The effect of repetitive magnetic stimulation on localized musculoskeletal pain

Current magnetic coil stimulators can efficiently activate neural structures without deep electrode placement and the local discomfort associated with transcutaneous electrical stimulation used in pain control. We tested the possibility of reducing pain in patients with localized musculoskeletal processes by applying repetitive magnetic stimulation on the tender body region. Thirty patients were randomized to receive 40 min of real or sham magnetic stimulation. After a single session, real magnetic stimulation significantly exceeded the sham effect: a 101-point pain score decreased by 59% in the treated group and 14% in sham-treated patients (z = -3.26, p = 0.001). The pain relief attained regularly persisted for several days. Results indicate that powerful magnetic coil stimulation can efficiently reduce pain originating from localized musculoskeletal injuries.

Effect of focal cerebellar lesions on procedural learning in the serial reaction time task

Prior studies have shown that procedural learning is severely impaired in patients with diffuse cerebellar damage (cortical degeneration) as measured by the serial reaction time task (SRTT). We hypothesize that focal cerebellar lesions can also have lateralized effects on procedural learning. Our objective was to assess the effects of focal cerebellar lesions in procedural learning as measured by the SRTT. We studied 14 patients with single, unilateral vascular lesions in the territory of the posterior-inferior or superior cerebellar artery, who were compared with ten age- and sex-matched controls in a one-handed version of the SRTT. Patients with lesions at any other level of the brain or posterior fossa were excluded by cranial magnetic resonance imaging. Our results show that patients do not acquire procedural knowledge when performing the task with the hand ipsilateral to the lesion, but show normal learning with the contralateral hand. No correlation was found with the side, size, or vascular territory of the lesion. We conclude that procedural learning is impaired in hemispheric cerebellar lesions and involves only the hand ipsilateral to the lesion, which suggests a critical role for the cerebellum and/or crossed cerebellar-prefrontal connections in this type of learning.

Linguistic processing during repetitive transcranial magnetic stimulation

To determine if linguistic processing could be selectively disrupted with repetitive transcranial magnetic stimulation (rTMS), rTMS was performed during a picture-word verification task. Seven right-handed subjects were trained in two conditions: picture-word verification, which required the subject to verify whether the picture of an object matched the subtitle name on the same page, and frame verification, which required subjects to verify whether there was a rectangular frame around the combined object picture and subtitle. Half of the trials were performed during rTMS. The effects of rTMS on performance were evaluated at the following four scalp positions: left anterior (the area where rTMS produced speech arrest), a mirror site on the right, and two positions in the left and right parietal region. Stimulation over the left deltoid muscle served as a control. Subjects had less difficulty in making picture-word matching decisions during unstimulated compared with stimulated trials at the left anterior and posterior positions. No significant difference in accuracy was detected in the frame verification condition, but response times in the frame verification condition were longer with stimulation at the left anterior position. Because rTMS of the dominant hemisphere affected linguistic processing independent of speech motor output, we confirm that rTMS may be used to investigate language and other cognitive functions.

Functional relevance of cross-modal plasticity in blind humans

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 tasks. Other studies have also shown that visual cortical areas can be activated by somatosensory input in blind subjects but not those with sight. 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.

Lateralized effects of self-induced sadness and happiness on corticospinal excitability

We studied the changes in excitability of the corticospinal projection evoked by self-induced sad and happy thoughts. Corticospinal excitability was probed using focal, single-pulse transcranial magnetic stimulation (TMS) applied to the optimal scalp position for evoking motor potentials in the contralateral first dorsal interosseus muscle. Fourteen right-handed subjects were studied while counting mentally, thinking sad thoughts, or thinking happy thoughts. In each of these three conditions TMS was applied in each subject randomly, 20 times to the right and 20 times to the left hemisphere. Sad thoughts resulted in a significant facilitation of the motor potentials evoked by left-hemispheric stimulation, while happy thoughts facilitated motor potentials evoked by right-hemispheric TMS, but decreased the amplitude of those evoked by left-hemispheric TMS. In two subjects an additional experiment using H-reflex measurements suggests that these changes are caused by changes in cortical rather than spinal excitability. These results further illustrate the lateralized control of mood in normal volunteers.

Melatonin levels in Parkinson's disease: drug therapy versus electrical stimulation of the internal globus pallidus

The objective of our work was to measure plasma melatonin levels in patients with Parkinson's Disease (PD) following electrical stimulation of the internal globus pallidus (GPi), and to compare these levels with groups of PD patients under drug therapy and healthy controls. The levels of melatonin were measured twice daily at 1000 and 1200. The GPi stimulation at 130 Hz lowered melatonin levels, while no changes were observed in the absence of stimulation. The melatonin levels from healthy subjects were lower than those observed in PD patients. The melatonin levels from PD patients under drug therapy were also measured during the night (2000-2400-0400) and at 0800 in order to observe their circadian changes. The Internal Globus Pallidus (GPi) stimulation was effective in lowering the melatonin levels during the day and, therefore returned these levels to those observed in normal subjects.

Transcranial magnetic stimulation as therapy for depression and other disorders

OBJECTIVE

To provide an overview of the progress and prospects of transcranial magnetic stimulation as a psychiatric therapy for depression.

METHOD

Published and unpublished studies of the usefulness of transcranial magnetic stimulation as a therapy for depression were assessed, and characterised in terms of a consistent measure of dosage. Additional information was obtained through correspondence, personal meetings and visits to facilities.

RESULTS

Transcranial magnetic stimulation, a means for inducing small regional currents in the brain, has been used in clinical neurology for some time, and can be used on conscious subjects with minimal side-effects. Early researchers noticed transient mood effects on people receiving this treatment, which prompted several inconclusive investigations of its effects on depressed patients. More recently, knowledge of functional abnormalities associated with depression has led to trials using repetitive transcranial magnetic stimulation to stimulate underactive left prefrontal regions, an approach which has produced short-term benefits for some subjects. The higher dosage delivered by high-frequency repetitive transcranial magnetic stimulation appears to produce greater benefits; scope exists for more conclusive studies based on extended treatment periods.

CONCLUSIONS

Repetitive transcranial magnetic stimulation is a promising technology. The reviewed evidence indicates that it may be useful in the treatment of depression, and perhaps other disorders which are associated with regional hypometabolism. Should repetitive transcranial magnetic stimulation prove an effective, non-invasive, drug-free treatment for depression, a range of disorders could be similarly treatable.

Reorganization of human cortical motor output maps following traumatic forearm amputation

We report the results of serial transcranial magnetic stimulation mapping of cortical motor outputs to the face and upper extremity in a subject studied before and repeatedly after traumatic amputation of the right arm immediately below the elbow. The results of the mapping studies illustrate the time course of plastic changes in the motor cortical representation in humans following a traumatic amputation and allow the correlation of subjective perceptions of phantom limbs with the reorganization of cortical outputs.

Rapid-rate transcranial magnetic stimulation of left dorsolateral prefrontal cortex in drug-resistant depression

BACKGROUND

Lesion and neuroimaging studies suggest that left prefrontal lobe dysfunction is pathophysiologically linked to depression. Rapid-rate transcranial magnetic stimulation (rTMS) to prefrontal structures has a lateralised effect on mood in normal volunteers, and several preliminary studies suggest a beneficial effect of rTMS on depression. However, adequately controlled studies have not been conducted.

METHODS

We have studied the effects of focal rTMS on the depressive symptoms in 17 patients with medication-resistant depression of psychotic subtype. The study was designed as a multiple cross-over, randomised placebo-controlled trial. Sham rTMS and stimulation of different cortical areas were used as controls.

FINDINGS

Left dorsolateral prefrontal cortex rTMS resulted in a significant decrease in scores on the Hamilton depression rating scale HDRS (from 25.2 to 13.8) and the self-rated Beck questionnaire BQ (from 47.9 to 25.7). 11 of the 17 patients showed pronounced improvement that lasted for about 2 weeks after 5 days of daily rTMS sessions. No patient experienced any significant undesirable side-effects.

INTERPRETATION

Our findings emphasise the role of the left dorsolateral prefrontal cortex in depression, and suggest that rTMS of the left dorsolateral prefrontal cortex might become a safe, non-convulsive alternative to electroconvulsive treatment in depression.

Activation of the primary visual cortex by Braille reading in blind subjects

Primary visual cortex receives visual input from the eyes through the lateral geniculate nuclei, but is not known to receive input from other sensory modalities. Its level of activity, both at rest and during auditory or tactile tasks, is higher in blind subjects than in normal controls, suggesting that it can subserve nonvisual functions; however, a direct effect of non-visual tasks on activation has not been demonstrated. To determine whether the visual cortex receives input from the somatosensory system we used positron emission tomography (PET) to measure activation during tactile discrimination tasks in normal subjects and in Braille readers blinded in early life. Blind subjects showed activation of primary and secondary visual cortical areas during tactile tasks, whereas normal controls showed deactivation. A simple tactile stimulus that did not require discrimination produced no activation of visual areas in either group. Thus in blind subjects, cortical areas normally reserved for vision may be activated by other sensory modalities.

Locating the motor cortex on the MRI with transcranial magnetic stimulation and PET

Transcranial magnetic stimulation with a focal coil was used to map the cortical representation of a hand muscle in four healthy subjects. In each subject, the three-dimensional locations of the magnetic stimulation positions and about 400 positions on the surface of the head were digitized. The amplitude-weighted center of gravity of each subject's map was found, and a line perpendicular to the local head surface was projected inward. The digitized heads were registered with the subjects' MRIs using the scalp contours. The coordinate transformations yielded by this process were used to map the stimulation positions and the perpendicular line into the MRIs. Brain areas imaged with positron emission tomography (PET) and 15O-labeled water, activated by movement of the same muscle, were registered with the MRIs using the brain contours. In all cases, the magnetic stimulation lines encountered the surface of the brain at the anterior lip of the central sulcus and ran along the precentral gyrus a few millimeters anterior to the central sulcus, coming within 5-22 mm of all the PET activation maxima. This technique demonstrates the accuracy of transcranial magnetic stimulation for locating the primary motor area.

Lateralized effect of rapid-rate transcranial magnetic stimulation of the prefrontal cortex on mood

We studied the effects of rapid-rate transcranial magnetic stimulation (rTMS) of different scalp positions on mood. Ten normal volunteers rated themselves before and after rTMS on five analog scales labeled "Tristeza" (Sadness), "Ansiedad" (Anxiety), "Alegria" (Happiness), "Cansancio" (Tiredness), and "Dolor/Malestar" (Pain/Discomfort). rTMS was applied to the right lateral prefrontal, left prefrontal, or midline frontal cortex in trains of 5 seconds' duration at 10 Hz and 110% of the subject's motor threshold intensity. Each stimulation position received 10 trains separated by a 25-second pause. No clinically apparent mood changes were evoked by rTMS to any of the scalp positions in any subject. However, left prefrontal rTMS resulted in a significant increase in the Sadness ratings (Tristeza) and a significant decrease in the Happiness ratings ("Alegria") as compared with right prefrontal and midfrontal cortex stimulation. These results show differential effects of rTMS of left and right prefrontal cortex stimulation on mood and illustrate the lateralized control of mood in normal volunteers.

Changes in mood and hormone levels after rapid-rate transcranial magnetic stimulation (rTMS) of the prefrontal cortex

Rapid-rate transcranial magnetic stimulation (rTMS) was administered to 10 healthy volunteers on different days over the right or left prefrontal cortex, midfrontal cortex, occipital cortex, or cerebellum. Mood (self-rated), reaction time, and hormone levels were serially measured. Consistent with a previous study, comparison of hemispheres revealed significant associations with decreased happiness after left prefrontal rTMS and decreased sadness after right prefrontal rTMS. Stimulation of all three prefrontal regions, but not the occipital or cerebellar regions, was associated with increases in serum thyroid-stimulating hormone. There was no effect on serum prolactin. rTMS applied to prefrontal cortex is safe and well tolerated and produces regionally and laterally specific changes in mood and neuroendocrine measures in healthy adults. rTMS is a promising tool for investigating prefrontal cortex functions.

The role of the dorsolateral prefrontal cortex in implicit procedural learning

We studied the role of the dorsolateral prefrontal cortex in procedural learning. Normal subjects completed several blocks of a serial reaction time task using only one hand without or with concurrent non-invasive repetitive transcranial magnetic stimulation. To disrupt their function transiently, stimulation was applied at low intensity over the supplementary motor area or over the dorsolateral prefrontal cortex contralateral or ipsilateral to the hand used for the test. Stimulation to the contralateral dorsolateral prefrontal cortex markedly impaired procedural implicit learning, as documented by the lack of significant change in response times during the task. Stimulation over the other areas did not interfere with learning. These results support the notion of a critical role of contralateral dorsolateral prefrontal structures in learning of motor sequences.

The role of reading activity on the modulation of motor cortical outputs to the reading hand in Braille readers

We studied the cortical motor output maps of the first dorsal interosseous (FDI) of both hands and the abductor digiti minimi of the reading hand in 6 blind proficient Braille readers. The maps were generated using transcranial magnetic stimulation. We compared the maps obtained on a day in which they worked as Braille proofreaders (reading Braille for approximately 6 hours) with the maps obtained on a day they took off from work. On the work day, the maps for the FDI of the reading hand were significantly larger after the working shift than in the morning after having been off work for 2 days. These changes were not seen for the same muscle on the day off work or on any of the 2 days in the other two muscles studied. These results illustrate the rapid modulation in motor cortical outputs in relation to preceding activity and emphasize the importance of precise timing in studies of the neurophysiological correlates of skill acquisition.

Role of intracortical mechanisms in the late part of the silent period to transcranial stimulation of the human motor cortex

Transcranial magnetic stimulation (TMS) and transcranial electrical stimulation (TES) of the human motor cortex produce a silent period (SP) following motor evoked potentials (MEPs). The early part of the SP can be explained by decreased alpha motor neuron excitability, whereas the late part is presumably due to suprasegmental mechanisms. In order to determine the level of the suprasegmental contribution of the generation of SPs, we recorded excitatory and inhibitory responses to TMS, TES and percutaneous electrical brainstem stimulation (PBS) in the voluntarily activated first dorsal interosseous muscle of the hand. Stimulus intensities were set so that PBS and TES induced MEPs with areas equal to or larger than those of MEPs obtained with TMS. This procedure revealed that SPs were 49% and 83% shorter with TES and PBS, respectively, than with TMS. As TMS is more effective than TES or PBS in activating cortical interneurons, these findings support the idea that a significant component of the SP arises from intracortical mechanisms.

Modulation of muscle responses evoked by transcranial magnetic stimulation during the acquisition of new fine motor skills

1. We used transcranial magnetic stimulation (TMS) to study the role of plastic changes of the human motor system in the acquisition of new fine motor skills. We mapped the cortical motor areas targeting the contralateral long finger flexor and extensor muscles in subjects learning a one-handed, five-finger exercise on the piano. In a second experiment, we studied the different effects of mental and physical practice of the same five-finger exercise on the modulation of the cortical motor areas targeting muscles involved in the task. 2. Over the course of 5 days, as subjects learned the one-handed, five-finger exercise through daily 2-h manual practice sessions, the cortical motor areas targeting the long finger flexor and extensor muscles enlarged, and their activation threshold decreased. Such changes were limited to the cortical representation of the hand used in the exercise. No changes of cortical motor outputs occurred in control subjects who underwent daily TMS mapping but did not practice on the piano at all (control group 1). 3. We studied the effect of increased hand use without specific skill learning in subjects who played the piano at will for 2 h each day using only the right hand but who were not taught the five-finger exercise (control group 2) and who did not practice any specific task. In these control subjects, the changes in cortical motor outputs were similar but significantly less prominent than in those occurring in the test subjects, who learned the new skill.(ABSTRACT TRUNCATED AT 250 WORDS)

Induction of errors in a delayed response task by repetitive transcranial magnetic stimulation of the dorsolateral prefrontal cortex

We used repetitive transcranial magnetic stimulation to study the function of the dorsolateral frontal cortex in a delayed response task in 10 normal volunteers. Unilateral stimulation of right or left prefrontal cortex during the delay period between instruction and execution cues led to a significantly greater number of response errors than stimulation of motor cortex or in unstimulated trials. Repetitive transcranial stimulation of appropriate intensity and frequency seems to disrupt the function of mnemonic cells in the prefrontal cortex and is useful for the study of the role of the prefrontal cortex in short-term motor memory.

Responses to rapid-rate transcranial magnetic stimulation of the human motor cortex

We applied trains of focal, rapid-rate transcranial magnetic stimulation (rTMS) to the motor cortex of 14 healthy volunteers with recording of the EMG from the contralateral abductor pollicis brevis, extensor carpi radialis, biceps brachii and deltoid muscles. Modulation of the amplitude of motor evoked potentials (MEPs) produced in the target muscle during rTMS showed a pattern of inhibitory and excitatory effects which depended on the rTMS frequency and intensity. With the magnetic coil situated over the optimal scalp position for activating the abductor pollicis brevis, rTMS led to spread of excitation, as evident from the induction of progressively larger MEPs in the other muscles. The number of pulses inducing this spread of excitation decreased with increasing rTMS frequency and intensity. Latency of the MEPs produced in the other muscles during the spread of excitation was significantly longer than that produced by single-pulse TMS applied to the optimal scalp positions for their activation. The difference in MEP latency could be explained by a delay in intracortical conduction along myelinated cortico-cortical pathways. Following rTMS, a 3-4 min period of increased excitability was demonstrated by an increase in the amplitude of MEPs produced in the target muscles by single-pulse TMS. Nevertheless, repeated rTMS trains applied 1 min apart led to similar modulation of the responses and to spread of excitation after approximately the same number of pulses. This suggests that the spread might be due to the breakdown of inhibitory connections or the recruitment of excitatory pathways, whereas the post-stimulation facilitation may be due to a transient increase in the efficacy of excitatory synapses.

Resetting of essential tremor and postural tremor in Parkinson's disease with transcranial magnetic stimulation

We studied the effects of transcranial motor cortex stimulation on the electromyographic characteristics of tremor in 9 patients with familial essential tremor and in 12 patients with postural tremor associated with Parkinson's disease. Transcranial magnetic stimulation reset both types of tremor equally. The resetting depended on the stimulus intensity, but was most closely correlated with the duration of the electromyographic silent period that followed the stimulus-induced motor evoked potential. Tremor resetting was present bilaterally even after focal, unilateral stimulation. Transcranial electrical stimulation failed to reset the tremor in either patient group. These results emphasize the role of central, intracortical structures in the generation of essential tremor and postural tremor in Parkinson's disease.

Cortical motor representation of the ipsilateral hand and arm

We sought to determine whether motor evoked potentials (MEPs) as well as silent periods could be produced in hand and shoulder muscles by transcranial magnetic stimulation (TMS) of the ipsilateral cerebral hemisphere and, if so, whether their cortical representations could be mapped with respect to those of contralateral muscles. In six normal subjects, we delivered ten stimuli each to a grid of sites 1 cm apart on the scalp. The EMG was recorded and averaged from the contralateral first dorsal interosseous (FDI) and risorius (facial) muscles at rest and the ipsilateral FDI muscle, which was voluntarily contracted. In four of these subjects and an additional subject, we used the same mapping technique and recorded from the deltoid muscle on the right and left sides and the contralateral FDI during activation of the ipsilateral deltoid. In all subjects, the cortical representation of the contralateral risorius was anterolateral to that of the FDI. The contralateral deltoid could be activated in only three subjects. In them, its representation was slightly medial to that of the FDI. All subjects had at least one scalp site where TMS produced MEPs in the ipsilateral FDI. Two subjects had rich ipsilateral hand representations with multiple ipsilateral MEP sites. Both had ipsilateral MEP sites near the representation of the contralateral FDI, but the largest ipsilateral MEPs occurred with TMS at more lateral sites, which were near the representation of the contralateral risorius. In these subjects, the ipsilateral deltoid was preferentially activated at sites medial and posterior to those activating the contralateral muscle. Ipsilateral TMS also produced silent periods in the FDI in all subjects. These silent periods were much more frequent than the ipsilateral MEPs and tended to occur with TMS near the representation of the contralateral FDI. The excitatory cortical representation of the ipsilateral arm muscles is accessible to TMS in normal subjects and is different from that of the homologous contralateral muscles. The hand may have two ipsilateral representations, one of which produces silent periods and the other MEPs at the same stimulus intensity.

Differentiation of sensorimotor neuronal structures responsible for induction of motor evoked potentials, attenuation in detection of somatosensory stimuli, and induction of sensation of movement by mapping of optimal current directions

Transcranial magnetic stimulation (TMS) of the sensorimotor cortex can evoke motor evoked potentials (MEPs), attenuation in detection of somatosensory stimuli (ADSS), and sensation of movement (SOM) referred to the same body part. In this study we tried to differentiate the substrates responsible for these effects. In 6 normal volunteers, TMS was applied with a nearly monopolar Dantec stimulator and a butterfly coil. Optimal scalp location and current direction were determined for induction of MEPs in abductor pollicis brevis (APB), first dorsal interosseous (FDI), and adductor digiti minimi (ADM); SOM in digits 2 and 5 in an ischemically paralyzed hand; and ADSS applied to digits 2 and 5. All 3 muscles' MEPs and SOM and ADSS in both digits were optimally activated from a single scalp position. In all subjects, optimal current directions for MEPs pointed anteriorly; those for ADSS and SOM pointed posteriorly. Optimal current directions showed the same progression in all subjects for MEPs (ADM, FDI, and APB from antero-lateral to antero-medial), ADSS (digit 5 postero-medial, 2 postero-lateral), and SOM (digit 1 through 5 postero-lateral to postero-medial). We conclude that neuronal networks targeting corticospinal neurons responsible for MEPs are different from those leading to SOM and ADSS (which could not be differentiated).

Induction of a recall deficit by rapid-rate transcranial magnetic stimulation

We used rapid-rate, repetitive transcranial magnetic stimulation (rTMS) for the noninvasive study of verbal recall. Five right-handed normal subjects were studied. Recall followed immediately after presentation of a 12-word list. Focal rTMS was applied with a figure eight coil in trains of 500 ms duration to F7, F8, T5, T6, P3, P4, or O1, O2 at latencies of 0, 250, 500, or 1000 ms during word list presentation. Recall was consistently significantly diminished only after left mid-temporal and bilateral dorsofrontal rTMS at both 0 and 250 ms latencies. We conclude that rTMS may be useful as a non-invasive tool for the study of verbal memory processes.

Akinesia in Parkinson's disease. I. Shortening of simple reaction time with focal, single-pulse transcranial magnetic stimulation

We studied the effects of transcranial magnetic stimulation (TMS) of the motor cortex on simple reaction time (RT) in 10 patients with Parkinson's disease compared with 10 age-matched normal controls. The subjects flexed their right elbow rapidly in response to a visual go-signal. In random trials, TMS was applied to the left motor cortex at varying delays after the go-signal. In trials without TMS, RT was longer in the patients. However, in the trials with subthreshold TMS, RT in the patients became as fast as RT in trials without TMS in the controls. This shortening was associated with normalization of the voluntary triphasic EMG pattern and the pre-movement cortical excitability increase.

Akinesia in Parkinson's disease. II. Effects of subthreshold repetitive transcranial motor cortex stimulation

We studied the effects of repetitive transcranial stimulation of the motor cortex (rTMS) on choice reaction time (cRT), movement time (MT), and error rate (ER) in a serial reaction-time task in six medicated patients with Parkinson's disease (PD) and 10 age-matched normal controls. In normal subjects, subthreshold 5-Hz rTMS did not significantly change cRT, slightly shortened MT, but increased ER. In the patients, rTMS significantly shortened cRT and MT without affecting ER. These effects did not impair procedural learning. Performance on a grooved peg-board test was improved by rTMS in the same PD patients, especially when they were off medications, but worsened in the normal subjects. Repetitive, subthreshold motor cortex stimulation can improve performance in patients with PD and could be useful therapeutically.

Abnormal facilitation of the response to transcranial magnetic stimulation in patients with Parkinson's disease

We studied the facilitation of the motor evoked potential (MEP) elicited with transcranial magnetic stimulation by increasing the stimulus intensity and the degree of voluntary activation of the target muscle in patients with Parkinson's disease (PD) and in normal volunteers. The threshold intensity for eliciting MEPs with the muscle at rest did not differ in PD patients and normal subjects. At rest, stimuli of similar intensity, related to the individual's threshold, elicited MEPs with amplitudes consistently larger in patients than in normal subjects, although when we compared the averaged MEP amplitude across all stimulus intensities, the differences reached only borderline statistical significance. Voluntary muscle activation elicited a smaller increase in the MEP area in PD patients than in normal subjects. Increasing the degree of voluntary muscle activation at fixed stimulus intensities elicited a smaller increase of MEP amplitude, duration, and area in PD patients than in normal subjects. These results suggest that control of the excitability of the motor system is abnormal in PD patients, with enhancement of excitability at rest and weak energization during voluntary muscle activation.

Modulation of cortical motor output maps during development of implicit and explicit knowledge

The excitability of the human motor cortex during the development of implicit and declarative knowledge of a motor task was examined. During a serial reaction time test, subjects developed implicit knowledge of the test sequence, which was reflected by diminishing response times. Motor cortical mapping with transcranial magnetic stimulation revealed that the cortical output maps to the muscles involved in the task became progressively larger until explicit knowledge was achieved, after which they returned to their baseline topography. These results illustrate the rapid functional plasticity of cortical outputs associated with learning and with the transfer of knowledge from an implicit to explicit state.

Induction of visual extinction by rapid-rate transcranial magnetic stimulation of parietal lobe

We used repetitive, rapid-rate transcranial magnetic stimulation (rTMS) for the noninvasive study of visual attention in humans. Six right-handed volunteers completed eight blocks of 20 single- and 10 double-visual-stimulus trials. The visual stimulus was a single asterisk on the right or left side of a computer screen or two asterisks presented simultaneously. The subject had to respond to the stimulus by pressing the right or left response key or both keys simultaneously. During six of the blocks, we applied focal rTMS in trains of five pulses at 25 Hz and 115% of the subject's motor threshold intensity to scalp positions O1, O2, P3, P4, T5, or T6. Occipital rTMS led to a large number of misses of the contralateral asterisk regardless of whether a single or double stimulus was presented. Parietal rTMS did not induce misses of single stimuli but led to a large number of misses of the contralateral asterisk in the double-stimulus condition. The effects of temporal rTMS were inconsistent. We conclude that rTMS to the occipital lobe causes a sensory detection block, whereas rTMS to the parietal lobe can induce selective extinction of contralateral visual stimuli during a simultaneous double stimulus.

Non-invasive differentiation of motor cortical representation of hand muscles by mapping of optimal current directions

Non-invasive mapping of human motor cortex by stimulating different scalp positions with a magnetic coil held at a constant orientation allows differentiation of proximal and distal arm muscles. This study describes a technique for more precise mapping of closely represented muscles using different orientations of a coil that delivers nearly monopolar current pulses. EMG was recorded from abductor pollicis brevis (APB), first dorsal interosseous (FDI), abductor digiti minimi (ADM), and flexor carpi radialis (FCR) of 9 normal volunteers. Stimuli were delivered from a Dantec stimulator through an 8-shaped coil. The center of the coil was kept flat on the scalp on a given position, and the coil rotated at different angles. The amplitudes of the motor evoked potentials were used for calculation of optimal current directions in the brain for activation of each muscle in each position. The optimal current direction for FCR activation pointed antero-medially. ADM, FDI and APB mapped progressively more antero-laterally. The relationship between current directions was constant across subjects and did not change in different scalp positions. This technique improves the spatial resolution of non-invasive cortical mapping and may express the differences in orientations of interneuronal nets in the precentral gyrus.

Topography of the inhibitory and excitatory responses to transcranial magnetic stimulation in a hand muscle

We studied the excitatory motor evoked potentials (MEPs) and the inhibitory (silent period) responses to focal transcranial magnetic stimulation (TMS) in the abductor pollicis brevis (APB) of 5 normal subjects to learn whether the scalp topography of the two responses differed. At the scalp location where stimulation produced the highest-amplitude MEP in the voluntarily activated APB, stimulus intensities below the MEP threshold produced silent periods with little or no preceding facilitation. The silent periods had a mean duration of 26.8 +/- 6.8 msec and a mean onset latency of 27.6 +/- 3.6 msec, which was 7.2 +/- 2.3 msec longer than the latency of MEPs produced in the APB by higher stimulus intensities. A period of excitation, with an onset latency of 50-80 msec, often followed the silent period. On averaged trials, a stimulus intensity just above the threshold of the MEP at its optimal position produced MEPs followed by silent periods at a cluster of scalp locations 1 cm apart on the central scalp (medial area) and silent periods with very slight or no preceding facilitation in 3-9 locations lateral to the MEP area (lateral area). This finding was confirmed in 3 subjects with maps constructed from statistical analysis of multiple trials. These maps also showed that MEPs produced from the medial area occurred 4-6 msec earlier than those produced from the lateral area. The integral of the silent period tended to be larger in the lateral area. The motor representation of APB, as defined by TMS, is not homogeneous but rather contains at least two components that differ physiologically and topographically.

Cortical tremor. A common manifestation of cortical myoclonus

Ten patients, three with postural tremor and seven with action myoclonus, had stereotyped involuntary rhythmic movements when attempting to execute a sustained isometric muscle contraction. The movements were characterized by rhythmic EMG bursts lasting less than 50 msec and appearing synchronously in agonist and antagonist muscles at a rate of 9 to 18 Hz. Backaveraging of the EEG activity related to the onset of the rhythmic EMG bursts identified a cortical potential preceding the EMG bursts in all patients. These symptoms and signs fit the description of "cortical tremor," a variant of cortical reflex myoclonus. Cortical tremor is common in patients with cortical myoclonus and may be a source of functional disability. In two patients in whom we studied the effects of graded levels of isometric force, force recruitment modulated the abnormal EMG bursting frequency, amplitude, and spatial distribution of the myoclonic jerks in the activated limb. Transcranial magnetic and electrical stimulation, but not peripheral nerve stimulation, influenced the abnormal EMG bursting pattern, implying a greater dependence of this rhythmic phenomenon on a central generator than on peripheral feedback loops.

Procedural learning in Parkinson's disease and cerebellar degeneration

We compared procedural learning, translation of procedural knowledge into declarative knowledge, and use of declarative knowledge in age-matched normal volunteers (n = 30), patients with Parkinson's disease (n = 20), and patients with cerebellar degeneration (n = 15) by using a serial reaction time task. Patients with Parkinson's disease achieved procedural knowledge and used declarative knowledge of the task to improve performance, but they required a larger number of repetitions of the task to translate procedural knowledge into declarative knowledge. Patients with cerebellar degeneration did not show performance improvement due to procedural learning, failed to achieve declarative knowledge, and showed limited use of declarative knowledge of the task to improve their performance. Both basal ganglia and cerebellum are involved in procedural learning, but their roles are different. The normal influence of the basal ganglia on the prefrontal cortex may be required for timely access of information to and from the working memory buffer, while the cerebellum may index and order events in the time domain and be therefore essential for any cognitive functions involving sequences.

Modulation of motor cortical outputs to the reading hand of braille readers

We used focal transcranial magnetic stimulation to map the motor cortical areas targeting the first dorsal interosseous and the abductor digiti minimi muscles bilaterally in 10 proficient braille readers and 10 blind controls who were matched for age (mean, 50.6 yr) and age at time of blindness (mean, 7.5 yr). The proficient braille readers had learned braille at age 8 to 14 years and used it daily for 5 to 10 hours. Controls had not learned braille until age 17 to 21 years and used it daily for < 1 hour. In the controls, motor representations of the right and left first dorsal interosseous and abductor digiti minimi muscles were not significantly different. However, in the proficient braille readers, the representation of the first dorsal interosseous muscle in the reading hand was significantly larger than that in the nonreading hand or in either hand of the controls. Conversely, the representation of the abductor digiti minimi muscle in the reading hand was significantly smaller than that in the nonreading hand or in either hand of the controls. These differences were not due to differences in motor thresholds. Our results suggest that the cortical representation of the reading finger in proficient braille readers is enlarged at the expense of the representation of other fingers.

Rapid modulation of human cortical motor outputs following ischaemic nerve block

The amplitudes of motor evoked potentials to transcranial magnetic stimulation from muscles immediately proximal to a temporarily anaesthetized (Bier's block) human forearm increase in minutes after the onset of anaesthesia and return to control values after the anaesthesia subsides. In order to determine the level at which the early modulation of human motor outputs takes place, we recorded maximal H reflexes, peripheral M responses, motor evoked potentials to transcranial magnetic stimulation, and motor evoked potentials to transcranial electrical stimulation and spinal electrical stimulation from a muscle immediately proximal to a limb segment made ischaemic by a pneumatic tourniquet. The amplitudes of motor evoked potentials to transcranial magnetic stimulation, but not to transcranial electrical stimulation and spinal electrical stimulation, were larger during ischaemia, implying that the site of change was in the motor cortex. The maximal H/M ratios were unaffected by ischaemia, indicating that alpha-motor neuron excitability to segmental Ia inputs remained unchanged. The map of cortical representation areas for this muscle obtained with transcranial magnetic stimulation was also enlarged. Taken together, our findings suggest that the temporary removal by ischaemic nerve block of myelinated afferent inputs reduces inhibition at the motor cortical level and that this disinhibition is responsible for the increased excitability of the corticospinal system.

Safety of rapid-rate transcranial magnetic stimulation in normal volunteers

In 9 normal volunteers, we studied the safety of rapid-rate transcranial magnetic stimulation (rTMS) applied to different scalp positions at various frequencies and intensities. Pure tone threshold audiometry showed temporary threshold shifts in 3 subjects. In the subject stimulated at the highest intensity, rTMS induced a focal, secondarily generalized seizure despite the absence of definite risk factors for seizures. Rapid-rate TMS did not result in any important changes in the neurological examination findings, cognitive performance, electroencephalogram, electrocardiogram, and hormone levels (prolactin, adrenocorticotropic hormone, thyroid-stimulating hormone, luteinizing hormone, and follicle-stimulating hormone). In 10 additional subjects, the electromyographic activity in several contralateral muscles showed that trains of rTMS applied to the motor cortex induced a spread of cortical excitability. The spread of excitability depended on the intensity and frequency of the stimuli and probably constituted an early epileptogenic effect of rTMS. Guidelines for preventing the undesirable side effects of rTMS are offered.

Plasticity of the sensorimotor cortex representation of the reading finger in Braille readers

We studied the organization of the somatosensory cortex in proficient Braille readers, recording somatosensory evoked potentials (SEPs) in 10 subjects and using transcranial magnetic stimulation (TMS) in five subjects, and compared the results with those of 15 control subjects. Somatosensory evoked potentials were elicited by a focal electrical stimulus to the tip of the index finger and recorded from a contralateral 4 x 4 grid of scalp electrodes centred around C3' and C4'. Transcranial magnetic stimulation, with an 8-shaped coil centred over the same scalp positions, was delivered simultaneously with, and at different intervals after, the finger stimulus. The results of the right index (reading) finger in Braille readers were compared with those of their left index (non-reading) finger and of the right and left index fingers of the control subjects. The scalp areas from which we recorded N20 and P22 components of the SEP with an amplitude of at least 70% of the maximal amplitude recorded in each trial were significantly larger in SEPs evoked from the reading fingers. Detection of the stimulus applied to the reading finger was blocked by TMS delivered over a larger contralateral scalp area and during a longer time window after the stimulus. These experiments suggest that reading Braille is associated with expansion of the sensorimotor cortical representation of the reading finger.

Postexercise depression of motor evoked potentials: a measure of central nervous system fatigue

Fatigue of voluntary muscular effort is a complex and multifaceted phenomenon. Fatigue of peripheral nervous system components, including the contractile apparatus and the neuromuscular junction, has been well studied. Central nervous system components also fatigue, but studies have lagged for want of objective methods. Transcranial magnetic stimulation is a relatively new technique that can be used to assess central nervous system excitability from the motor cortex to the alpha-motoneuron. In six normal volunteers, including four of the investigators, the amplitudes of motor evoked potentials elicited by transcranial magnetic stimulation were transiently decreased after exercise, indicating fatigue of motor pathways in the central nervous system. The decrease in amplitude was associated with a feeling of fatigue. The mechanism of this phenomenon is apparently decreased efficiency in the generation of the motor command in the motor cortex.

Human motor evoked responses to paired transcranial magnetic stimuli

We studied the changes in motor pathway excitability induced by transcranial magnetic stimulation of the motor cortex, using paired stimuli (conditioning and test stimulus) and varying interstimulus interval (ISI). The effects induced depended on the stimulus intensity. At a low intensity, there was inhibition of the response to the test stimulus at ISIs of 5-40 msec, followed by facilitation at ISIs of 50-90 msec. At a high intensity, there was facilitation at ISIs of 25-50 msec, followed by inhibition at ISIs of 60-150 msec and, occasionally, by another phase of facilitation at ISIs of more than 200 msec. Only tentative explanations are currently possible for these effects: the inhibition observed at low intensities and short ISIs may be due to activation of cortical inhibitory mechanisms. The facilitation that follows may arise from the coincidence of various factors that transiently increase the excitability in alpha motoneurons. 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. The profound inhibition that follows probably results from a combination of both segmental and suprasegmental inhibitory mechanisms.

Focal transcranial magnetic stimulation and response bias in a forced-choice task

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.

Effects of focal transcranial magnetic stimulation on simple reaction time to acoustic, visual and somatosensory stimuli

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.

Rapid reversible modulation of human motor outputs after transient deafferentation of the forearm: a study with transcranial magnetic stimulation

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.