized that the potential may arise as a result of the synchronized recruitment of motor units in the extraocular muscles prior to the commencement of the saccade.

Adult↗
PubMed1985

Dipole source derivation. Application to the half-field pattern evoked potential.

Dipole modelling is a technique for estimating the location of sources of electrical activity in the brain responsible for the generation of surface recorded potentials on the human scalp. A detailed description of this technique is given with a modification that allows for possible activity at the reference electrode during the surface potential measurement. The application of this technique to localisation of the source of the principal components of the left half-field pattern visual evoked potential is described.

Adult↗
PubMed1985

Cerebral events preceding self-paced and visually triggered saccades. A study of presaccadic potentials.

The cerebral potentials preceding self-paced and visually triggered saccadic eye movements were studied using multichannel recording and spatio-temporal mapping techniques, and the contribution to these potentials of stimulus-evoked activity and the electro-oculographic field was investigated. A premotor positivity (PMP) and negativity (PMN) comparable to those associated with movements of the extremities were found with both self-paced and triggered saccades, but there were differences in the amplitude, duration and topography of these potentials in the different types of eye movement. The slope of the PMP was greater for visually triggered than for self-paced saccades and its slope and duration were influenced by the predictability of the triggering stimulus, the slope being greater and the duration shorter with predictable than with unpredictable stimuli. The characteristics of this potential are compatible with an origin from saccade-related and visually sensitive neurons in parietal and occipital cortex and it is suggested that in a visually triggered eye movement, the PMP may be a correlate of visuomotor interactions. A PMN was associated with both self-paced and visually triggered saccades, and in the case of triggered saccades is thought to reflect anticipation of the cuing stimulus in addition to preparatory movement-related activity. When the precise timing of the cuing stimulus was not known by the subject, a PMN still developed but then plateaued until the arrival of the stimulus and execution of the movement. In this situation the PMN therefore appears to reflect an increasing level of arousal and preparedness to move which is then maintained pending the final motor command.

Adult↗
PubMed1985

Cerebral potentials accompanying visually triggered finger movement in man.

The cerebral potentials associated with visually triggered movements of the right index finger were studied in 5 normal subjects using multichannel scalp recordings and separate stimulus and movement-locked averaging. The visually locked wave forms showed enhanced components of the passive visual evoked potential (P1-N1) with a more widespread distribution than the VEP. The amplitude of the P1-N1 complex was influenced by the reaction time but its timing was independent of the timing of the movement. In a 'move'/'no move' choice paradigm, the positive components following the P1-N1 complex were attenuated with movement and subtraction of the 'move' and 'no move' wave forms showed a movement-related negativity building up over the contralateral parietal and central regions. With movement-locked averaging, a pre-motor negativity (PMN) of comparable topography and timing to the readiness potential was found when the timing of the cuing stimulus was regular. With randomly timed stimuli, the PMN still developed but then plateaued prior to movement. It is suggested that in this situation the PMN is related to an increasing level of arousal, anticipation and preparedness to move which is then maintained pending the arrival of the stimulus and the final motor command.

Adult↗
PubMed1984

Spatio-temporal mapping of evoked cerebral activity.

A technique is described for the colour-coded display of averaged scalp electrical activity at 40 instants in time. An application of this technique to the pattern reversal visual evoked potential is discussed, showing the value of spatio-temporal mapping in the interpretation of multichannel evoked potential recordings.

Brain↗
PubMed1984

Source derivation: application to topographic mapping of visual evoked potentials.

The application of a source derivation technique to topographic mapping of the pattern-reversal visual evoked potential is described. Comparisons were made with non-cephalic and common average reference recordings. Source derivation yielded wave forms which were less influenced by activity of reference origin. Source derivation wave forms were similar to those obtained with common average referencing but contour plots showed less spread of regions of maximal activity in the case of source derivation. It is suggested that source derivation may be useful in further studies of the topography and sources of cerebral evoked potentials.

Adult↗
PubMed1984

F-response studies: computer analysis and recovery cycle.

A computerized technique has been used for the analysis of F responses in normal subjects. Differences between the dominant and non-dominant upper limb have been found in some subjects and potentiation of the F response by the Jendrassik manoeuvre has been demonstrated. The recovery cycle of the F response has been studied in human subjects and in the rat and has been found to be similar to that of the H reflex. Such studies may be useful for studying alpha motor neurone excitability.

Adult↗