Hirofumi Sekiguchi

Differences in recruitment properties of the corticospinal pathway between lengthening and shortening contractions in human soleus muscle.

The purpose of this study was to investigate how the recruitment properties of the corticospinal pathway are modulated in the soleus muscle of the lower limb during lengthening (LEN) and shortening (SHO) contractions by comparing the shape of the input-output (I/O) relation of the corticospinal pathway. To this end, we investigated the relationship between various stimulus intensities applied via transcranial magnetic stimulation and the size of motor-evoked potentials in 14 healthy subjects during voluntary plantarflexion and dorsiflexion (active lengthening) with a similar background activity (BGA) level. The shape of this relationship was sigmoidal and was characterized by a plateau value, maximum slope and threshold. The plateau value was clearly lower during LEN contractions than during SHO contractions. Likewise, the maximum slope was lower during LEN contractions. However, the threshold did not differ significantly between the two tasks. Since the plateau value and the maximum slope clearly differed between LEN and SHO contractions despite the similarity of their BGA levels, the central nervous system appears to have a different activation strategy for each of these tasks. Namely, the relative balance between excitatory and inhibitory components of the corticospinal volleys, as well as the subliminal fringe of the corticospinal pathway, were reduced during LEN contractions compared with SHO contractions. These strategies may help to avoid reflexive contractions brought about by higher discharge of muscle spindles and enable fine motor actions in voluntary lengthening contractions.

Lower excitability of the corticospinal tract to transcranial magnetic stimulation during lengthening contractions in human elbow flexors.

The purpose of this study was to characterize the neuromuscular control during shortening (SHO) and lengthening (LEN) contractions by investigating the input-output (I/O) property in the corticospinal tract. To this end, the relation between various stimulus intensities applied via transcranial magnetic stimulation and the size of motor evoked potentials was investigated in six healthy subjects during elbow flexion and extension. The measured I/O property demonstrates a sigmoidal shape, and is characterized by a plateau value, maximum slope and threshold. The results demonstrated that both the plateau value and maximum slope were significantly lower during LEN contraction than during SHO contraction (P<0.05), whereas the threshold was found not to be significantly different. These results suggest that both the maximum excitation level and the gain of the corticospinal tract are reduced during LEN contractions.

Effects of loading and unloading of lower limb joints on the soleus H-reflex in standing humans

OBJECTIVE To investigate the effects of loading and unloading of the lower limb joints on the soleus H-reflex in standing humans. METHODS H-reflexes were elicited in the soleus muscle in subjects standing on a force platform in a water tank under the following loading conditions of the ankle and knee joints: control condition; reduced loads of -10 and -20 N; imposed loads of 10 and 20 N. The joint loading was altered by changing the combinations of buoys and weights attached to the lower limb segments, while total body weight was kept constant. RESULTS As the ankle- or knee-joint load was reduced, the H-reflex was significantly enhanced compared to that under the control condition. In contrast, the H-reflex was decreased as the ankle- or knee-joint load was increased. In both cases, similar levels of background activity were recorded. CONCLUSIONS The present results suggest that joint afferents might mediate the suppression of the soleus H-reflex in standing humans. However, the identification of the receptors and/or the mechanisms cannot be addressed under the current experimental set up. SIGNIFICANCE The results of this study give some basic insights into reflex control in an upright posture.

An fMRI study of musicians with focal dystonia during tapping tasks.

Musician’s dystonia is a type of task specific dystonia for which the pathophysiology is not clear. In this study, we performed functional magnetic resonance imaging to investigate the motor-related brain activity associated with musician’s dystonia. We compared brain activities measured from subjects with focal hand dystonia and normal (control) musicians during right-hand, left-hand, and both-hands tapping tasks. We found activations in the thalamus and the basal ganglia during the tapping tasks in the control group but not in the dystonia group. For both groups, we detected significant activations in the contralateral sensorimotor areas, including the premotor area and cerebellum, during each tapping task. Moreover, direct comparison between the dystonia and control groups showed that the dystonia group had greater activity in the ipsilateral premotor area during the right-hand tapping task and less activity in the left cerebellum during the both-hands tapping task. Thus, the dystonic musicians showed irregular activation patterns in the motor-association system. We suggest that irregular neural activity patterns in dystonic subjects reflect dystonic neural malfunction and consequent compensatory activity to maintain appropriate voluntary movements.

Inhibition of the human soleus Hoffman reflex during standing without descending commands

The purpose of the present study was to ascertain the contribution of peripheral sensory inputs to posture-related Hoffman reflex (H-reflex) modulation in the human soleus muscle. The soleus H-reflexes were elicited in the sitting (SI) and passive standing (ST) conditions in patients with clinically complete spinal cord injuries (SCI) and in neurologically normal subjects. The results clearly showed suppression of the H-reflex amplitude during the ST compared with the SI condition especially in the SCI group. Considering the lack of a descending neural command in the SCI patients, our findings suggest that peripheral sensory inputs primarily contribute to the reduction of the soleus H-reflex during the upright standing posture.

The role of the dorsolateral prefrontal cortex in the inhibition of stereotyped responses

Stereotyped behaviors should be inhibited under some circumstances in order to encourage appropriate behavior. Psychiatrists have used the modified rock-paper-scissors (RPS) task to examine the inhibition of stereotyped behavior. When subjects are required to lose in response to a gesture, it is difficult for them to lose, and they have a tendency to win involuntarily. It is thought that the win response is the stereotyped response in the RPS task, and the difficulty in making positive attempts to lose is due to the requirement for inhibition of the stereotyped response. In this study, we investigated the brain regions related to inhibition of the stereotyped response using functional magnetic resonance imaging (fMRI). Subjects were assigned to one of two groups: the “win group” or the “lose group.” The lose group showed higher activation of the left dorsolateral prefrontal cortex (DLFPC) when compared to the win group. We also delivered transcranial magnetic stimulation (TMS) while the subjects performed the modified RPS task to investigate whether the left DLPFC (middle frontal gyrus, Brodmann area, BA 9) was directly involved in the inhibition of the stereotyped response. When TMS was delivered before onset of the visual stimulus, the subjects displayed increased response errors. In particular, the subjects had a tendency to win erroneously in a lose condition even though they were required to lose. These results indicate involvement of the left DLPFC in inhibition of the stereotyped responses, which suggests that this region is associated with inhibition of the preparatory setting for stereotyped responses rather than inhibition of ongoing processing to produce a stereotyped response.

TMS-induced artifacts on EEG can be reduced by rearrangement of the electrode’s lead wire before recording

OBJECTIVE Our purpose was to establish a technique to reduce residual artifacts after transcranial magnetic stimulation (TMS) from electroencephalographic (EEG) signals. METHODS We investigated the effects of coil direction and stimulus intensity on residual artifacts in an artificial circuit, and tested whether or not the size of the circuit area affects the residual artifact (the model study). Based on the results, the optimization by rearranging the electrodes lead wire was tested on the human scalp (the human study). RESULTS The residual artifact after TMS was dependent on the direction of the figure-of-eight coil, and on the artificial circuit area size. CONCLUSIONS In accordance with the model study, the scalp EEG shows that TMS-induced artifacts can be reduced dramatically before the amplifier input stages in TMS-EEG experiments by a step-wise procedure rearranging the lead wires relative to the fixed coil orientation. SIGNIFICANCE Our technique makes it possible to significantly reduce the residual artifacts from recordings of short-latency TMS-evoked potentials.

Posture-related modulation of cortical excitability in the tibialis anterior muscle in humans

Corticospinal excitability in the lower leg muscles is enhanced during standing as compared to other postures. In the present study, we investigated how the excitability of intracortical circuits that control the tibialis anterior muscle (TA) is modulated during standing. Short-interval intracortical inhibition (SICI) and intracortical facilitation (ICF) were assessed by the paired-pulse transcranial magnetic stimulation technique during standing (STD) and sitting (SIT) with a comparable background activity level in both the soleus and the TA muscle. The results demonstrated that SICI was less effective during STD than during SIT, whereas ICF was more effective during STD than during SIT. These findings suggest that the excitabilities of these cortical neural circuits are modulated depending on posture. A decrease in SICI and an increase in ICF may reflect subliminal enhancement of the cortical excitability in the TA muscle during standing as compared with that during sitting.

Recruitment gain of antagonistic motoneurons is higher during lengthening contraction than during shortening contraction in man

The purpose of this study was to investigate how the recruitment gain, as derived from the Hoffmann (H)-reflex measurement, of antagonistic motoneurons is modulated during voluntary lengthening (LEN) and shortening (SHO) contractions in the human tibialis anterior (TA) muscle. To this end, the parameters of the ratios of the slope in each ascending part of the H and M recruitment curves (Hslp/Mslp) in the antagonist muscle were compared between LEN and SHO contractions in nine young, healthy subjects. Although there were no differences in the levels of background activity (BGA) between LEN and SHO contractions in the agonist (TA) and antagonist (soleus) muscles, the Hslp/Mslp of the antagonist muscle was significantly higher during LEN contractions than during SHO contractions (P<0.01). It was therefore demonstrated that the recruitment gain of the antagonistic motoneurons to the la afferent inputs was higher during LEN contractions than during SHO contractions despite similar BGA levels. This result might reflect differences in the extent of the reciprocal inhibition from the agonist to the antagonist muscles and/or in the neural mechanism underlying the length-changing manners of the antagonist muscle itself.

Dissociating the neural correlates of tactile temporal order and simultaneity judgements

Perceiving temporal relationships between sensory events is a key process for recognising dynamic environments. Temporal order judgement (TOJ) and simultaneity judgement (SJ) are used for probing this perceptual process. TOJ and SJ exhibit identical psychometric parameters. However, there is accumulating psychophysical evidence that distinguishes TOJ from SJ. Some studies have proposed that the perceptual processes for SJ (e.g., detecting successive/simultaneity) are also included in TOJ, whereas TOJ requires more processes (e.g., determination of the temporal order). Other studies have proposed two independent processes for TOJ and SJ. To identify differences in the neural activity associated with TOJ versus SJ, we performed functional magnetic resonance imaging of participants during TOJ and SJ with identical tactile stimuli. TOJ-specific activity was observed in multiple regions (e.g., left ventral and bilateral dorsal premotor cortices and left posterior parietal cortex) that overlap the general temporal prediction network for perception and motor systems. SJ-specific activation was observed only in the posterior insular cortex. Our results suggest that TOJ requires more processes than SJ and that both TOJ and SJ implement specific process components. The neural differences between TOJ and SJ thus combine features described in previous psychophysical hypotheses that proposed different mechanisms.