Showing papers by "Mark Hallett published in 1997"
TL;DR: Spread of excitation, which may be a warning sign for seizures, occurred in one subject and was not accompanied by increased MEP amplitude, suggesting that spread ofexcitation and amplitude changes are different phenomena and also indicating the need for adequate monitoring even with stimulations at low frequencies.
Abstract: We studied the effects of low-frequency transcranial magnetic stimulation (TMS) on motor cortex excitability in humans. TMS at 0.1 Hz for 1 hour did not change cortical excitability. Stimulation at 0.9 Hz for 15 minutes (810 pulses), similar to the parameters used to induce long-term depression (LTD) in cortical slice preparations and in vivo animal studies, led to a mean decrease in motor evoked potential (MEP) amplitude of 19.5%. The decrease in cortical excitability lasted for at least 15 minutes after the end of the 0.9 Hz stimulation. The mechanism underlying this decrease in excitability may be similar to LTD. TMS-induced reduction of cortical excitability has potential clinical applications in diseases such as epilepsy and myoclonus. Spread of excitation, which may be a warning sign for seizures, occurred in one subject and was not accompanied by increased MEP amplitude, suggesting that spread of excitation and amplitude changes are different phenomena and also indicating the need for adequate monitoring even with stimulations at low frequencies.
2,013 citations
TL;DR: The scale proposed involves a compartimentalized quantification of postural and stance disorders, limb ataxia, dysarthria and oculomotor disorders, in order that a subscore concerning these symptoms may be separately studied.
1,162 citations
TL;DR: It is concluded that blindness from an early age can cause the visual cortex to be recruited to a role in somatosensory processing and proposed that this cross-modal plasticity may account in part for the superior tactile perceptual abilities of blind subjects.
Abstract: Functional imaging studies of people who were blind from an early age have revealed that their primary visual cortex can be activated by Braille reading and other tactile discrimination tasks1. Other studies have also shown that visual cortical areas can be activated by somatosensory input in blind subjects but not those with sight2,3,4,5,6,7. The significance of this cross-modal plasticity is unclear, however, as it is not known whether the visual cortex can process somatosensory information in a functionally relevant way. To address this issue, we used transcranial magnetic stimulation to disrupt the function of different cortical areas in people who were blind from an early age as they identified Braille or embossed Roman letters. Transient stimulation of the occipital (visual) cortex induced errors in both tasks and distorted the tactile perceptions of blind subjects. In contrast, occipital stimulation had no effect on tactile performance in normal-sighted subjects, whereas similar stimulation is known to disrupt their visual performance. We conclude that blindness from an early age can cause the visual cortex to be recruited to a role in somatosensory processing. We propose that this cross-modal plasticity may account in part for the superior tactile perceptual abilities of blind subjects.
870 citations
TL;DR: These placebo-controlled results suggest that daily left prefrontal repetitive transcranial magnetic stimulation has antidepressant activity when administered at these parameters.
Abstract: Objective: Preliminary studies have indicated that daily left prefrontal repetitive transcranial magnetic stimulation might have antidepressant activity. The authors sought to confirm this finding by using a double-blind crossover design. Method: Twelve depressed adults received in random order 2 weeks of active treatment (repetitive transcranial magnetic stimulation, 20 Hz at 80% motor threshold) and 2 weeks of sham treatment. Results: Changes from the relevant phase baseline in scores on the 21-item Hamilton depression scale showed that repetitive transcranial magnetic stimulation significantly improved mood over sham treatment. During the active-treatment phase, Hamilton depression scale scores decreased 5 points, while during sham treatment the scores increased or worsened by 3 points. No adverse effects were noted. Conclusions: These placebo-controlled results suggest that daily left prefrontal repetitive transcranial magnetic stimulation has antidepressant activity when administered at these parameters. Further controlled studies are indicated to explore optimal stimulation characteristics and location, potential clinical applications, and possible mechanisms of action. (Am J Psychiatry 1997; 154:1752‐1756)
524 citations
TL;DR: The left hemisphere plays a greater role in timing ipsilateral complex sequences than the right hemisphere and may be more involved in the processing of complex motor programs.
Abstract: Functional imaging and behavioral studies suggest involvement of the ipsilateral hemisphere in hand movements, particularly of the left hand. If this is so, transient disturbance of the motor cortex (M1) with repetitive transcranial magnetic stimulation (rTMS) may affect ipsilateral motor sequences, and the effects may differ on the two sides. We studied 15 right-handed subjects who played a simple and a complex piano sequence for 8 seconds each. Two seconds after the beginning of each sequence, rTMS was delivered to the ipsilateral or contralateral M1, or directed away from the head (control trial). Ipsilateral M1 stimulation on either side induced timing errors in both sequences, and with the complex sequence induced more timing errors in the left hand than in the right hand. Errors of the right hand with both sequences occurred in the stimulation period only, but errors of the left hand with the complex sequence occurred in both the stimulation and poststimulation periods. We conclude that the ipsilateral M1 is involved in fine finger movements. The left hemisphere plays a greater role in timing ipsilateral complex sequences than the right hemisphere and may be more involved in the processing of complex motor programs.
323 citations
TL;DR: High-frequency repetitive transcranial magnetic stimulation (rTMS) was used to study the role of the mesial frontocentral cortex (including the supplementary motor area) in the organization of sequential finger movements of different complexity in humans, indicating a critical role in the organizations of forthcoming movements in complex motor sequences that are rehearsed from memory and fit into a precise timing plan.
Abstract: We used high-frequency repetitive transcranial magnetic stimulation (rTMS) to study the role of the mesial frontocentral cortex (including the supplementary motor area) in the organization of sequential finger movements of different complexity in humans. In 15 subjects, rTMS was randomly applied to the scalp overlying the region of the supplementary motor area and over other positions, including the contralateral primary motor cortex (hand area) during the performance of three overlearned finger sequences on an electronic piano. In all trials, rTMS (frequency 15-20 Hz) started 2 s after the first key press and lasted for approximately 2 s. All sequences were metronome-paced at 2 Hz and retrieved from memory. The 'simple' sequence consisted of 16 repeated index finger key presses, the 'scale' sequence of four times four sequential key presses of the little, ring, middle and index fingers, and the 'complex' sequence of a much less systematic and, therefore, more difficult series of 16 key presses. To measure the effects of rTMS interference with regional cortical function, we analysed rTMS-induced accuracy errors in the movement sequences. Stimulation over the supplementary motor area induced accuracy errors only in the complex sequence, while stimulation over the primary motor cortex induced errors in both the complex and scale sequences, and stimulation over other positions (e.g. F3, F4, FCz, P3, P4) did not interfere with sequence performance at all. Stimulation over the supplementary motor area interfered with the organization of subsequent elements in the complex sequence of movements, with error induction occurring approximately 1 s later than with stimulation over the primary motor cortex. Our findings are in keeping with recent results in non-human primates (Tanji J, Shima K. Nature, 1994; 371: 413-6) indicating a critical role of the supplementary motor area in the organization of forthcoming movements in complex motor sequences that are rehearsed from memory and fit into a precise timing plan.
322 citations
TL;DR: Findings implicate the frontal lobes in control of movement planning and execution and the involvement of different frequency bands with different timings may represent parallel changes in the cortical network.
308 citations
TL;DR: Induction of a seizure in a normal subject with trains of repetitive transcranial magnetic stimulation applied in close succession suggested that short inter-train intervals, a parameter not considered in previous safety studies, may not be safe.
Abstract: Induction of a seizure in a normal subject with trains of repetitive transcranial magnetic stimulation (rTMS) applied in close succession suggested that short inter-train intervals, a parameter not considered in our previous safety studies, may not be safe Here, we evaluate the safety of different inter-train intervals for rTMS in 10 healthy volunteers Ten rTMS trains at 20 Hz for 16 s and a stimulus intensity of 110% of motor threshold (MT) were found to be safe at the inter-train interval of 5 s However, inter-train intervals of 1 s or less were unsafe for trains of 20 Hz for 16 s and stimulus intensities higher than 100% of MT Based on these results, we propose safety guidelines for inter-train intervals at different stimulus intensities We also analyzed the stimulus parameters, used in 3 studies, that led to seizures in normal subjects One seizure was due to short inter-train intervals, one was likely related to intense individual rTMS trains close to the limit of our previous safety recommendations, and one was likely due to a combination of these two factors To provide an additional safety margin, we suggest reducing the duration for individual rTMS trains by 25% from our previous recommendations Updated safety tables currently in use at our institution are provided
238 citations
TL;DR: The inhibitory effects induced by magnetic stimulation are reduced in patients with writer's cramp, but only on the symptomatic side during muscle activation, which may relate to the overflow of muscle activity that characterizes this condition.
Abstract: We used paired transcranial magnetic stimulation (TMS) to evaluate inhibitory mechanisms in eight patients with writer's cramp during rest and isometric wrist extension. Both stimuli were 110% of the motor threshold; the interstimulus intervals (ISIs) were 20 to 200 ms in increments of 10 ms. Surface EMG was recorded from wrist extensors. In the symptomatic hemisphere, there was no significant difference in the amplitude of the test (second) motor evoked-potential (MEP) between patients and age-matched controls at rest. However, with voluntary muscle activation, inhibition of the test MEP by the conditioning MEP was significantly less in writer's cramp patients than in controls (p = 0.02). The difference was most prominent at ISIs of 60 to 80 ms in which inhibition is maximum. In the asymptomatic hemisphere, there was no significant difference between patients and controls in both rest and active conditions. The silent period was shorter in patients than controls on the symptomatic side (p = 0.003) but not on the asymptomatic side. We conclude that the inhibitory effects induced by magnetic stimulation are reduced in patients with writer's cramp, but only on the symptomatic side during muscle activation. This may relate to the overflow of muscle activity that characterizes this condition.
223 citations
TL;DR: The predicted rCBF changes in premotor and prefrontal areas during arbitrary mapping tasks are confirmed and suggest that a broad frontoparietal network may show decreased synaptic activity as arbitrary rules become more familiar.
Abstract: Deiber, M-P, S P Wise, M Honda, M J Catalan, J Grafman, and M Hallett Frontal and parietal networks for conditional motor learning: a positron emission tomography study J Neurophysiol
217 citations
TL;DR: Phenytoin elevated motor thresholds to transcranial magnetic stimulation in all subjects, but had no effects on motor-evoked potential amplitudes, silent period durations, and intracortical excitability tested by paired TMS during rest and voluntary muscle activation.
Abstract: We studied the effects of a loading dose of phenytoin on motor cortex excitability in five healthy volunteers. Phenytoin elevated motor thresholds to transcranial magnetic stimulation (TMS) in all subjects, but had no effects on motor-evoked potential amplitudes, silent period durations, and intracortical excitability tested by paired TMS during rest and voluntary muscle activation. These results are consistent with the hypothesis that blockade of voltage-gated sodium channels decreases membrane excitability and elevates the threshold to TMS, but will not reduce intracortical excitability.
TL;DR: PD patients' underestimation of the amplitudes of finger perturbations was significantly greater than that of normal subjects, but not for task K; this implies that the sensorimotor apparatus is “set” smaller in PD patients than in normal subjects.
Abstract: Kinesthesia may be defective in patients with Parkinson's disease (PD), and this defect conceivably has a role in parkinsonian hypokinetic symptoms. In the present study, PD patients used kinesthetic perception to estimate the amplitude of passive angular displacements of the index finger about the metacarpophalangeal joint and to scale them as a percentage of a reference stimulus. The reference stimulus was either a standard kinesthetic stimulus preceding each test stimulus (task K) or a visual representation of the standard kinesthetic stimulus (task V). In task V, the PD patients' underestimation of the amplitudes of finger perturbations was significantly greater than that of normal subjects, but not for task K. PD patients' underestimation was also greater in task V than in task K; the difference between the underestimations was significantly greater than for normal subjects. These results suggest that, when kinesthesia is used to match a visual target, distances are perceived to be shorter by the PD patients. Assuming that visual perception is normal, kinesthesia is "reduced" in PD patients. This reduced kinesthesia, when combined with the well-known reduced motor output and probably reduced corollary discharges, implies that the sensorimotor apparatus is "set" smaller in PD patients than in normal subjects.
TL;DR: The combination of progressively increasing signal intensity with an area that increases to 2 Hz and declines at faster frequencies explains the PET finding of plateau of rCBF at the faster frequencies.
Abstract: To evaluate the effect of the repetition rate of a simple movement on the magnitude of neuronal recruitment in the primary sensorimotor cortex, we used a blood flow-sensitive, echo planar functional magnetic resonance imaging (fMRI) sequence in six normal volunteers. Three of the volunteers also had [15O]water positron emission tomography (PET) studies using the same paradigm. Previous PET studies had shown an increase in regional CBF (rCBF) with movement frequencies up to 2 Hz and then a plateau of regional cerebral blood flow (rCBF) at faster frequencies. To evaluate the extent of the activation, the correlation coefficient (cc) of the Fourier-transformed time-signal intensity change with the Fourier-transformed reference function was calculated pixel by pixel. The degree of activation was measured as the signal percent change of each region of interest with a cc > 0.5. The left primary sensorimotor cortex was constantly activated at 1, 1.5, 2, and 4 Hz, while there was only inconsistent activation at 0.25 and 0.5 Hz. Percent change in signal intensity linearly increased from 1 to 4 Hz. Area of activation increased up to 2 Hz and showed a tendency to decrease at higher frequencies. Individual analysis of PET data showed activation in the same location as that revealed by fMRI. The combination of progressively increasing signal intensity with an area that increases to 2 Hz and declines at faster frequencies explains the PET finding of plateau of rCBF at the faster frequencies. Functional magnetic resonance imaging shows similar results to PET, but is better able to dissociate area and magnitude of change.
TL;DR: The results suggest that the visual and motor components of reaching may have a different functional organization and that many brain regions represent both limb of reach and field of reach.
Abstract: Positron emission tomography (PET) was used to identify the brain areas involved in visually guided reaching by measuring regional cerebral blood flow (rCBF) in six normal volunteers while they were fixating centrally and reaching with the left or right arm to targets presented in either the right or the left visual field. The PET images were registered with magnetic resonance images from each subject so that increases in rCBF could be localized with anatomical precision in individual subjects. Increased neural activity was examined in relation to the hand used to reach, irrespective of field of reach (hand effect), and the effects of target field of reach, irrespective of hand used (field effect). A separate analysis on intersubject, averaged PET data was also performed. A comparison of the results of the two analyses showed close correspondence in the areas of activation that were identified. We did not find a strict segregation of regions associated exclusively with either hand or field. Overall, significant rCBF increases in the hand and field conditions occurred bilaterally in the supplementary motor area, premotor cortex, cuneus, lingual gyrus, superior temporal cortex, insular cortex, thalamus, and putamen. Primary motor cortex, postcentral gyrus, and the superior parietal lobule (intraparietal sulcus) showed predominantly a contralateral hand effect, whereas the inferior parietal lobule showed this effect for the left hand only. Greater contralateral responses for the right hand were observed in the secondary motor areas. Only the anterior and posterior cingulate cortices exhibited strong ipsilateral hand effects. Field of reach was more commonly associated with bilateral patterns of activation in the areas with contralateral or ipsilateral hand effects. These results suggest that the visual and motor components of reaching may have a different functional organization and that many brain regions represent both limb of reach and field of reach. However, since posterior parietal cortex is connected with all of these regions, we suggest that it plays a crucial role in the integration of limb and field coordinates.
TL;DR: There is increasing evidence from studies using different methodologies such as rTMS, functional imaging and movement-related cortical potentials, that M1 is involved in ipsilateral hand movements, with greater involvement in more complex tasks and the left hemisphere playing a greater role than the right.
Abstract: The ipsilateral primary motor cortex (M1) plays a role in voluntary movement. In our studies, we used repetitive transcranial magnetic stimulation (rTMS) to study the effects of transient disruption of the ipsilateral M1 on the performance of finger sequences in right-handed normal subjects. Stimulation of the M1 ipsilateral to the movement induced timing errors in both simple and complex sequences performed with either hand, but with complex sequences, the effects were more pronounced with the left-sided stimulation. Recent studies in both animals and humans have confirmed the traditional view that ipsilateral projections from M1 to the upper limb are mainly directed to truncal and proximal muscles, with little evidence for direct connections to distal muscles. The ipsilateral motor pathway appears to be an important mechanism for functional recovery after focal brain injury during infancy, but its role in functional recovery for older children and adults has not yet been clearly demonstrated. There is increasing evidence from studies using different methodologies such as rTMS, functional imaging and movement-related cortical potentials, that M1 is involved in ipsilateral hand movements, with greater involvement in more complex tasks and the left hemisphere playing a greater role than the right.
TL;DR: Electrophysiologic findings support previous studies which have demonstrated that motor activity changes as behavior changes over the course of learning.
TL;DR: The efficacy of surgery in the case of hemiballismus demonstrates that pallidotomy can be an effective treatment for this condition and suggests that patterned neuronal activity in the GPi is important in the mechanism of hyperkinetic disorders.
Abstract: A patient with unremitting, medically intractable hemiballismus underwent a pallidotomy that abolished his involuntary movements. Firing rates of cells in the internal segment of the globus pallidus (GPi) recorded during this procedure were significantly lower than those observed during pallidotomy for Parkinson's disease, either "on" or "off" medication. Firing patterns in hemiballismus were characterized by low-frequency modulation of the firing rate. These results are consistent with the hyperkinetic model, which suggests that hemiballismus results from decreased inhibition of the pallidal relay nucleus of the thalamus by the GPi. The efficacy of surgery in the case of hemiballismus demonstrates that pallidotomy can be an effective treatment for this condition and suggests that patterned neuronal activity in the GPi is important in the mechanism of hyperkinetic disorders.
TL;DR: Homosynaptic depression localized at the presynaptic terminal seems to be the mechanism behind the H-reflex depression in humans.
Abstract: The H-reflex is depressed for seconds if elicited following a single H-reflex or train of H-reflexes Presynaptic inhibition from flexor afferents (tibialis anterior) onto soleus Ia afferents elicited by either single or trains of stimuli had no effect on the soleus H-reflex on a time scale of seconds Postsynaptic inhibition was also excluded by magnetic stimulation tests that showed that the excitability of the motoneuron pool was not changed at latencies within a range of seconds Homosynaptic depression localized at the presynaptic terminal seems to be the mechanism behind the H-reflex depression in humans
TL;DR: Analysis of muscle recruitment patterns when the same focal movement was made under behavioral conditions of a self-paced task and a reaction-time task supports the theory that anticipatory postural adjustments are planned in detail.
TL;DR: The anterior supplementary motor area and rostral cingulate motor area in the midline, prefrontal cortices bilaterally, and lobus parietalis inferior on the ipsilateral side were more active during the self-paced movements and increases in rCBF in those areas may be related to the increased time devoted to planning the movement in this condition.
Abstract: To evaluate the hypothesis that self-paced movements are mediated primarily by the supplementary motor area, whereas externally triggered movements are mainly affected by the lateral premotor cortex, different movements in 6 healthy volunteers were studied while changes in regional cerebral blood flow (rCBF) were measured using positron emission tomography (PET) and 15O-labeled water. Subjects made a series of finger opposition movements initiated in a self-paced manner every 4 to 6 seconds, and separately, made continuous finger opposition movements at a frequency of 2 Hz paced by a metronome. The primary motor cortex, lateral area 6, cerebellum on both sides, and caudal cingulate motor area, and the putamen and thalamus on the contralateral side were more active during the metronome-paced movements. The increases in rCBF in these areas are likely the result of the larger number of movements per minute made with the externally triggered task. The anterior supplementary motor area and rostral cingulate motor area in the midline, prefrontal cortices bilaterally, and lobus parietalis inferior on the ipsilateral side were more active during the self-paced movements. Increases in rCBF in those areas, which include medial premotor structures, may be related to the increased time devoted to planning the movement in this condition.
TL;DR: Evidence from behavioral studies of patients with cerebellar atrophy implies that the cerebellum plays a role in visuomotor learning and adaptation, planning, strategic thinking, time processing, and associative learning and substantiates the hypothesis that the Cerebellum acts in concert with other structures as part of a frontal subcortical system devoted to the storage and organization of timed sequential behaviors.
Abstract: Publisher Summary Evidence from behavioral studies of patients with cerebellar atrophy implies that the cerebellum plays a role in visuomotor learning and adaptation, planning, strategic thinking, time processing, and associative learning. Evidence from studies using functional neuroimaging supports this implication and substantiates the hypothesis that the cerebellum acts in concert with other structures as part of a frontal subcortical system devoted to the storage and organization of timed sequential behaviors. The role of the cerebellum in timed sequential cognitive processing may be analogous to its role in motor processing and suggests a mechanism by which cognitive events become sequenced and temporally labeled. Motor learning is a complex phenomenon with many components. Depending on the particular task, different anatomical structures are involved. The cerebellum takes the principal part in adaptation learning. The role of the cerebellum in timed sequential cognitive processing is analogous to its role in motor processing and suggests a mechanism by which cognitive events become sequenced and temporally labeled.
TL;DR: It is concluded that post-exercise depression is detectable only after prolonged muscle activation in MEPs elicited to transcranial magnetic stimulation.
Abstract: Post-exercise facilitation and post-exercise depression are phenomena described in motor evoked potentials (MEPs) elicited to transcranial magnetic stimulation. Brief, non-fatiguing muscle activation produces post-exercise facilitation, and prolonged fatiguing muscle activation produces post-exercise depression. We studied 12 normal subjects to determine whether post-exercise depression occurs before fatigue is reached. We recorded MEPs from the resting extensor carpi radialis muscle after increasing the duration of isometric wrist extension, at 50% of maximum voluntary contraction, until the muscle fatigued. Fatigue was defined as the inability to maintain that force. The mean exercise duration before the muscle fatigued was 130 s, and post-exercise depression occurred only beyond 90 s of exercise. We conclude that post-exercise depression is detectable only after prolonged muscle activation.
TL;DR: Steady-state MRCPs require actual recording times of less than 10 min, and show excellent inter-session reproducibility, which may make them convenient for studying sensorimotor cortex activity experimentally and clinically.
TL;DR: To determine the presence of post-exercise facilitation after exercise of a contralateral muscle, MEPs were recorded from the resting right extensor carpi radialis (ECR) muscle while the left ECR muscle was activated, then immediately after brief left E CR activation, and, finally, immediately after Brief right ECR activation.
Abstract: We have previously described post-exercise facilitation and post-exercise depression of motor evoked potentials (MEPs) to transcranial magnetic stimulation (TMS). To determine the presence of post-exercise facilitation after exercise of a contralateral muscle, MEPs were recorded from the resting right extensor carpi radialis (ECR) muscle while the left ECR muscle was activated, then immediately after brief left ECR activation, and, finally, immediately after brief right ECR activation. We repeated the experiment using the first dorsal interosseous (FDI) muscle. To determine the presence of post-exercise depression after exercise of a contralateral muscle, MEPs were recorded from the right ECR after prolonged exercise of the left ECR, followed by right ECR recording after its fatigue. The mean MEP amplitudes from the right ECR and the right FDI after brief activation were 187% and 266% of their pre-exercise values, respectively. There were no significant changes in MEPs recorded from the right ECR or FDI muscles during or immediately after brief activation of their left counterparts. The mean amplitude of MEPs recorded from the right ECR after it fatigued was approximately half the pre-exercise value, but there was no significant change in MEPs recorded from the right ECR after prolonged exercise of the left ECR. Therefore, neither post-exercise facilitation nor post-exercise depression occurred after contralateral homologous muscle exercise.
TL;DR: Simulation studies evaluated the effects of varying the injected dose, speed of injection, and scan duration to maximize the sensitivity of noninvasive activation studies with 15O-water and three-dimensional positron emission tomography and found that a slow infusion permits longer scan acquisitions with only a small loss in S/N.
Abstract: We investigated the effects of varying the injected dose, speed of injection, and scan duration to maximize the sensitivity of noninvasive activation studies with 15O-water and three-dimensional positron emission tomography. A covert word generation task was used in four subjects with bolus injections of 2.5 to 30 mCi of 15O-water. The noise equivalent counts (NEC) for the whole brain peaked at an injected dose of 12 to 15 mCi. This was lower than expected from phantom studies, presumably because of the effect of radioactivity outside of the brain. A 10 mCi injection gave an NEC of 92.4 ± 2.2% of the peak value. As the scan duration increased from 60 to 90 to 120 seconds, the areas of activation decreased in size or were no longer detected. Therefore, we selected a 1 minute scan using 10 mCi for bolus injections. We then performed simulation studies to evaluate, for a given CBF change, the effect on signal-to-noise ratio (S/N) of longer scan duration with slow tracer infusions. Using a measured arterial input function from a bolus injection, new input functions for longer duration injections and the corresponding tissue data were simulated. Combining information about image noise derived from Hoffman brain phantom studies with the simulated tissue data allowed calculation of the S/N for a given CBF change. The simulation shows that a slow infusion permits longer scan acquisitions with only a small loss in S/N. This allows the investigator to choose the injection duration, and thus the time period during which scan values are sensitive to regional CBF.
TL;DR: Because of the limited time available to perfomr the kinesthetic triggering task, the role for cutaneous mechanoreceptors, to provide singals for on-line coordination of movement appears limited compared with muscle spindle afferents.
Abstract: A coordinated triggering task requiring use of kinesthetic information was employed to assess the timing of use of kinesthetic information in normal subjects and patients with cerebellar dysfunction. Passive movements of varying velocity were imposed in the flexor direction about the metacarpophalangeal joint of the right index finger. Subjects attempted to depress a switch with their left thumb when the index finger moved, past a specified angle that was learned during a training session. The velocities ranged from 10°/s to 88°/s in 2°/s increments. After 200 trials, subjects were then instructed instead to react as quickly as possible (reaction-time task) to the onset of movement for an additional 200 trials. For the same movements, the timing of onset of responses of muscle spindle afferents and cutaneous mechanoreceptors was determined by recording the responses of these afferents using microneurography. For slow velocities, patients were able to perform similarly to normals but at faster velocities patients triggered too late compared with normals. Patients required more time to use kinesthetic information than did normal subjects. An estimate of kinesthetic processing was not longer in patients. The chief explanation for the prolonged time required to use kinesthetic information in patients was that their reaction times were prolonged by 93 ms. In addition, the movement time was also prolonged, but this accounted for only 23 ms. Impaired motor performance in tasks requiring the use of kinesthetic information in cerebellar patients can be explained largely by their prolonged reaction times. Muscle spindle afferents responded on average much sooner than cutaneous mechanoreceptors. Because of the limited time available to perfomr the kinesthetic triggering task, the role for cutaneous mechanoreceptors, to provide singals for on-line coordination of movement appears limited compared with muscle spindle afferents.
TL;DR: It is concluded that in patients with cerebellar degeneration, postex exercise MEP facilitation is significantly reduced, whereas postexercise MEP depression after fatigue is similar to that of normal subjects.
Abstract: We studied the effects of exercise on motor evoked potentials (MEPs) elicited by transcranial magnetic stimulation (TMS) in 18 normal (control) subjects and 11 medication-free patients with cerebellar degeneration. Subjects performed repeated sets of isometric exercise of the extensor carpi radialis muscle until the muscle fatigued (subject became unable to maintain half maximal force). MEPs were recorded before and after each exercise set and for up to 30 minutes after the last set. The mean amplitude of MEPs recorded from the resting muscle immediately after each exercise set was 218% of the mean preexercise MEP amplitude in normal subjects and 132% in cerebellar patients, indicating postexercise MEP facilitation in both groups. However, postexercise MEP facilitation, compared with the mean preexercise MEP amplitude, was not significant in the patients but was significant in the normal subjects. The amplitudes of MEPs recorded within the first few minutes after fatigue were 44% of the mean preexercise MEP amplitudes in both groups. We conclude that in patients with cerebellar degeneration, postexercise MEP facilitation is significantly reduced, whereas postexercise MEP depression after fatigue is similar to that of normal subjects.
TL;DR: The different effects of TES and TMS on the 2 phases of reciprocal inhibition provide evidence of the presynaptic nature of the second phase, and in both cases the disinhibitory effects had essentially the same time course as the facilitatory effect of T MS on the uninhibited H-reflex.
Abstract: We studied the effects of transcranial electrical stimulation (TES) and transcranial magnetic stimulation (TMS), delivered at intensities below the threshold for evoking an electromyographic response, on the disynaptic and presynaptic phases of reciprocal inhibition in 8 healthy subjects. After TES, the H-reflex evoked in the flexor carpi radialis (FCR) muscle was strongly facilitated when the cortical stimulus was given 4.0-4.5 ms after the test stimulus (median nerve stimulus). TES reduced the disynaptic phase of reciprocal inhibition most strongly when the cortical stimulus followed the test stimulus by 3.0-3.5 ms. TES also reduced presynaptic inhibition, but with a time course that was identical to that of the facilitation of the uninhibited H-reflex. After subthreshold TMS, the facilitation of the H-reflex showed at least 2 peaks, one occurring when the cortical stimulus was given 2 ms after the test stimulus and the other when the cortical stimulus followed the test stimulus by 0.5 to -1.5 ms. The effects of TMS on the 2 phases of reciprocal inhibition were similar, and in both cases the disinhibitory effects had essentially the same time course as the facilitatory effect of TMS on the uninhibited H-reflex. The different effects of TES on the 2 phases of reciprocal inhibition provide evidence of the presynaptic nature of the second phase. The absence of a difference in the effect of TMS on the 2 phases could be due to the more temporally dispersed descending volley after TMS.