Showing papers by "Mark Hallett published in 1992"
TL;DR: The early facilitation observed at high intensities seems to be a consequence of a rise in cortical excitability induced by the conditioning stimulus, causing an increase in the number or size, or both, of descending volleys from the test stimulus.
649 citations
TL;DR: The effects of coil orientation, stimulus intensity, and shape of the induced current pulse on the amplitudes of motor evoked potentials in the left abductor pollicis brevis of 10 normal adults who had transcranial magnetic stimulation are studied.
Abstract: We studied the effects of coil orientation, stimulus intensity, and shape of the induced current pulse on the amplitudes of motor evoked potentials in the left abductor pollicis brevis of 10 normal adults who had transcranial magnetic stimulation. The optimal direction of currents induced in the brain is approximately perpendicular to the central sulcus, flowing diagonally from back to front. The most effective coil orientation depends on the shape of the induced current pulse and, when the first and second phases of the pulse are of similar size, also on the intensity of stimulation. Optimal mapping of the human motor cortex with magnetic stimulation requires knowledge of the influences of all these factors.
614 citations
TL;DR: Transcranial magnetic stimulation was used to map the cortical representations of 4 upper extremity muscles of 10 normal subjects and showed a somatotopic progression on the scalp of proximal to distal muscles along a posteromedial to anterolateral axis.
569 citations
TL;DR: Neither age, sex, education level, severity of dementia, word fluency, response time, memory, nor visuomotor procedural learning predicted CA or CCA performance, but a deficit in cognitive planning suggests a functional link between the cerebellum, basal ganglia, and the frontal lobe concerning specific cognitive processes.
Abstract: We compared the performance of 12 patients with cerebellar atrophy (CA) and 12 normal controls matched for age and education on the Tower of Hanoi, a nine-problem task that requires cognitive planning. CA patients performed significantly worse than controls on this task despite no difference in planning and between-move pause times. A reanalysis of the data using just the subgroup of patients with pure cerebellar cortical atrophy (CCA) (N = 9) replicated the above results and also showed that CCA patients had significantly increased planning times compared with controls. Neither age, sex, education level, severity of dementia, word fluency, response time, memory, nor visuomotor procedural learning predicted CA or CCA performance. This deficit in cognitive planning suggests a functional link between the cerebellum, basal ganglia, and the frontal lobe concerning specific cognitive processes. However, the exact role of the cerebellum in cognitive planning remains undetermined.
370 citations
TL;DR: There was an inverse relationship between %M recruited by transcranial magnetic stimuli in different subjects and the variability in MEP amplitude and latency, and Latency variability was less pronounced than amplitude variability.
340 citations
TL;DR: The amplitudes of MEPs from biceps, which was the muscle immediately proximal to the block, gradually increased with anesthesia and then returned to preanesthesia levels within approximately 20 minutes after anesthesia was ended, strongly suggest unmasking of preexisting synaptic connections.
Abstract: Reorganization of corticospinal pathways after spinal cord injury and amputations leads to increased excitability of motor pathways targeting muscles proximal to the level of interruption of efferents from the CNS. To study the timing of these changes, we have recorded motor evoked potentials (MEPs) in the arm muscles of three normal subjects before, during, and after anesthetic block of the forearm and hand. The amplitudes of MEPs from biceps, which was the muscle immediately proximal to the block, gradually increased with anesthesia and then returned to preanesthesia levels within approximately 20 minutes after anesthesia was ended. MEPs from the contralateral arm were unaffected. Such rapid changes strongly suggest unmasking of preexisting synaptic connections, due to disinhibition at cortical or subcortical levels, as the mechanism underlying acute modulation of motor outputs.
250 citations
TL;DR: It is concluded that magnetic stimulation shortens RT by inducing an earlier initiation of this excitability increase in the motor cortex, similar to what was found with suprathreshold stimulation to the ipsilateral motor cortex.
Abstract: In a simple reaction time (RT) paradigm, magnetic stimulation of different intensities was delivered over different scalp positions and at variable delays before (negative) or after (positive) the go-signal. Magnetic stimulation shortened RT to different go-signals (auditory, visual and somatosensory stimuli) by approximately 30 ms when delivered over the motor cortex contralateral to the responding arm at intensities below motor threshold. This effect was maximal at a delay of approximately + 10 ms. A similar effect was found with suprathreshold stimulation to the ipsilateral motor cortex. Magnetic stimulation over other scalp areas did not affect RT regardless of the delay. No differences were found between the effects on elbow flexion and thumb abduction. The shortening of RT was not associated with changes in the timing development of premovement excitability increase in the motor cortex. We conclude that magnetic stimulation shortens RT by inducing an earlier initiation of this excitability increase
191 citations
TL;DR: The absence of changes in the excitability of the alpha motoneuron pool in the presence of motor reorganization targeting muscles proximal to the stump suggests that reorganization occurs proximalto the alpha Motoneuron level.
142 citations
TL;DR: Signs of changes in N30 amplitude may be indicative of abnormal excitatory effects on cortex resulting from disorders of the basal ganglia in patients with dystonia and normal controls.
121 citations
TL;DR: Risk of hearing loss from the acoustic artifact of magnetic stimulation, as evaluated by audiograms, tympanograms, acoustic reflexes, and auditory evoked potentials, seems to be small in humans.
Abstract: Prompted by the description of hearing loss in rabbits exposed to the acoustic artifact of magnetic stimulation, we compared the results of audiologic studies before and after exposure to transcranial magnetic stimulation in humans. We found no evidence of temporary or permanent threshold shifts in any of the subjects, even in those exposed to transcranial magnetic stimulation repeatedly for several years. Risk of hearing loss from the acoustic artifact of magnetic stimulation, as evaluated by audiograms, tympanograms, acoustic reflexes, and auditory evoked potentials, seems to be small in humans.
119 citations
TL;DR: Reaction time to Suprathreshold transcranial stimulation to the motor cortex seems to transiently inhibit the neurons responsible for initiation of motor programs involving muscles in which motor evoked potentials have been induced, thereby prolonging the reaction time.
Abstract: We studied the effect of different go-signals on the reaction time in nine normal human subjects trained to respond by rapidly flexing one arm. Reaction times to auditory stimuli were shorter than those to visual or somatosensory stimuli, and were inversely correlated with the stimulus intensity. The reaction time was longest to a transcranial (magnetic or electric) stimulus delivered over the contralateral motor cortex that was sufficiently strong to induce a motor evoked potential in the responding biceps. Conversely, reaction time was shortest to either subthreshold transcranial stimulation over the same scalp position or to transcranial stimulation over the ipsilateral motor cortex regardless of intensity. Suprathreshold transcranial stimulation to the motor cortex seems to transiently inhibit the neurons responsible for initiation of motor programs involving muscles in which motor evoked potentials have been induced, thereby prolonging the reaction time. On the other hand, a subthreshold stimulus either disinhibits or directly activates such neurons leading to a shorter reaction time. Transcallosal connections between the motor cortices may account for the short reaction time to ipsilateral transcranial stimulation.
TL;DR: In this article, the authors studied the excitation thresholds for motor and sensory fibers in the ulnar, median and tibial nerves using both electric and magnetic stimulation, and they found that for short duration electrical stimuli (0.1 msec) the threshold for motor fibers is lower than for sensory fibers.
TL;DR: The effects of transcranial magnetic stimulation on the performance of a warned, forced-choice response time task by normal adults were studied in this article, where subjects were asked to choose the right or left finger only after the go-signal was delivered.
Abstract: The effects of transcranial magnetic stimulation were studied on the performance of a warned, forced-choice response time task by normal adults. The task consisted of extension of the index finger in response to the click produced by the discharge of the magnetic coil (go-signal). The subjects were asked to choose the right or left finger only after the go-signal was delivered. Single magnetic stimuli were delivered to the prefrontal or motor area, and in the control situation, away from the head. Magnetic stimulation affected hand preference only when it was delivered to the motor area. With stimulation of this area, subjects more often chose the hand contralateral to the site stimulated with response times that were mainly less than 200 ms. With longer response times (between 200 and 1100 ms), magnetic stimulation had no effect on hand preference regardless of the site stimulated. Stimulation of prefrontal areas yielded results similar to the control situation. These results suggest that response bias in this paradigm is caused by an effect of magnetic stimulation on neural structures within, or closely related to, the motor areas of the brain. Although the response bias was clear and predictable, the subjects were unaware of its existence. It is possible to influence endogenous processes of movement preparation externally without disrupting the conscious perception of volition.
TL;DR: The temperature of electrodes and metal disks positioned close to a coil was measured during rapid-rate magnetic stimulation and the temperature rise ranged from a fraction of a degree to almost half a degree per stimulus pulse and increased with the electrical conductivity of the metal, the square of the electrode radius and thesquare of the stimulus strength.
TL;DR: Five patients from a kindred with hereditary hyperekplexia had physiological testing, and testing at graded stimulus intensities indicated an increase in the gain of the acoustic startle reflex.
Abstract: Five patients from a kindred with hereditary hyperekplexia had physiological testing. The surface-recorded electromyographic pattern of audiogenic muscle jerks was identical to that of the normal acoustic startle reflex. Testing at graded stimulus intensities indicated an increase in the gain of the acoustic startle reflex. Nose-tap stimuli resulted in short-latency generalized electromyographic bursts that were similar to the R1 component of the blink reflex. Electrical stimulation of peripheral nerves elicited a pattern of generalized muscle jerks that was similar to that of the acoustic startle reflex. Somatosensory evoked potentials, brainstem auditory evoked potentials, and cortical auditory evoked potentials were normal. The primary physiological abnormality in hereditary hyperekplexia is widespread elevated gain of vestigial withdrawal reflexes in the brainstem and possibly the spinal cord, most likely resulting from increased excitability of reticular neurons.
TL;DR: In this paper, a magnetic coil was moved along the median nerve in the region of the elbow, thereby ensuring that the properties of the nerve and surrounding volume conductor were uniform, and the site of stimulation was determined to be 3.0 ± 0.5 cm from the center of an 8-shaped coil.
TL;DR: Cutaneous reflexes are compatible with the loss of an inhibitory spinal mechanism elicited by cutaneous afferents, and can be a partial explanation for increased tone in PD.
Abstract: The physiology of rigidity in Parkinson's disease (PD) can be investigated by the study of reflexes. Cutaneous reflexes (CR) were measured in 10 patients with PD and in 10 age- and sex-matched normal volunteers. EMG activity was recorded from the first dorsal interosseous muscle with surface electrodes, rectified and averaged. The index finger was stimulated with an intensity four times the sensory threshold. The subjects abducted the index finger with 20% of maximal force. While the latencies of the different reflex components and the amplitudes of the excitatory peaks were not different in the two groups, the first inhibitory components was less pronounced in patients with PD as compared with normals. This effect is partially reversed with dopaminergic drug treatment. The results are compatible with the loss of an inhibitory spinal mechanism elicited by cutaneous afferents, and can be a partial explanation for increased tone in PD.
TL;DR: Analyzing glucose metabolism in the posterior fossa in 67 patients with primary unilateral supratentorial brain tumors found that glucose utilization was lower in the contralateral cerebellar cortex (compared with the ipsilateral one), consistent with the selection criterion, and the “dentate sparing” suggests preservation of afferent input to the largest of the deep Cerebellar nuclei from the Purkinje cells in the cortex.
Abstract: A unilateral supratentorial lesion may cause hypometabolism in the contralateral cerebellar hemisphere (crossed cerebellar diaschisis). We analyzed glucose metabolism, measured by PET-FDG, in the posterior fossa in 67 patients (78 PET studies) with primary unilateral supratentorial brain tumors selected for visually obvious metabolic asymmetry in the cerebellar hemispheres. We found that glucose utilization was 17% lower in the contralateral cerebellar cortex (compared with the ipsilateral one), consistent with the selection criterion, and 19% lower in the ipsilateral pons, wherein lie the first order synapses of the corticopontocerebellar pathway. This finding helps to validate the prevalent view that cerebellar diaschisis is due to interruption of afferent input from the corticopontocerebellar pathway. However, glucose metabolism in the contralateral dentate nucleus was relatively preserved--only 2% less than the ipsilateral dentate. This "dentate sparing" suggests preservation of afferent input to the largest of the deep cerebellar nuclei from the Purkinje cells in the cortex, despite interruption of the major excitatory input to the Purkinje cells.
TL;DR: The findings suggest that the quantitive assessment of perioral reflexes may provide information about the excitability of brainstem interneurons in cranial dystonia that is complementary to blink reflex studies.
Abstract: In order to assess the clinical utility of trigemino-facial reflexes in lower facial muscles, we studied perioral reflexes to mechanical and electrical stimulation in 13 patients with spasmodic dysphonia and orofacial dyskinesia and in 7 healthy subjects. Mechanical stimulation of the upper lip of all patients and electrical stimulation of the infraorbital nerve of patients with orofacial dyskinesia elicited larger perioral reflexes than in controls. In the majority of patients, hyperexcitable perioral reflexes were accompanied by increased gain of the blink reflex. In 4 patients, however, trigemino-facial reflexes were enhanced selectively in either the perioral muscles or orbicularis oculi. Our findings suggest that the quantitative assessment of perioral reflexes may provide information about the excitability of brainstem interneurons in cranial dystonia that is complementary to blink reflex studies.
TL;DR: AntI was more pronounced during movements with lower velocity and greater size, and when performed in the direction of heavier background loads, but its variation could not be related to any single kinematic or kinetic variable in all circumstances.