Biomedical subjects

Yasoichi Nakajima

Publications and source records attributed to Yasoichi Nakajima.

5 recordsLinked to original sources

PubMed2006

Modulation of corticospinal excitability during lengthening and shortening contractions in the first dorsal interosseus muscle of humans.

Lengthening and shortening contractions are the fundamental patterns of muscle activation underlying various movements. It is still unknown whether or not there is a muscle-specific difference in such a fundamental pattern of muscle activation. The purpose of this study was, therefore, to investigate whether or not the relationship between lengthening and shortening contractions in the modulation of corticospinal excitability in the first dorsal interosseus (FDI) muscle is the same as that of previously tested muscles because the hand muscles are anatomically and functionally different from the other muscles. To this end, we investigated the relationship between the input-output curves of the corticospinal pathway (i.e., the relationship between the stimulus intensities vs. the area of motor-evoked potentials) during lengthening and shortening contractions in 17 healthy subjects. The shape of this relationship was sigmoidal and characterized by a plateau value, maximum slope, and threshold. The plateau value was at the same level between lengthening and shortening contractions. However, the maximum slope (P < 0.01) and threshold (P < 0.01) were significantly higher during lengthening contractions than during shortening contractions. These findings were different from the results of other muscles tested in previous studies (i.e., the soleus muscle and the elbow flexors). That is to say, the plateau value and the maximum slope during lengthening contractions were significantly lower than those during shortening contractions in previous studies. This study provides tentative evidence that the relationship between lengthening and shortening contractions in the modulation of corticospinal excitability differs between muscles, indicating that the underlying neural control is not necessarily the same even though the fundamental patterns of muscle activation are carried out.

Action Potentials↗

PubMed2005

Testing Bayesian models of human coincidence timing.

A sensorimotor control task often requires an accurate estimation of the timing of the arrival of an external target (e.g., when hitting a pitched ball). Conventional studies of human timing processes have ignored the stochastic features of target timing: e.g., the speed of the pitched ball is not generally constant, but is variable. Interestingly, based on Bayesian theory, it has been recently shown that the human sensorimotor system achieves the optimal estimation by integrating sensory information with prior knowledge of the probabilistic structure of the target variation. In this study, we tested whether Bayesian integration is also implemented while performing a coincidence-timing type of sensorimotor task by manipulating the trial-by-trial variability (i.e., the prior distribution) of the target timing. As a result, within several hundred trials of learning, subjects were able to generate systematic timing behavior according to the width of the prior distribution, as predicted by the optimal Bayesian model. Considering the previous studies showing that the human sensorimotor system uses Bayesian integration in spacing and force-grading tasks, our result indicates that Bayesian integration is fundamental to all aspects of human sensorimotor control. Moreover, it was noteworthy that the subjects could adjust their behavior both when the prior distribution was switched from wide to narrow and vice versa, although the adjustment was slower in the former case. Based on a comparison with observations in a previous study, we discuss the flexibility and adaptability of Bayesian sensorimotor learning.

Adult↗

PubMed2004

1/f-type fluctuation in human visuomotor transformation.

In the absence of vision of the limb, movements toward a visual target exhibit substantial errors which are considered to originate mainly in the visuomotor transformation process. To determine the time-dependent property of human visuomotor transformation, we investigated the error sequences in movements toward visual target using scaling analyses. When subjects could see their controlling limb, the error sequences could not be distinguished from a random sequence. On the other hand, when the controlling limb was invisible, the error sequences were not random in order, but exhibited 1/f-type time correlation. This finding that the variation in human visuomotor transformation shows 1/f-type fluctuation provides a significant index for mathematical modeling and system identification in human visuomotor control.

Adult↗

PubMed2004

Selective activation and deactivation of the human brain structures between speeded and precisely timed tapping responses to identical visual stimulus: an fMRI study.

We investigated the difference between brain activities in speeded and precisely timed responses to identical visual stimulus using fMRI. Stimulus used was a row of seven light-emitting diodes (LEDs) lightened up one after another with constant speed within a trial but with various speeds between trials. Subjects were asked to execute finger-thumb tapping with the right hand in response to the onset of the first LED light in the reaction time (RT) task and in anticipation of the onset of the last (i.e., seventh) LED light in the timing task. In control condition, they were asked to passively view the stimulus without motor response. Results showed that various movement-related areas including contralateral cingulate motor cortex were commonly activated for both tasks relative to the control condition, suggesting these structures are involved in general perception and response execution rather than specific function for speeded or precisely timed responses. In the RT task, the presupplementary motor area extending to the cingulate sulcus was activated more strongly than in the timing task probably to focus attention to the onset of the first LED light unpredictably presented after random foreperiods. The lateral occipital area extending to the temporo-parieto-occipital junction was activated more strongly in the timing task than in the RT task; the same area was deactivated in the RT task relative to the control condition. Auditory-related areas were also deactivated in the both tasks. This inter- and intramodal task-specific modification including deactivation underscores significance of the context for perception and action and can have an important role in dexterous or skilled performance.

Adult↗

PubMed2003

Sustained muscle contractions maintained by autonomous neuronal activity within the human spinal cord.

It is well known that muscle contraction can be easily evoked in the human soleus muscle by applying single-pulse electrical stimulation to the tibial nerve at the popliteal fossa. We herein reveal the unexpected phenomenon of muscle contractions that can be observed when train stimulation is used instead. We found, in 11 human subjects, that transient electrical train stimulation (1-ms pulses, 50 Hz, 2 s) was able to induce sustained muscle contractions in the soleus muscle that outlasted the stimulation period for greater than 1 min. Subjects were unaware of their own muscle activity, suggesting that this is an involuntary muscle contraction. In fact, the excitability of the primary motor cortex (M1) with the sustained muscle contractions evaluated by transcranial magnetic stimulation was lower than the excitability with voluntary muscle contractions even when both muscle contraction levels were matched. This finding indicates that M1 was less involved in maintaining the muscle contractions. Furthermore, the muscle contractions did not come from spontaneous activity of muscle fibers or from reverberating activity within closed neuronal circuits involving motoneurons. These conclusions were made based on the respective evidence: 1) the electromyographic activity was inhibited by stimulation of the common peroneal nerve that has inhibitory connections to the soleus motoneuron pool and 2) it was not abolished after stopping the reverberation (if any) for approximately 100 ms by inducing the silent period that followed an H-reflex. These findings indicate that the sustained muscle contractions induced in this study are most likely to be maintained by autonomous activity of motoneurons and/or interneurons within the human spinal cord.

Adult↗