Transcranial Magnetic Brain Stimulation Research Articles

Transcranial magnetic stimulation and cortical evoked potentials: A TMS/EEG co-registration study

Objective In recent years, a promising tool has been introduced which allows the co-registration of electroencephalographic (EEG) activity during brain transcranial magnetic stimulation (TMS). The aims of the present study are to identify eventual stimulus-related artefacts, and to confirm and extend previous EEG/TMS findings about the possible networks generating EEG responses evoked by TMS. Methods Focal TMS was delivered to the left primary motor cortex (MI), with different coils (real and sham) and orientations (45 and 135° in respect to the sagittal plane), in six healthy subjects. EEG and motor evoked potentials (MEPs) were simultaneously recorded from 19 scalp electrodes. Results TMS, with coil oriented at 45°, induced EEG responses characterized by a sequence of positive deflections peaking at approximately 14, 30, 60 and 190 ms and negative deflections peaking at approximately 10, 18, 40 and 100 ms post-TMS. The negative components were recorded at the recording electrode corresponding with the stimulation site (N10, N18), as well as at recording electrodes over the frontal region of the contralateral, unstimulated, hemisphere (N40) and bilaterally over the central hemispheres with its maximal representation at the stimulation site (N100). The positive components were instead detected at the frontal region of the right, unstimulated, hemisphere (P14), over the central electrodes Cz, Fz and the frontal region of the right hemisphere (P30), at the stimulation site (P60), and over the frontal regions of both hemispheres. When TMS was delivered with the coil oriented at 135°, no MEPs were recorded from the right target muscle. Nonetheless, all the TMS-induced EEG components were still evoked apart from the N20–P30. Finally, TMS with the sham coil over left MI did not induce either significant EEG responses or MEPs. Conclusions In conclusion, the TMS evoked components we have obtained by recording in continuous mode strikingly fit with those already described by other authors for both their latencies and the spatio-temporal pattern of scalp distribution. Significance This experiment is a farther validation of the combined EEG/TMS recording technique as a promising tool for experimental and clinical purposes.

How a Lateralized Brain Supports Symmetrical Bimanual Tasks

A large repertoire of natural object manipulation tasks require precisely coupled symmetrical opposing forces by both hands on a single object. We asked how the lateralized brain handles this basic problem of spatial and temporal coordination. We show that the brain consistently appoints one of the hands as prime actor while the other assists, but the choice of acting hand is flexible. When study participants control a cursor by manipulating a tool held freely between the hands, the left hand becomes prime actor if the cursor moves directionally with the left-hand forces, whereas the right hand primarily acts if it moves with the opposing right-hand forces. In neurophysiological (electromyography, transcranial magnetic brain stimulation) and functional magnetic resonance brain imaging experiments we demonstrate that changes in hand assignment parallels a midline shift of lateralized activity in distal hand muscles, corticospinal pathways, and primary sensorimotor and cerebellar cortical areas. We conclude that the two hands can readily exchange roles as dominant actor in bimanual tasks. Spatial relationships between hand forces and goal motions determine hand assignments rather than habitual handedness. Finally, flexible role assignment of the hands is manifest at multiple levels of the motor system, from cortical regions all the way down to particular muscles.

Accuracy of Stereotaxic Positioning of Transcranial Magnetic Stimulation

In cognitive neuroscience, optically tracked frameless stereotaxic navigation has been successfully used to precisely guide transcranial magnetic stimulation (TMS) to desired cortical areas for brain-mapping purposes. Thereby, potential sources of imprecision are the fixation of a reference frame to the head of the subject and the referencing procedure according to certain landmarks (LM). The aim of our study was to evaluate the accuracy of frameless stereotaxic coil positioning in a standard experimental setting. A parameter for accuracy is the reproducibility of LM coordinates. In order to test the stability of the referencing for stereotaxic positioning within a single TMS session (within-session stability), the coordinates of six predefined facial LM in nine subjects were recorded first after the initial registration and second after a 20 minutes TMS session. The two sets of coordinates were then compared. The reliability of the positioning coordinates between different TMS sessions (inter-session repeatability) was addressed by registering the subjects LM coordinates in two independent TMS sessions. The variance of the recorded coordinates was analyzed. Altogether, LM were registered 1728 times (192 measures per subject). Within-session stability: The mean Euclidean distance (MED) between the LM position coordinates before and after a TMS session was 1.6 mm, when pooling over all LM. Inter-session repeatability: The MED between the LM positions recorded after the reference procedures of two different sessions showed an average deviation of 2.5 mm. In conclusion, optically tracked frameless stereotaxic coil positioning is from the technical viewpoint of high stability and repeatability. It is therefore a precise method for TMS brain mapping studies or for repeated TMS treatments, with the need of topographically exact stimulation.

TMS-responses during anticipatory postural adjustment in bimanual unloading in humans

Transcranial magnetic brain stimulation (TMS) was used to assess the influence of the corticospinal system on motor output during forearm unloading in humans. Unloading was obtained either “passively” by the experimenter, or “actively” with the subjects’ own contralateral arm. Anticipatory postural adjustments consisted of changes in the activity of a forearm flexor muscle prior to active unloading of the limb and acted to stabilize the forearm position. Motor evoked potentials (MEPs) were recorded in the forearm flexor at different times during active and passive unloading, static forearm loading, and during lifting of an equivalent weight by the contralateral arm while the ipsilateral forearm was statically loaded and held stationary. In active unloading, MEP amplitude decreased with the decrease of muscle activity. Passive unloading resulted in a similar decrease of MEP as with active unloading. During stationary forearm loading, the change in MEP corresponded to the degree of loading. If during static loading the contralateral arm has lifted a separate, equivalent weight, the amplitude of MEP decreased. A possible role of direct corticospinal volley and the motor command mediated by subcortical structures in anticipatory postural adjustments is discussed.

Uncrossed cortico-muscular projections in humans are abundant to facial muscles of the upper and lower face, but may differ between sexes

It is a popular concept in clinical neurology that muscles of the lower face receive predominantly crossed cortico-bulbar motor input, whereas muscles of the upper face receive additional ipsilateral, uncrossed input. To test this notion, we used focal transcranial magnetic brain stimulation to quantify crossed and uncrossed cortico-muscular projections to 6 different facial muscles (right and left Mm. frontalis, nasalis, and orbicularis oris) in 36 healthy right-handed volunteers (15 men, 21 women, mean age 25 years). Uncrossed input was present in 78% to 92% of the 6 examined muscles. The mean uncrossed: crossed response amplitude ratios were 0.74/0.65 in right/left frontalis, 0.73/0.59 in nasalis, and 0.54/0.71 in orbicularis oris; ANOVA p>0.05). Judged by the sizes of motor evoked potentials, the cortical representation of the 3 muscles was similar. The amount of uncrossed projections was different between men and women, since men had stronger left-to-left projections and women stronger right-to-right projections. We conclude that the amount of uncrossed pyramidal projections is not different for muscles of the upper from those of the lower face. The clinical observation that frontal muscles are often spared in central facial palsies must, therefore, be explained differently. Moreover, gender specific lateralization phenomena may not only be present for higher level behavioural functions, but may also affect simple systems on a lower level of motor hierarchy.

Ellen R. Grass Lecture: Motor Evoked Potential Monitoring

ABSTRACT.Because somatosensory evoked potentials (SSEPs) recorded from the brain are carried solely within the dorsal columns of the spinal cord, SSEP monitoring may fail to detect damage to the spinal cord motor pathways, and techniques for directly monitoring the motor pathways have been developed. Transcranial magnetic brain stimulation is useful for extraoperative evaluation of the motor system, but anesthetic effects on cortical synaptic activity limit its usefulness for intraoperative monitoring. Another proposed monitoring technique, rostral spinal cord stimulation with recording from peripheral nerves, predominantly reflects retrograde conduction within the dorsal columns; like SSEPs, this technique may fail to detect motor pathway damage. The true motor evoked potential (MEP) monitoring technique preferred by most institutions involves transcranial electrical brain stimulation. This stimulates the corticospinal tract axons directly, producing a D-wave in them. There may be additional corticospinal tract volleys, reflecting indirect activation of the pyramidal cells via synapses from cortical interneurons; these I-waves are largely suppressed by anesthesia. Temporal summation at the alpha motor neuron synapse is necessary to elicit myogenic MEPs (M-waves) under anesthesia; this is accomplished with brief trains of stimuli that produce multiple pyramidal tract volleys. High stimulus intensities are required to stimulate the brain through an intact skull; with proper precautions, this is acceptably safe. Myogenic MEPs are quite sensitive to anesthesia and show considerable run-to-run variability. Therefore, criteria for evaluating MEPs differ from those for SSEPs, and the anesthetic regimen and degree of neuromuscular blockade must be carefully controlled. SSEPs should be monitored concurrently with MEPs, because the dorsal columns may be affected without compromise of the motor tracts, and also because this provides redundancy, with two independent assessments of spinal cord function.

Concept formation: 'object' attributes dynamically inhibited from conscious awareness.

We advance a dominant neural strategy for facilitating conceptual thought. Concepts are groupings of "object" attributes. Once the brain learns such critical groupings, the "object" attributes are inhibited from conscious awareness. We see the whole, not the parts. The details are inhibited when the concept network is activated, ie. the inhibition is dynamic and can be switched on and off. Autism is suggested to be the state of retarded concept formation. Our model predicts the possibility of accessing nonconscious information by artificially disinhibiting (turning off) the inhibiting networks associated with concept formation, using transcranial magnetic brain stimulation (TMS). For example, this opens the door for the restoration of perfect pitch, for recalling detail, for acquiring accent-free second languages beyond puberty, and even for enhancing creativity. The model further shows how unusual autistic savant skills as well as certain psychopathologies can be due respectively to privileged or inadvertent access to information that is normally inhibited from conscious awareness.

Three-dimensional probabilistic anatomical cranio-cerebral correlation via the international 10–20 system oriented for transcranial functional brain mapping

The recent advent of multichannel near-infrared spectroscopy (NIRS) has expanded its technical potential for human brain mapping. However, NIRS measurement has a technical drawback in that it measures cortical activities from the head surface without anatomical information of the object to be measured. This problem is also found in transcranial magnetic stimulation (TMS) that transcranially activates or inactivates the cortical surface. To overcome this drawback, we examined cranio-cerebral correlation using magnetic resonance imaging (MRI) via the guidance of the international 10–20 system for electrode placement, which had originally been developed for electroencephalography. We projected the 10–20 standard cranial positions over the cerebral cortical surface. After examining the cranio-cerebral correspondence for 17 healthy adults, we normalized the 10–20 cortical projection points of the subjects to the standard Montreal Neurological Institute (MNI) and Talairach stereotactic coordinates and obtained their probabilistic distributions. We also expressed the anatomical structures for the 10–20 cortical projection points probabilistically. Next, we examined the distance between the cortical surface and the head surface along the scalp and created a cortical surface depth map. We found that the locations of 10–20 cortical projection points in the standard MNI or Talairach space could be estimated with an average standard deviation of 8 mm. This study provided an initial step toward establishing a three-dimensional probabilistic anatomical platform that enables intra- and intermodal comparisons of NIRS and TMS brain imaging data.

Effector-independent representations of simple and complex imagined finger movements: a combined fMRI and TMS study.

Kinesthetic motor imagery and actual execution of movements share a common neural circuitry. Functional magnetic resonance imaging was used in 12 right-handed volunteers to study brain activity during motor imagery and execution of simple and complex unimanual finger movements of the dominant and the nondominant hand. In the simple task, a flexible object was rhythmically compressed between thumb, index and middle finger. The complex task was a sequential finger-to-thumb opposition movement. Premotor, posterior parietal and cerebellar regions were significantly more active during motor imagery of complex movements than during mental rehearsal of the simple task. In 10 of the subjects, we also used transcranial magnetic brain stimulation to examine corticospinal excitability during the same motor imagery tasks. Motor-evoked potentials increased significantly over values obtained in a reference condition (visual imagery) during imagery of the complex, but not of the simple movement. Imagery of finger movements of either hand activated left dorsal and ventral premotor areas and the supplementary motor cortex regardless of task complexity. The effector-independent activation of left premotor areas was particularly evident in the simple motor imagery task and suggests a left hemispherical dominance for kinesthetic movement representations in right-handed subjects.

Dexterity is not affected by fatigue-induced depression of human motor cortex excitability

Following a fatiguing muscle contraction, motor evoked potentials (MEPs) evoked by transcranial magnetic brain stimulation remain depressed for many minutes, reflecting a reduction in the excitability of the corticospinal projection. No functional significance has been linked to this observation. We postulated that dexterity would be affected when MEPs are depressed. MEPs were recorded from the first dorsal interosseous muscle of 11 healthy subjects in resting muscles before and after a fatiguing maximum voluntary contraction. This induced significant MEP depression in all subjects. No change in dexterity was seen when MEPs were depressed or at any time point throughout the experiment. We conclude that fatigue-induced MEP depression is not associated with a decline in dexterity.

Mechanisms of motor‐evoked potential facilitation following prolonged dual peripheral and central stimulation in humans

1. Repetitive electrical peripheral nerve or muscle stimulation can induce a lasting increase in the excitability of the corticomotor projection. By pairing peripheral stimulation with transcranial magnetic brain stimulation it is possible to shorten the duration of stimulation needed to induce this effect. This ability to induce excitability changes in the motor cortex may be of significance for the rehabilitation of brain-injured patients. The mechanisms responsible for the increases in excitability have not been investigated thoroughly. 2. Using two paired transcranial magnetic stimuli protocols we investigated the excitability of intracortical inhibitory and excitatory systems before and following a period of repetitive dual muscle and brain stimulation. The dual stimulation consisted of motor point stimulation of first dorsal interosseous (FDI; 10 Hz trains of 1 ms square waves for 500 ms) delivered at one train every 10 s, paired with single transcranial magnetic stimulation given 25 ms after the onset of the train. 3. Following 30 min of dual stimulation, motor-evoked potentials (MEPs) were significantly increased in amplitude. During this period of MEP facilitation there was no significant difference in the level of intracortical inhibition. There was, however, a significant increase in the intracortical facilitation demonstrated with paired magnetic stimuli. The increase in facilitation was seen only at short interstimulus intervals (0.8-2.0 ms). These intervals comprised a peak in the time course of facilitation, which is thought to reflect I wave interaction within the motor cortex. 4. The relevance of this finding to the MEP facilitation seen following dual peripheral and central stimulation is discussed.

Magnetic Brain Stimulation in ADHD

Transcranial magnetic brain stimulation was performed in 27 children and adolescents, aged 4 to 18 years, with ADHD in the Services of Pediatric Neurology and Clinical Neurophysiology, Miguel Servet Hospital, Zaragoza, Spain.

The excitability of human cortical inhibitory circuits responsible for the muscle silent period after transcranial brain stimulation.

The silent period after transcranial magnetic brain stimulation mainly reflects the activity of inhibitory circuits in the human motor cortex. To assess the excitability of the cortical inhibitory mechanisms responsible for the silent period after transcranial stimulation, we studied, in 15 healthy human subjects, the recovery cycle of the silent period evoked by transcranial and mixed nerve stimulation delivered with a paired stimulation technique. The recovery cycle is defined as the time course of the changes in the size or duration of a conditioned test response when pairs of stimuli (conditioning and test) are used at different conditioning-test intervals. The recovery cycle of the duration of the silent period in the first dorsal interosseous (FDI) muscle during maximum voluntary contraction after transcranial magnetic stimulation was studied by delivering paired magnetic shocks (a conditioning shock and a test shock) at 120% motor-threshold intensity. Conditioning-test intervals ranged from 20-550 ms. The recovery cycle of the silent period in the FDI muscle during maximum voluntary contraction after nerve stimulation was evaluated by paired, supramaximum bipolar electrical stimulation of the ulnar nerve at the wrist (conditioning-test intervals ranging from 20 to 550 ms). Electromyographic activity was recorded by a pair of surface-disk electrodes over the FDI muscle. The recovery cycle of the silent period after transcranial magnetic stimulation delivered through the large round coil showed two phases of facilitation (lengthening of the silent period), one at 20-40 ms and the other at 180-350 ms conditioning-test intervals, with an interposed phase of inhibition (shortening of the silent period) at 80-160 ms. The conditioning magnetic shock left the size of the test motor-evoked potentials statistically unchanged during maximum voluntary contraction. Paired transcranial stimulation with a figure-of-eight coil increased the duration of the test silent period only at short conditioning-test intervals. Conditioning nerve stimulation left the silent period produced by test nerve stimulation unchanged. In conclusion, after a single transcranial magnetic shock, inhibitory circuits in the human motor cortex undergo distinctive short-term changes in their excitability, probably involving different mechanisms.

Variations on the musical brain.

If intelligent extraterrestrials ever intercept Voyager, the first message they decode will be Glenn Gould playing Bachl. Music, an ancient and peculiarly human pastime, is valued by all cultures. For highly visual primates, we are surprisingly susceptible to these abstract acoustic stimuli with no obvious survival value2. Neurologists have long been fascinated by musical curiosities they occasionally meet in their clinical practice. With the advent of techniques such as functional magnetic resonance imaging (fMRI), positron emission tomography (PET) and transcranial magnetic brain stimulation (TMS) it has become possible to study the perception and production of music in detail. A brief set of variations may serve to outline the theme.

Task-dependent effects on motor-evoked potentials and on the following silent period.

The silent period (SP) after transcranial stimulation is used as a diagnostic tool in various central nervous system disorders although no standardized experimental setup has been established. The aim of this study was to demonstrate the influence of an isotonic compared to an isometric experimental condition. The SP after transcranial magnetic brain stimulation in the biceps brachii and brachioradialis muscle was up to 130% longer when elicited during a maintain-position (isotonic) task as compared to a maintain-force task (isometric) when stimulus intensities of 5% to 25% above threshold were used. The mean SP duration in these muscles was positively correlated to the mean contraction time in both tasks. However, no such relationship was observed for the trials within the individual subjects. We speculate that the invariably longer SP of the maintain-position task was due to the different "motor set" which predictively determined the muscle behavior after the stimulus. In the maintain-position trials, the stimulus-induced long-lasting flexion movement is counteracted by a motor set aiming to relax the elbow flexors immediately after the stimulus. In the maintain-force task the contraction twitch is short and a force drop below the preset level must be prevented by a motor set aiming to contract the elbow flexors immediately after the stimulus. The latter may increase the synaptic input to the motoneuron pool and facilitate the reoccurrence of the electromyogram terminating the SP. At high-stimulus intensities the SP duration increased in both tasks, and the task-dependent differences disappeared. Therefore, when using the SP duration for diagnostic purposes, isometric conditions and high-stimulus intensities should be used.

Shortening of the cortical silent period following transcranial magnetic brain stimulation during an experimental paradigm for generating contingent negative variation (CNV)

Objectives: We investigated changes in the cortical silent period (CSP) following transcranial magnetic cortical stimulation (TCMS) during a standard paradigm which was designed to evoke contingent negative variation (CNV) in ten normal subjects.Methods: We recorded the motor evoked potentials (MEP) and CSP during the inter-stimulus interval (ISI) of a CNV paradigm in ten normal subjects. The CNV paradigm consisted of a visual warning stimulus (S1) followed by a visual response stimulus (S2). The CSP following TCMS on the hand motor area was recorded from mildly contracted first dorsal interosseous muscles.Results: The CSP was significantly shortened during the ISI (P<0.01, t test) with a highly significant correlation with the TCMS timing during the ISI (P<0.02, Spearman's correlation coefficient), while the MEP amplitude and latency were unchanged.Conclusions: The results suggested that shortening of the CSP was associated with neural processes related to preparation for voluntary movement during the paradigm.

Prognostic value of motor evoked potential obtained by transcranial magnetic brain stimulation in motor function recovery in patients with acute ischemic stroke.

The early prognostic application of transcranial magnetic brain stimulation (TMS) for assessing motor and functional recovery in ischemic stroke patients has yielded contradictory results. We performed a prospective study of patients with acute ischemic stroke and motor deficit to evaluate the early prognostic value of TMS in motor and functional recovery. Fifty patients with different degrees of hemiparesis were studied in the first week after ischemic stroke and evaluated by clinical scales (Medical Research Council Scale, Canadian Neurological Scale, Barthel Index), with clinical follow-up over 6 months. TMS (Magstim 200) was performed at the same time, recording the motor evoked potential (MEP) in the thenar eminence muscles, with facilitation by voluntary contraction. Of the total group of 50 patients, MEP was absent in 20 and present in 30 (17 with normal and 13 with delayed central conduction time [CCT]). The patients with MEP showed better motor and functional recovery than those without. The MEP provided information on patient recovery, regardless of the initial strength and/or Barthel values. The degree of recovery was better in those patients with normal CCT than in those with delayed CCT. MEP obtained by TMS in patients with hemiparesis after acute ischemic stroke is useful as an early prognostic indicator of motor and functional recovery. This technique would allow the early identification of those patients who will have a good recovery, particularly among those with severe initial paresis.

Inhibitory action of forearm flexor muscle afferents on corticospinal outputs to antagonist muscles in humans.

1. To find out whether muscle afferents influence the excitability of corticospinal projections to antagonist muscles, we studied sixteen healthy subjects and one patient with a focal brain lesion. 2. Using transcranial magnetic and electrical brain stimulation we tested the excitability of corticomotoneuronal connections to right forearm muscles at rest after conditioning stimulation of the median nerve at the elbow. Somatosensory potentials evoked by median nerve stimulation were also recorded in each subject. 3. Test stimuli delivered at 13-19 ms after median nerve stimulation significantly inhibited EMG responses elicited in forearm extensor muscles by transcranial magnetic stimulation, but did not inhibit responses to electrical stimulation. In contrast, magnetically and electrically elicited responses in forearm flexor muscles were suppressed to the same extent. 4. The higher the intensity of the test shocks, the smaller was the amount of median nerve-elicited inhibition. Inhibition in extensor muscles was also smaller during tonic wrist extension, or if the induced electrical stimulating current in the brain flowed from posterior to anterior over the motor strip rather than vice versa. Test responses evoked by magnetic transcranial stimulation in the first dorsal interosseous and in brachioradialis muscles were not inhibited after median nerve stimulation at the elbow. Stimulation of digital nerves failed to inhibit motor potentials in extensor muscles. 5. Test stimuli delivered at 15 and 17 ms after radial nerve stimulation significantly inhibited EMG responses elicited in forearm flexor muscles by magnetic transcranial stimulation. 6. In the patient with a focal thalamic lesion, who had dystonic postures and an absent N20 component of the somatosensory-evoked potentials but normal strength, median nerve stimulation failed to inhibit magnetically evoked responses in forearm extensor muscles. 7. We propose that activation of median nerve muscle afferents can suppress the excitability of cortical areas controlling the antagonist forearm extensor muscles acting on the hand. The inhibitory effect occurs at short latency and might assist spinal pathways mediating reciprocal inhibition by contrasting the co-activation of antagonistic pools of corticospinal cells.

Latency of changes in spinal motoneuron excitability evoked by transcranial magnetic brain stimulation in spinal cord injured individuals

Objectives: To examine the basis for delay in the excitatory effects of transcranial magnetic stimulation (TMS) of motor cortex on motoneuron pools of muscles left partially-paralyzed by traumatic spinal cord injury (SCI). Methods: The effect of subthreshold transcranial magnetic stimulation (TMS) on just-suprathreshold H-reflex amplitude was examined in subjects ( n=10) with incomplete cervical SCI, and in able-bodied (AB) subjects ( n=20) for comparison. EMG activity was recorded from the soleus and the abductor hallucis muscles, and H-reflex was elicited by stimulation of the tibial nerve behind the knee. Comparison of the peak-to-peak amplitude of the TMS-conditioned H-reflex to that of the H-reflex alone (i.e. unconditioned H-reflex) was made for different conditioning-test intervals with multivariate analysis of variance and (when called for) t testing. Results: The absolute latencies of motor responses to suprathreshold TMS delivered during a weak voluntary contraction of the soleus and abductor hallucis were significantly prolonged in the SCI group relative to AB subjects. For the TMS-conditioned H-reflex, the time-course effect of TMS on the H-reflex amplitude in different AB subjects included an early effect (typically facilitation, but occasionally inhibition) seen between −5 and 0 ms, followed by a later period (i.e. >5 ms) of H-reflex facilitation. In contrast, the earliest indication of a TMS effect on H-reflex excitability in SCI subjects was between 5 and 10 ms after TMS. This difference between SCI and AB subjects of approximately 10 ms was similar to the prolongation of TMS-evoked response latencies in the soleus and the abductor hallucis muscles of the SCI subjects. Conclusions: The results suggest that motor conduction slowing after traumatic SCI most likely occurs across the population of the descending tract axons mediating the TMS-evoked motor responses.

Modulation of Corticospinal Output to Human Hand Muscles Following Deprivation of Sensory Feedback

Excitability and conductivity of corticospinal tracts of 10 volunteers were investigated by motor-evoked potentials (MEPs) to transcranial magnetic brain stimulation, before and after anesthetic block of right median (sensory + motor) and radial (sensory) nervous fibers at the wrist. MEPs were simultaneously recorded from two ulnar-supplied muscles during full relaxation and voluntary contraction. These muscles maintained an intact strength following anesthesia, but they were in a remarkably different condition with respect to the surrounding skin: the first dorsal interosseous muscle (FDI) was totally “enveloped” within the anesthetized area but was still dispatching a normal proprioceptive feedback; the abductor digiti minimi (ADM) was preserving both cutaneous and proprioceptive information. Spinal and peripheral nerve excitability were monitored as well. The sensory deprivation induced short-term changes which selectively took place within the hemisphere connected to the anesthetized hand. The physiological latency “anticipation” of MEPs recorded during active contraction versus relaxation was reduced (P< 0.001) in the FDI, but not in the ADM, when values during anesthesia were compared with preanesthesia values. The FDI cortical representation—as analyzed by a mapping procedure of the motor cortex via focal stimuli of several scalp positions—was significantly (P< 0.002) reduced, while the ADM representation remained either unchanged or enlarged. MEP and F-wave variability significantly decreased in the FDI but not in the ADM. F-waves were also affected due to changes in the motoneuronal excitability at spinal level. Peripheral nerve and root stimulation showed no modifications. Results are discussed in view of the short-term modifications of the corticospinal pathway somatotopy produced by the selective reduction of the sensory flow. Implications of the sensory feedback in motor control are also discussed.