Motor unit firing variability and synchronization during short-term light-load training in older adults

We compared motor unit synchronization and firing rate variability within and across synergistic hand muscles during a pinching task following short-term light-load training to improve force steadiness in older adults. A total of 183 motor unit pairs before training and 158 motor unit pairs after training were recorded with intramuscular fine-wire electrodes within and across the first dorsal interosseous (FDI) and adductor pollicis (AdP) muscles during a pinch task performed by ten older adults before and after a 4-week short-term light-load training program. Nine younger adults performed the same experimental sessions 4 weeks apart with no training intervention. Two-minute sustained contractions of 2, 4, 8, and 12% maximal voluntary contraction (MVC) were performed with the non-dominant hand. The coefficient of variation (CV) of force was greater in older than in younger adults and was lower at the 2 and 4% MVC levels in both the finger (0.12 ± 0.01 vs. 0.08 ± 0.01, and 0.08 ± 0.01 vs. 0.05 ± 0.01, respectively) and thumb (0.11 ± 0.01 vs. 0.08 ± 0.01, and 0.09 ± 0.01 vs. 0.05 ± 0.01, respectively) compared to higher force levels following training in the older adults. There were no changes in CIS or k’-1 values following training. Motor unit firing rate variability significantly decreased at low force levels in the FDI muscle and also tended to decrease with training in the AdP muscle (p = 0.06). No changes occurred in the younger control group. These findings are the first to show that motor unit synchronization does not change during light-load training. Thus, it is likely that force steadiness in older adults improves by reducing motor unit firing variability rather than by changing motor unit synchronization.     

Age-related differences of saccade induced cortical activation

Recent functional MRI (fMRI) studies have described the increased task-related brain activation in older subjects during motor, cognitive and perceptual tasks. Age affects the ability to control saccadic eye movements. To investigate the age-related changes of oculomotor control, we studied the representation of saccades in 11 young (median age 29 years) and 11 older (median age 62 years) healthy individuals using fMRI. Brain activation was measured during a visually guided prosaccade trial. Differences in activation between rest and saccades as well as between younger and older subjects were assessed with statistical parametric mapping (SPM). In both age groups, activation of a frontoparietal network was observed. Older subjects showed increased activation compared to younger subjects with overactivation in bilateral parietal eye fields, the right frontal eye field, as well as in the right extrastriate cortex. We conclude that older adults increase activation in an extended oculomotor and visual network to maintain performance during simple prosaccades. This observation also underlines the importance of using appropriate age-matched control groups in fMRI studies after brain lesions.     

Modulating neural networks with transcranial magnetic stimulation applied over the dorsal premotor and primary motor cortices

Our study uses the combined transcranial magnetic stimulation/positron emission tomography (TMS/PET) method for elucidating neural connectivity of the human motor system. We first altered motor excitability by applying low-frequency repetitive TMS over two cortical motor regions in separate experiments: the dorsal premotor and primary motor cortices. We then assessed the consequences of modulating motor excitability by applying single-pulse TMS over the primary motor cortex and measuring: 1) muscle responses with electromyography and 2) cerebral blood flow with PET. Low-frequency repetitive stimulation reduced muscle responses to a similar degree in both experiments. To map networks of brain regions in which activity changes reflected modulation of motor excitability, we generated t-statistical maps of correlations between reductions in muscle response and differences in cerebral blood flow. Low-frequency repetitive stimulation altered neural activity differently in both experiments. Neural modulation occurred in multiple brain regions after dorsal premotor cortex stimulation; these included motor regions in the frontal cortex as well as more associational regions in the parietal and prefrontal cortices. In contrast, neural modulation occurred in a smaller number of brain regions after primary motor cortex stimulation, many of these confined to the motor system. These findings are consistent with the known differences between the dorsal premotor and primary motor cortices in the extent of cortico-cortical anatomical connectivity in the monkey.     

Efficient Mapping of the Motor Cortex with Navigated Biphasic Paired-Pulse Transcranial Magnetic Stimulation

Navigated transcranial magnetic stimulation (nTMS) can be applied to locate cortical muscle representations. Usually, single TMS pulses are targeted to the motor cortex with the help of neuronavigation and by measuring motor evoked potential (MEP) amplitudes from the peripheral muscles. The efficacy of single-pulse TMS to induce MEPs has been shown to increase by applying facilitatory paired-pulse TMS (ppTMS). Therefore, the aim was to study whether the facilitatory ppTMS could enable more efficient motor mapping. Biphasic single-pulse TMS and ppTMS with inter-stimulus intervals (ISIs) of 1.4 and 2.8 ms were applied to measure resting motor thresholds (rMTs) as a percentage of the maximal stimulator output and to determine the cortical representation areas of the right first dorsal interosseous muscle in healthy volunteers. The areas, shapes, hotspots, and center of gravities (CoGs) of the representations were calculated. Biphasic ppTMS with ISI of 1.4 ms resulted in lower rMTs than those obtained with the other protocols (p?=?0.001). With ISI of 2.8 ms, rMT was lower than with single-pulse TMS (p?=?0.032). The ppTMS mapping was thus performed with lower intensity than when using single-pulse TMS. The areas, shapes, hotspots, and CoGs of the muscle representations were in agreement. Hence, biphasic ppTMS has potential in the mapping of cortical hand representations in healthy individuals as an alternative for single-pulses, but with lower stimulation intensity by utilizing cortical facilitatory mechanism. This could improve application of nTMS in subjects with low motor tract excitability.     

Evidence for motor cortex dedifferentiation in older adults

Older adults (OA) show more diffuse brain activity than young adults (YA) during the performance of cognitive, motor, and perceptual tasks. It is unclear whether this overactivation reflects compensation or dedifferentiation. Typically, these investigations have not evaluated the organization of the resting brain, which can help to determine whether more diffuse representations reflect physiological or task-dependent effects. In the present study we used transcranial magnetic stimulation (TMS) to determine whether there are differences in motor cortex organization of both brain hemispheres in young and older adults. We measured resting motor threshold, motor evoked potential (MEP) latency and amplitude, and extent of first dorsal interosseous representations, in addition to a computerized measure of reaction time. There was no significant age difference in motor threshold, but we did find that OA had larger contralateral MEP amplitudes and a longer contralateral MEP latency. Furthermore, the spatial extent of motor representations in OA was larger. We found that larger dominant hemisphere motor representations in OA were associated with higher reaction times, suggesting dedifferentiation rather than compensation effects.     

Motor unit synchronization measured by cross-correlation is not influenced by short-term strength training of a hand muscle

The purpose of the study was to quantify the strength of motor unit synchronization and coherence from pairs of concurrently active motor units before and after short-term (4–8 weeks) strength training of the left first dorsal interosseous (FDI) muscle. Five subjects (age 24.8 ± 4.3 years) performed a training protocol three times/week that consisted of six sets of ten maximal isometric index finger abductions, whereas three subjects (age 27.3 ± 6.7 years) acted as controls. Motor unit activity was recorded from pairs of intramuscular electrodes in the FDI muscle with two separate motor unit recording sessions obtained before and after strength training (trained group) or after 4 weeks of normal daily activities that did not involve training (control group). The training intervention resulted in a 54% (45.2 ± 8.3 to 69.5 ± 13.8 N, P = 0.001) increase in maximal index finger abduction force, whereas there was no change in strength in the control group. A total of 163 motor unit pairs (198 single motor units) were examined in both subject groups, with 52 motor unit pairs obtained from 10 recording sessions before training and 51 motor unit pairs from 10 recording sessions after training. Using the cross-correlation procedure, there was no change in the strength of motor unit synchronization following strength training (common input strength index; 0.71 ± 0.41 to 0.67 ± 0.43 pulses/s). Furthermore, motor unit coherence z scores at low (0–10 Hz; 3.9 ± 0.3 before to 4.4 ± 0.4 after) or high (10–30 Hz; 1.7 ± 0.1 before to 1.9 ± 0.1 after) frequencies were not influenced by strength training. These motor unit data indicate that increases in strength following several weeks of training a hand muscle are not accompanied by changes in motor unit synchronization or coherence, suggesting that these features of correlated motor unit activity are not important in the expression of muscle strength.Dawson J. Kidgell and Martin V. Sale contributed equally to the study.     

Short and long duration transcranial direct current stimulation (tDCS) over the human hand motor area

The aim of the present paper is to study effects of short and long duration transcranial direct current stimulation (tDCS) on the human motor cortex. In eight normal volunteers, motor evoked potentials (MEPs) induced by transcranial magnetic stimulation (TMS) were recorded from the right first dorsal interosseous muscle, and tDCS was given with electrodes over the left primary motor cortex (M1) and the contralateral orbit. We performed two experiments: one for short duration tDCS (100 ms, 1, 3 or 5 mA) and the other for long duration tDCS (10 min, 1 mA). The stimulus onset asynchrony (SOA) between the onset of tDCS and TMS were 1–7 and 10–120 ms for the former experiment. In the latter experiment, TMS was given 0–20 min after the end of 10 min tDCS. We evaluated the effect of tDCS on the motor cortex by comparing MEPs conditioned by tDCS with control MEPs. Cathodal short duration tDCS significantly reduced the size of responses to motor cortical stimulation at SOAs of 1–7 ms when the intensity was equal to or greater than 3 mA. Anodal short duration tDCS significantly increased MEPs when the intensity was 3 mA, but the enhancement did not occur when using 5 mA conditioning stimulus. Moreover, both anodal and cathodal short duration tDCS decreased responses to TMS significantly at SOAs of 20–50 ms and enhanced them at an SOA of 90 ms. Long duration cathodal tDCS decreased MEPs at 0 and 5 min after the offset of tDCS and anodal long duration tDCS increased them at 1 and 15 min. We conclude that the effect at SOAs less than 10 ms is mainly caused by acute changes in resting membrane potential induced by tDCS. The effect at SOAs of 20–100 ms is considered to be a nonspecific effect of a startle-like response produced by activation of skin sensation at the scalp. The effect provoked by long duration tDCS may be short-term potentiation or depression like effects.     

Motor unit synchronization in young and elderly adults

In an effort to determine whether aging might alter the manner in which pairs of motor units are coactivated, the extent of motor unit synchronization was assessed in the first dorsal interosseous (FDI) muscle in seven young (mean 28 years) and eight older adults (mean 75 years). During constant-force isometric contractions at either 50% or 100% of maximal effort, motor units were recorded using a four-wire needle electrode and a multi-channel recording technique. Customized software was utilized to identify the occurrences of motor unit action potentials from the resultant three-channel signals. The magnitude of motor unit synchronization was determined using six variables that have been described previously in the literature. The extent of motor unit synchronization was similar in both young and older adults, occurring in nearly every motor unit pair observed. During the 50% maximum voluntary contractions, the CIS synchronization measure (the number of synchronized discharges per unit time) averaged 2.5 in the young subjects and 2.4 in the older individuals. The intensity of motor unit synchronization was somewhat greater during maximal force isometric contractions. We conclude that motor unit synchronization in the FDI muscle is a ubiquitous phenomenon that is not affected by the aging process. Accepted: 7 October 1999     

Relationships between motor unit size and recruitment threshold in older adults: implications for size principle

As a part of the aging process, motor unit reorganization occurs in which small motoneurons reinnervate predominantly fast-twitch muscle fibers that have lost their innervation. We examined the relationship between motor unit size and the threshold force for recruitment in two muscles to determine whether older individuals might develop an alternative pattern of motor unit activation. Young and older adults performed isometric contractions ranging from 0 to 50% of maximal voluntary contraction in both the first dorsal interosseous (FDI) and tibialis anterior (TA) muscles. Muscle fiber action potentials were recorded with an intramuscular needle electrode and motor unit size was computed using spike-triggered averaging of the global EMG signal (macro EMG), which was also obtained from the intramuscular needle electrode. As expected, older individuals exhibited larger motor units than young subjects in both the FDI and the TA. However, moderately strong correlations were obtained for the macro EMG amplitude versus recruitment threshold relationship in both the young and older adults within both muscles, suggesting that the size principle of motor unit recruitment seems to be preserved in older adults.     

Measurement of spontaneous signal fluctuations in fMRI: adult age differences in intrinsic functional connectivity

Functional connectivity (FC) reflects the coherence of spontaneous, low-frequency fluctuations in functional magnetic resonance imaging (fMRI) data. We report a behavior-based connectivity analysis method, in which whole-brain data are used to identify behaviorally relevant, intrinsic FC networks. Nineteen younger adults (20–28 years) and 19 healthy, older adults (63–78 years) were assessed with fMRI and diffusion tensor imaging (DTI). Results indicated that FC involving a distributed network of brain regions, particularly the inferior frontal gyri, exhibited age-related change in the correlation with perceptual-motor speed (choice reaction time; RT). No relation between FC and RT was evident for younger adults, whereas older adults exhibited a significant age-related slowing of perceptual-motor speed, which was mediated by decreasing FC. Older adults’ FC values were in turn associated positively with white matter integrity (from DTI) within the genu of the corpus callosum. The developed FC analysis illustrates the value of identifying connectivity by combining structural, functional, and behavioral data.     

Hemoglobin concentration changes in the contralateral hemisphere during and after theta burst stimulation of the human sensorimotor cortices

Using near infrared spectroscopy and repetitive transcranial magnetic stimulation (rTMS), we studied interhemispheric interactions between bilateral motor and sensory cortices in humans. RTMS consisted of a triple-pulse burst (50 Hz) repeated every 200 m for 2 s (10 bursts, 30 pulses); one kind of theta burst TMS (TBS) (Huang et al. in Neuron 45:201–206, 2005). The hemoglobin concentration changes were recorded at the right prefrontal cortex, premotor area (PM), primary hand motor area (M1) and primary sensory area (S1) during and after TBS over the left PM, M1 and S1 or sham stimulation in eight normal volunteers. In addition, motor evoked potentials (MEPs) to TMS over the right M1 were recorded from the left first dorsal interosseous muscle after the conditioning TBS over left S1. TBS over PM induced a significant oxy-Hb decrease at the contralateral PM. TBS over M1 elicited a significant oxy-Hb decrease at the contralateral S1, and TBS over S1 significant oxy-Hb decreases at the contralateral M1 and S1. MEPs to TMS of the right M1 were significantly suppressed by the conditioning TBS over the left S1. These results suggest that there are mainly inhibitory interactions between bilateral PMs and bilateral sensorimotor cortices in humans. Those are partly compatible with the previous findings. In addition to between the primary motor cortices, bilateral connection is requisite for smooth bimanual coordination between the sensory cortices or premotor cortices.     

Task-dependent changes of intracortical inhibition

 The motor-evoked potential (MEP) to transcranial magnetic stimulation (TMS) is inhibited when preceded by a subthreshold TMS stimulus at short intervals (1–6 ms; intracortical inhibition, ICI) and is facilitated when preceded by a subthreshold TMS at longer intervals (10–15 ms; intracortical facilitation, ICF). We studied changes in ICI and ICF associated with two motor tasks requiring a different selectivity in fine motor control of small hand muscles (abductor pollicis brevis muscle, APB, and fourth dorsal interosseous muscle, 4DIO). In experiment 1 (exp. 1), nine healthy subjects completed four sets (5 min duration each) of repetitive (1 Hz) thumb movements. In experiment 2 (exp. 2), the subjects produced the same number of thumb movements, but complete relaxation of 4DIO was demanded. Following free thumb movements (exp. 1), amplitudes of MEPs in response to both single and paired TMS showed a trend to increase with the number of exercise sets in both APB and 4DIO. By contrast, more focal, selective thumb movementsinvolving APB with relaxation of 4DIO (exp. 2) caused an increase in MEP amplitudes after single and paired pulses only in APB, while a marked decrease in MEPs after paired pulses, but not after single TMS, in the actively relaxed 4DIO. This effect was more prominent for the interstimulus interval (ISI) of 1–3 ms than for longer ISIs (8 ms, 10 ms, and 15 ms). F-wave amplitudes reflecting excitability of the alpha motoneuron pool were unaltered in APB and 4DIO, suggesting a supraspinal origin for the observed changes. We conclude that plastic changes of ICI and ICF within the hand representation vary according to the selective requirements of the motor program. Performance of more focal tasks may be associated with a decrease in ICI in muscles engaged in the training task, while at the same time ICI may be increased in an actively relaxed muscle, also required for a focal performance. Additionally, our data further supports the idea that ICI and ICF may be controlled independently. Received: 20 September 1996 / Accepted: 1 October 1997     

The effect of test TMS intensity on short-interval intracortical inhibition in different excitability states

The paired-pulse transcranial magnetic stimulation (TMS) paradigm is increasingly employed to examine intracortical inhibitory processes in different motor tasks. Short-interval intracortical inhibition (SICI) has been shown to vary with the size of the MEP elicited by the test TMS pulse. This suggests that factors that alter MEP size, such as changes in cortical excitability, may confound the interpretation of SICI. However, the effect of excitability on SICI has not been systematically investigated. The present study examined SICI in 11 volunteers. The effect of test TMS intensities ranging from 90 to 150% resting motor threshold (RMT) on SICI was examined in three excitability states in the right first dorsal interosseous muscle: rest, isometric abduction of the left index finger (Contra) and isometric abduction of the right index finger (Active). For all excitability states SICI was not observed when test TMS intensity was less than 110% resting motor threshold. This was true even for the Active condition in which 90 and 100% test TMS intensities elicited large and consistent MEPs. For all conditions moderately suprathreshold test TMS intensities (110–120% RMT) yielded the greatest measure of SICI; increasing test TMS intensities resulted in a progressive reduction in the estimate of SICI. These results suggest that estimates of SICI are systematically affected by the intensity of the test TMS pulse, regardless of excitability state. The results suggest that SICI should be examined using a constant test TMS intensity regardless of changes in cortical excitability and test MEP size.     

Effects of age on virtual environment place navigation and allocentric cognitive mapping

This study assessed age differences in navigational behavior in a virtual Morris water maze (vMWM) and examined the ability of older adults to develop cognitive maps after vMWM experience. Compared with younger participants, older volunteers traversed a longer linear distance to locate the hidden platform. On the probe trial, younger volunteers spent a greater proportion of their total distance traveled in proximity to the platform and had more platform intersections. Analysis of map reproductions demonstrated that older participants used proximal objects to locate the goal but did not use room-geometry cues to aid navigation. These findings demonstrate age-related deficits on a laboratory measure of place learning and suggest that deficiencies in allocentric mapping may contribute to these deficits.     

Remote effects of self-paced teeth clenching on the excitability of hand motor area

We studied remote effects of teeth clenching on motor cortical and spinal cord excitability using transcranial magnetic stimulation (TMS), brainstem electrical stimulation (BES), and ulnar nerve stimulation (F-wave) in eight normal volunteers. The TMS, BES, and ulnar nerve stimulation at the wrist were given at different intervals (0–200 ms) after the onset of masseter contraction. Surface electromyographic responses were recorded from the first dorsal interosseous muscle. Responses at different intervals were compared with the response elicited when the subject made no teeth clenching (control response). In TMS, conditioned responses (during teeth clenching) were significantly larger than the control at all intervals. In contrast, in BES and F-waves, conditioned responses were not larger than the control at an early phase (intervals shorter than 50 ms), whereas they were larger than the control at later intervals (longer than 50 ms). These results suggest that facilitation occurs in the hand motor area at the early phase of teeth clenching, and spinal facilitation dominates at its late phase. This time course of facilitation may indicate that the motor cortex must regulate hand muscles finely at the early phase of teeth clenching, and spinal cord may stabilize them firmly at the late phase. The excitability changes of the hand motor area may be in parallel with that of the masseter motor area which reflects the pattern of masseter contraction when the subject activates the masseter muscle phasically at the early phase and sustains that contraction at the late phase. Electronic Publication     

Age-related differences in corticospinal excitability and inhibition during coordination of upper and lower limbs

The ability to coordinate upper and lower limbs--a prerequisite for many everyday activities--is known to decline with age. Here we report 2 experiments in which transcranial magnetic stimulation (TMS) was used to assess corticospinal excitatory and inhibitory processes in younger and older adults during cyclical hand-foot movements. In experiment 1, motor evoked potentials (MEP) and silent period (SP) durations were measured from the active right extensor carpi radialis (ECR) muscle while it executed rhythmic oscillations in conjunction with the right or left foot. Younger adults exhibited increased SP with ipsilateral limb combinations and decreased SP with contralateral limb combinations, relative to a baseline hand only condition. Strikingly, older adults exhibited a reduced SP when ipsilateral limbs moved in opposite directions. This effect was found to be most pronounced in those older adults who exhibited poor coordination performance, suggesting that the inability to regulate inhibitory processes may underlie age-related degradation of task performance. Experiment 2 examined motor evoked potentials and SP duration in the left extensor carpi radialis which maintained a tonic contraction while the coordination task was undertaken by the right arm and right or left foot. For younger adults, coordination of ipsilateral limbs was accompanied by increased inhibition in the ipsilateral motor cortex than during the coordination of contralateral limbs. No differences in SP between conditions were noted for the older adults. In summary, older adults' reduced ability to coordinate upper and lower limbs may be related to the capacity to regulate inhibitory function in both hemispheres. This study suggests for the first time a direct link between age-related differences in interlimb coordination and the control of corticospinal inhibitory processes.     

Intense training overcomes effects of the val<Superscript>66</Superscript>met BDNF polymorphism on short-term plasticity

The val⁶⁶met polymorphism in the brain-derived neurotrophic factor (BDNF) gene impacts activity-dependent secretion of BDNF and modifies short-term cortical plasticity. The current study examined whether sustained training overcomes polymorphism effects on short-term plasticity and also examined polymorphism effects on long-term plasticity. Twenty-four subjects completed a 12-day protocol of daily training on a marble navigation task that required intense use of the first dorsal interosseus (FDI) muscle. In parallel, transcranial magnetic stimulation (TMS) mapping was used to assess serial measures of short-term cortical motor map plasticity, plus long-term cortical motor map plasticity, of the cortical FDI map. On Day 1, subjects with the polymorphism did not show significant short-term cortical motor map plasticity over 30 min of FDI activity, but subjects without the polymorphism did. After 5 days of intense training, a genotype-based difference in short-term cortical motor map plasticity was no longer found, as both groups showed short-term plasticity across the 30 min of FDI activity. Also, across 12 days of training, map area decreased significantly, in a manner that did not vary in relation to genotype. Training of sufficient intensity and duration overcomes effects that the val⁶⁶met polymorphism has on short-term cortical motor map plasticity. The polymorphism-related differences seen with short-term plasticity are not found with long-term cortical motor map plasticity.     

Diminished task-related adjustments of common inputs to hand muscle motor neurons in older adults

The purpose of this study was to quantify correlated motor unit activity during isometric, shortening and lengthening contractions of a hand muscle in older adults. Thirteen old subjects (69.6±5.9 years, six women) lifted and lowered a light load with abduction–adduction movements of the index finger over 10° using 6-s shortening and lengthening contractions of the first dorsal interosseus muscle. The task was repeated 10–20 times while activity in 23 pairs of motor units was recorded with intramuscular electrodes. The data were compared with 23 motor-unit pairs in 15 young (25.9±4.6 years, five women) subjects obtained using a similar protocol in a previous study. Correlated motor unit activity was quantified using time-domain (synchronization index; Common Input Strength) and frequency-domain (coherence) analyses for the same motor-unit pairs. For all contractions, there was no difference with age for the strength of motor-unit synchronization, although age-related differences were observed for synchronous peak widths (young, 17.6±7.4 ms; old, 13.7±4.9 ms) and motor-unit coherence at 6–9 Hz (z score for young, 3.0±1.8; old, 2.2±1.5). Despite increased synchrony during lengthening contractions and narrower peak widths for shortening contractions in young subjects, there was no difference in the strength of motor unit synchronization (CIS ~0.8 imp/s), or the width of the synchronous peak (~14 ms) during the three tasks in old subjects. Furthermore, no significant differences in motor-unit coherence were observed between tasks at any frequency for old adults. These data suggest that the strategy used by the central nervous system to control isometric, shortening, and lengthening contractions varies in young adults, but not old adults. The diminished task-related adjustments of common inputs to motor neurons are a likely consequence of the neural adaptations that occur with advancing age.     

Cortical hemoglobin-concentration changes under the coil induced by single-pulse TMS in humans: a simultaneous recording with near-infrared spectroscopy

We measured cortical hemoglobin-concentration changes under the coil induced by single-pulse transcranial magnetic stimulation (TMS) using a technique of simultaneous recording with near-infrared spectroscopy (NIRS). Single-pulse TMS was delivered over the hand area of the left primary motor cortex at an intensity of 100, 120, or 140% of the active motor threshold (AMT). NIRS recordings were also made during sham stimulation. These four different stimulation sessions (TMS at three intensities and sham stimulation) were performed both when the subject slightly contracted the right first dorsal interosseous muscle and when relaxed it (active and resting conditions). Under the active condition with TMS at 100% AMT, we observed a transient increase in oxy-hemoglobin (oxy-Hb), which was significantly larger than sham stimulation. Under the resting conditions with TMS at 120 and 140% AMT, we observed significant decreases in both deoxy-hemoglobin (deoxyHb) and total-hemoglobin (total-Hb) as compared to sham stimulation. We suggest that the increase of oxy-Hb concentration at 100% AMT under the active condition reflects an add-on effect by TMS to the active baseline and that decrease of deoxy-Hb and total-Hb concentrations at 120 and 140% AMT under the resting condition are due to reduced baseline firings of the corticospinal tract neurons induced by a lasting inhibition provoked by a higher intensity TMS.     

Reduced interhemispheric inhibition in musicians

In vivo magnetic resonance imaging has revealed that the anterior half of the corpus callosum is larger in musicians trained intensively from an early age than in untrained subjects. The corpus callosum is crucial for the coordination of bimanual motor activity, but neurophysiological correlates of morphological differences in the corpus callosum of musicians are not known. In the present study we have used transcranial magnetic stimulation (TMS) to assess interhemispheric inhibition in six adult professional musicians who began musical training at an early age. Conditioning TMS was applied to the hand area of the motor cortex of one hemisphere, followed 4-16 ms later by a test stimulus applied to the other hemisphere. Tests were performed at rest, and with the first dorsal interosseous muscle contralateral to the conditioning hemisphere voluntarily active. Conditioning TMS in musicians was 29% less effective at reducing the size of the test MEP at rest, and 63% less effective in the active condition, compared with control subjects. We conclude that transcallosal interhemispheric inhibitory circuits activated by TMS are less effective in musicians than in controls.