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Biomedical subjects

M Hallett

Publications and source records attributed to M Hallett.

At least 19 recordsLinked to original sources
PubMed1999

A PET study of human auditory spatial processing.

To learn more about human auditory spatial processing, we used positron emission tomography (PET) to measure regional cerebral blood flow in human volunteers engaged in sound localization tasks. Spectral and binaural cues of localized sound were reproduced by a sound system and delivered via headphones. During localization tasks, subjects activated inferior parietal lobules (IPL) bilaterally. In a second experiment, matched in design to the first, subjects made non-spatial auditory discriminations based on frequency, activating the IPL bilaterally with left hemispheric predominance. A between-study comparison revealed that the right IPL was significantly more activated during the sound localization task compared with the feature discrimination task, suggesting a preferential role for the right IPL in auditory spatial processing.

Adult↗
PubMed1999

Simultaneous repetitive transcranial magnetic stimulation does not speed fine movement in PD.

OBJECTIVE: To reevaluate the effect of subthreshold repetitive transcranial magnetic stimulation (rTMS) on concurrent fine movement in PD. BACKGROUND: A previous study showed a beneficial effect of rTMS on the performance of six patients with PD on the Grooved Pegboard test. METHODS: The authors repeated this experiment in 11 patients with idiopathic PD who performed the test while the stimulating coil discharged continuously at 5 Hz, either over the contralateral motor cortex at just below the threshold for movement, or in the air near the head as a control. Patients were tested twice under both conditions. RESULTS: Although some patients performed faster with rTMS, others showed the opposite effect. There was no significant effect of rTMS in the group, nor did baseline performance or the order of conditions interact with the effect of rTMS. CONCLUSIONS: Based on this larger and more comprehensive study, simultaneous, 5-Hz subthreshold rTMS over the motor cortex does not have consistent or potentially therapeutic effects on movement in PD.

Aged↗
PubMed1999

Blink reflex recovery in facial weakness: an electrophysiologic study of adaptive changes.

OBJECTIVE: To study the electrophysiologic effects of unilateral facial weakness on the excitability of the neuronal circuitry underlying blink reflex, and to localize the site of changes in blink reflex excitability that occur after facial weakness. BACKGROUND: Eyelid kinematic studies suggest that adaptive modification of the blink reflex occurs after facial weakness. Such adaptations generally optimize eye closure. A report of blepharospasm following Bell's palsy suggests that dysfunctional adaptive changes can also occur. METHODS: Blink reflex recovery was evaluated with paired stimulation of the supraorbital nerve at different interstimulus intervals. Comparisons were made between normal control subjects and patients with Bell's palsy who either recovered facial strength or who had persistent weakness. RESULTS: Blink reflex recovery was enhanced in patients with residual weakness but not in patients who recovered facial strength. Facial muscles on weak and unaffected sides showed enhancement. In patients with residual weakness, earlier blink reflex recovery occurred when stimulating the supraorbital nerve on the weak side. Sensory thresholds were symmetric. CONCLUSION: Enhancement of blink reflex recovery is dependent on ongoing facial weakness. Faster recovery when stimulating the supraorbital nerve on the paretic side suggests that sensitization may be lateralized, and suggests a role for abnormal afferent input in maintaining sensitization. Interneurons in the blink reflex pathway are the best candidates for the locus of this plasticity.

Adaptation, Physiological↗
PubMed1999

Feedforward ankle strategy of balance during quiet stance in adults.

1. We studied quiet stance investigating strategies for maintaining balance. Normal subjects stood with natural stance and with feet together, with eyes open or closed. Kinematic, kinetic and EMG data were evaluated and cross-correlated. 2. Cross-correlation analysis revealed a high, positive, zero-phased correlation between anteroposterior motions of the centre of gravity (COG) and centre of pressure (COP), head and COG, and between linear motions of the shoulder and knee in both sagittal and frontal planes. There was a moderate, negative, zero-phased correlation between the anteroposterior motion of COP and ankle angular motion. 3. Narrow stance width increased ankle angular motion, hip angular motion, mediolateral sway of the COG, and the correlation between linear motions of the shoulder and knee in the frontal plane. Correlations between COG and COP and linear motions of the shoulder and knee in the sagittal plane were decreased. The correlation between the hip angular sway in the sagittal and frontal planes was dependent on interaction between support and vision. 4. Low, significant positive correlations with time lags of the maximum of cross-correlation of 250-300 ms were found between the EMG activity of the lateral gastrocnemius muscle and anteroposterior motions of the COG and COP during normal stance. Narrow stance width decreased both correlations whereas absence of vision increased the correlation with COP. 5. Ankle mechanisms dominate during normal stance especially in the sagittal plane. Narrow stance width decreased the role of the ankle and increased the role of hip mechanisms in the sagittal plane, while in the frontal plane both increased. 6. The modulation pattern of the lateral gastrocnemius muscle suggests a central program of control of the ankle joint stiffness working to predict the loading pattern.

Adult↗
PubMed1999

Period of susceptibility for cross-modal plasticity in the blind.

Cross-modal plasticity in blind subjects contributes to sensory compensation when vision is lost early in life, but it is not known if it does so when visual loss occurs at an older age. We used H2(15)O positron emission tomography to identify cerebral regions activated in association with Braille reading, and repetitive transcranial magnetic stimulation to induce focal transient disruption of function during Braille reading, in 8 subjects who became blind after age 14 years (late-onset blind), after a lengthy period of normal vision. Results were compared with those previously reported obtained from congenitally and early-onset blind subjects. As shown by H2(15)O positron emission tomographic scanning, the occipital cortex was strongly activated in the congenitally blind and early-onset blind groups but not in the late-onset blind group. Occipital repetitive transcranial magnetic stimulation disrupted the Braille reading task in congenitally blind and early-onset blind subjects but not in late-onset blind subjects. These results indicate that the susceptible period for this form of functionally relevant cross-modal plasticity does not extend beyond 14 years.

Adult↗
PubMed1999

Parkinsonism after a wasp sting.

A 49-year-old man had mild parkinsonism after being stung by a wasp, a member of the Hymenoptera order. His clinical course was stable for 6 months after which his condition rapidly progressed to a severe akinetic-rigid syndrome with evidence, on a magnetic resonance imaging brain scan, of marked destruction of the basal ganglia. The symptoms did not respond to standard antiparkinsonian medications. Repeated courses of plasmapheresis followed by monthly intravenous infusions of immunoglobulin and long-term administration of azathioprine halted and appeared to partially reverse his deterioration. The literature on the neurologic, particularly the extrapyramidal, manifestations of stings by insects of the Hymenoptera order is reviewed and the possible pathophysiological mechanisms of injury are discussed. Hymenoptera stings should be included in the differential diagnosis of acute and chronic extrapyramidal syndromes.

Animals↗
PubMed1999

Effect of muscle activity immediately after botulinum toxin injection for writer's cramp.

Animal and human studies have shown that nerve stimulation enhances some effects of botulinum toxin (btx A) injection. Voluntary muscle activity might work similarly and would focus the effect of an injection into the active muscles. We studied the effects of exercise immediately after btx A injection in eight patients with writer's cramp with established response to btx A over two injection cycles with a single-blinded, randomized, crossover design. Immediately after the first study injection, they were randomly assigned to write continuously for 30 min or have their hand and forearm immobilized for 30 min. Following the second injection, they were assigned the alternate condition. Patients were assessed just before each injection, and at 2 weeks, 6 weeks, and 3 months post-injection. Assessment included objective strength testing, self-reported rating of benefit and weakness, and blinded evaluation of videotapes and writing samples of the patients writing a standard passage. Strength testing showed that the maximum weakness occurred at 2 weeks post-injection, but the benefit was maximum at 6 weeks post-injection. The "write" condition resulted in greater reduction in strength than the "rest" condition. Btx A treatment led to improvement in self-reported ratings, writer's cramp rating scale scores by blinded raters, and reduction in writing time, but the differences between the "write" and "rest" conditions were not significant. We conclude that voluntary muscle activity immediately after btx A injection leads to greater reduction in muscle strength. Our findings raise the possibility that voluntary muscle activation may allow reduction of btx A doses and favorably alter the balance of benefit and side effects of btx A injections.

Adult↗
PubMed1999

Kinematic characteristics of standing disequilibrium: reliability and validity of a posturographic protocol.

OBJECTIVE: To investigate retest reliability and concurrent validity of the fundamental measurements made of a posturographic protocol that employs quiet standing to quantify the severity and the nature of patients' postural disturbances. STUDY DESIGN: Retrospective complete block design. SETTING: Geriatric rehabilitation department. PARTICIPANTS: Thirty-six participants (age range, 67 to 86 yrs) having normal, moderate, or severe levels of disequilibrium. METHODS: Quiet standing was evaluated on three occasions using a three-dimensional motion analysis system and a force platform. Eight testing conditions, designed to vary task difficulty by controlling the contributions of vision, foot proprioception, and base-of-support width, were administered. MAIN OUTCOME MEASURES: Retest reliability of body sway, joint alignment, body position, and motor coordination indicators were evaluated by intraclass correlation coefficients (ICCs). Concurrent validity of protocol measures was evaluated by the prediction of disequilibrium from a stepwise linear discriminant analysis. RESULTS: ICCs indicated high level of retest reliability for all variables but those of motor coordination, which was not influenced by testing conditions. Discriminant analysis resulted in a four-factor discriminator that included measures of body sway, position, alignment, and motor coordination. The derived linear discriminate function correctly classified 96% of the patients' level of disequilibrium. CONCLUSIONS: The posturographic protocol has the potential to be a useful tool for evaluating severity and nature of postural instability and the effects of pharmacologic and rehabilitative treatment. Results also indicate that combining direct body measurements with force-plate data has the potential to expose the underlying impairments that cause disequilibrium, determine their pathogenesis, and evaluate compensatory strategies.

Aged↗
PubMed1999

Two periods of processing in the (circum)striate visual cortex as revealed by transcranial magnetic stimulation.

To determine the timing of visual processing in the (circum)striate visual cortex, we examined the effect of single pulse transcranial magnetic stimulation over the occipital pole of healthy subjects who were engaged in a forced-choice visual letter identification task. Single letters, subtending a visual angle of 0.35 degrees, were foveally presented for 10 ms and were immediately followed by a mask. We investigated 30 different delays (d), defined as the time between the onset of the visual stimulus and the onset of the magnetic stimulus, from d = -100 ms to d = +190 ms, and 4 different midsagittal coil positions (x), defined as the distance between the lower edge of the coil and the upper edge of the inion, from x = 1 cm to x = 7 cm. Three out of four subjects showed three distinct delay intervals (dips) at which application of TMS resulted in an impairment of the task. The first dip was centred around d = -50 ms and occurred independently of the coil position; the second dip was centred around d = 0 ms and was elicitable only with the two lowest coil positions; the third dip was centred around d = 100 ms and was also elicitable only with the lower coil positions. In the fourth subject, only the first and the third dip were found. We conclude that there are two distinct periods when the activity in the (circum)striate visual cortex is necessary for the identification of visually presented letters.

Electromagnetic Phenomena↗
PubMed1999

A PET study of sequential finger movements of varying length in patients with Parkinson's disease.

To study the difficulty that patients with Parkinson's disease have in performing long sequential movements, we used H2(15)O PET to assess the regional cerebral blood flow (rCBF) associated with the performance of simple repetitive movements, well-learned sequential finger movements of varying length and self-selected movements. Sequential finger movements in the Parkinson's disease patients were associated with an activation pattern similar to that found in normal subjects, but Parkinson's disease patients showed relative overactivity in the precuneus, premotor and parietal cortices. Increasing the complexity of movements resulted in increased rCBF in the premotor and parietal cortices of normal subjects; the Parkinson's disease patients showed greater increases in these same regions and had additional significant increases in the anterior supplementary motor area (SMA)/cingulate. Performance of self-selected movements induced significant activation of the anterior SMA/cingulate in normal subjects but not in Parkinson's disease patients. We conclude that in Parkinson's disease patients more cortical areas are recruited to perform sequential finger movements; this may be the result of increasing corticocortical activity to compensate for striatal dysfunction.

Adult↗
PubMed1999

Comparison of PET [15O]water studies with 6-minute and 10-minute interscan intervals: single-subject and group analyses.

The authors recently showed that [15O]water PET data obtained with a short interscan interval (6 minutes) produced similar results whether or not the residual background from the previous scan is subtracted. The purpose of the present study was to compare scans obtained during motor activation using a short (6-minute) interscan interval protocol with those obtained with a standard (10-minute) protocol in the same scanning session. Single-subject and group analyses were performed using Worsley's method, which uses a pooled variance estimate and statistical parametric mapping with a local variance estimate. High consistency in both the activation maps, i.e., the number of activated motor brain structures and the Talairach coordinates of peak intensities of the activated regions, was obtained in the 6- and 10-minute studies in both single-subject and group analyses. However, in comparison to the 6-minute studies, a larger cluster size of activated brain regions and an approximately 20% higher peak activation in these regions were observed in the 10-minute studies with the same number of replicates. Analysis of these results suggests that using a 6-minute interval with an increased number of replications, i.e., without changing the subject's total study duration, should produce comparable statistical power to that of the 10-minute interval for group analysis and increased statistical power for single-subject analyses that use a local variance estimate because of increased degrees of freedom. Alternatively, with a small increase in the number of scans and the use of a 6-minute interscan interval, a comparable level of statistical significance may be achieved for single-subject experiments that use a local variance estimate, with an overall shortening of the study duration.

Adult↗
PubMed1998

Perception of timing of kinesthetic stimuli.

A psychophysical method was used to estimate the timing of perception of kinesthetic stimuli with different velocities in normal volunteers. A 1 ms auditory click occurred randomly before or after an imposed flexion movement at either 20, 40 or 60 deg/s of the metacarpophalangeal joint. Subjects reported whether the click was perceived before or after the movement onset (experiment 1) or perception of movement velocity (experiment 2). The time at which there was a 50% chance that subjects reported movement or velocity perception after the click was taken as an estimate of the time subjects perceived the stimuli. The difference in time of perceived movement velocity discrimination and movement onset was only significant when the velocity was 20 deg/s (52 ms). This suggests that movement onset and identification of the velocity of the faster movements are perceived nearly simultaneously.

Acoustic Stimulation↗
PubMed1998

Mechanisms of deafferentation-induced plasticity in human motor cortex.

Deafferentation induces rapid plastic changes in the cerebral cortex, probably via unmasking of pre-existent connections. Several mechanisms may contribute, such as changes in neuronal membrane excitability, removal of local inhibition, or various forms of short- or long-term synaptic plasticity. To understand further the mechanisms involved in cortical plasticity, we tested the effects of CNS-active drugs in a plasticity model, in which forearm ischemic nerve block (INB) was combined with low-frequency repetitive transcranial magnetic stimulation (rTMS) of the deafferented human motor cortex. rTMS was used to upregulate the plastic changes caused by INB. We studied six healthy subjects. In two control sessions without drug application, INB plus rTMS increased the motor-evoked potential (MEP) size and decreased intracortical inhibition (ICI) measured with single- and paired-pulse TMS in the biceps brachii muscle proximal to INB. A single oral dose of the benzodiazepine lorazepam (2 mg) or the voltage-gated Na+ and Ca2+ channel blocker lamotrigine (300 mg) abolished these changes. The NMDA receptor blocker dextromethorphan (150 mg) suppressed the reduction in ICI but not the increase in MEP size. With sleep deprivation, used to eliminate sedation as a major factor of these drug effects, INB plus rTMS induced changes similar to that seen in the control sessions. The findings suggest that (1) the INB plus rTMS-induced increase in MEP size involves rapid removal of GABA-related cortical inhibition and short-term changes in synaptic efficacy dependent on Na+ or Ca2+ channels and that (2) the long-lasting (>60 min) reduction in ICI is related to long-term potentiation-like mechanisms given its duration and the involvement of NMDA receptor activation.

Adult↗
PubMed1998

Inhibitory influence of the ipsilateral motor cortex on responses to stimulation of the human cortex and pyramidal tract.

1. The ability of the primary motor cortex (M1) to modulate motor responses in ipsilateral hand muscles seems to be important for normal motor control and potentially also for recovery after brain lesions. It is not clear which pathways mediate this ipsilateral modulation. Transcallosal connections have been proposed, but are known to be sparse between cortical hand motor representations in primates. The present study was performed to determine whether descending ipsilateral modulation of motor responses might also be mediated below the cortical level in humans. 2. A paired-pulse protocol was used, in which motor-evoked potentials (MEPs) were produced by cortical transcranial magnetic stimulation (cTMS) or by electrical stimulation of the pyramidal tract at the level of the pyramidal decussation (pdTES), in both preactivated and relaxed hand muscles. Paired stimuli were applied at various interstimulus intervals (ISIs) between 2 and 100 ms. The conditioning stimulus (CS) was always magnetic, and delivered to the M1 ipsilateral to the target hand, prior to the test stimulus (TS). The magnetic TS was delivered to the M1 contralateral to the target hand; the electrical TS was applied through electrodes placed over the mastoid process bilaterally. Further experiments included cortical electrical stimulation and H-reflexes. The MEP amplitudes were averaged separately for each ISI and the control condition (no CS), and expressed as a percentage of the unconditioned response. 3. Conditioning stimulation of the ipsilateral M1 resulted in significant inhibition of magnetically evoked MEPs, and also of MEPs produced by pdTES. Inhibition occurred at ISIs between 6 and 50 ms, and was observed in preactivated and relaxed muscles. Higher CS intensities caused greater inhibition of both cTMS- and pdTES-evoked MEPs. 4. While the conditioning effects on magnetically evoked muscle responses could be explained by a transcallosal mechanism, the effects on pdTES-evoked MEPs cannot, because they are elicited subcortically and are therefore not susceptible to inhibitory mechanisms transmitted at the cortico-cortical level. 5. In conclusion, the present results provide novel evidence that the inhibitory influence of the human M1 on ipsilateral hand muscles is to a significant extent mediated below the cortical level, and not only through cortico-cortical transcallosal connections. They point to a concept of inhibitory interaction between the two primary motor cortices that is relayed at multiple levels along the neuroaxis, thus perhaps providing a structurally redundant system which may become important in case of lesions.

Adult↗
PubMed1998

Motor skill learning in patients with cerebellar degeneration.

To explore the role of the cerebellum in learning a complex motor task, we studied nineteen patients with cerebellar degeneration and sixteen healthy subjects who attempted to improve their performance in generating a trajectory connecting five via points on a data tablet. Multijoint arm movements were performed at a constant total movement time, and spatial error was measured. Subjects performed 100 trials at a movement time of 3.5 s (slow movements), and another 100 trials at maximum speed (fast movements). With slow movements, patients and normal subjects reduced the error over trials to the same extent, but in patients, the rate of improvement was slightly slower. With fast movements, patients showed less improvement than normal subjects. When tested 24 h later, patients demonstrated significant retention of acquired skill and tended to improve more rapidly when performing both slow and fast movements than during the first session. We conclude that patients with cerebellar degeneration can exhibit almost normal performance in skill learning with slow movements, but with fast movements, their performance improves to a lesser extent. The problem may be difficulty in the refinement of motor execution, which is more of a requirement for fast movements than for slow ones.

Adult↗
PubMed1998

Mechanisms of cortical reorganization in lower-limb amputees.

The human motor system undergoes reorganization after amputation, but the site of motor reorganization and the mechanisms involved are unknown. We studied the site and mechanisms of motor reorganization in 16 subjects with traumatic lower-limb amputation. Stimulation at different levels in the CNS was used to determine the site of reorganization. The mechanisms involved were evaluated by measuring the thresholds for transcranial magnetic stimulation (TMS) and by testing intracortical inhibition and facilitation. With TMS, the threshold for muscle activation on the amputated side was lower than that of the intact side, but with transcranial electrical stimulation there was no difference in motor threshold between the two sides. TMS at the maximal output of the stimulator activated a higher percentage of the motor neuron pool (%MNP) on the amputated side than on the intact side. The %MNP activated by spinal electrical stimulation was similar on the two sides. Paired TMS study showed significantly less intracortical inhibition on the amputated side. Our findings suggest that motor reorganization after lower-limb amputation occurs predominately at the cortical level. The mechanisms involved are likely to include reduction of GABAergic inhibition.

Adult↗
PubMed1998

The neurophysiology of dystonia.

Any model for the physiology of dystonia must be able to explain how dystonia can be produced in various circumstances. Brain lesions can cause dystonia; responsible sites include the basal ganglia, brainstem, and thalamus, but the most common site is the putamen. Dystonia can be hereditary, and genetic linkage has been found for both generalized and focal dystonia. The only genetic dystonia for which the gene product is known is Segawa disease, a hereditary progressive dystonia with marked diurnal fluctuation. The defect is in guanosine triphosphate cyclohydrolase I, a gene that makes a cofactor for the synthesis of dopamine, which explains why this form of dystonia should be amenable to treatment with levodopa. Another example of dystonia in which a disorder of dopamine pharmacology appears responsible is the dystonia occurring in Parkinson disease, either spontaneously or as a result of treatment. Curiously, the dystonia occurs at both peak and trough dopamine levels.

Animals↗