Tonic suppression of the soleus H-reflex during rhythmic movement of the contralateral ankle.

[Purpose] We investigated the effect of rhythmic ankle movement on the contralateral soleus H-reflex. The H-reflex was evoked from the right soleus muscle. [Subjects and Methods] Healthy humans rhythmically moved the left ankle (movement condition) or held the left ankle stationary (stationary condition) at one of three positions corresponding to the ankle positions at which the H-reflex was evoked in the movement condition. The background electromyographic amplitude in the right soleus muscle was maintained at 10% of the maximum voluntary contraction level, and that in the right tibialis anterior muscle was matched between the stationary and movement conditions. [Results] The soleus H-reflex was suppressed throughout all phases of contralateral rhythmic ankle movement. [Conclusion] Rhythmic movement of the contralateral joint suppresses the H-reflex in the muscle that is the prime mover of the joint homologous to the rhythmically moving joint. This inhibitory mechanism may be activated during unilateral rhythmic movement to isolate the motor control of the moving ankle from that of the contralateral stationary ankle.

The effect of eye movement on the control of arm movement to a target.

The purpose of this study was to investigate the effect of eye movement on the control of arm movement to a target. Healthy humans flexed the elbow to a stationary target in response to a start tone. Simultaneously, the subject moved the eyes to the target (saccade eye movement), visually tracked a laser point moving with the arm (smooth pursuit eye movement), or gazed at a stationary start point at the midline of the horizontal visual angle (non-eye movement--NEM). Arm movement onset was delayed when saccade eye movement accompanied it. The onset of an electromyographic burst in the biceps muscle and the onset of saccade eye movement were almost simultaneous when both the arm and the eyes moved to the target. Arm movement duration during smooth pursuit eye movement was significantly longer than that during saccade eye movement or NEM. In spite of these findings, amplitudes of motor-evoked potential in the biceps and triceps brachii muscles were not significantly different among the eye movement conditions. These findings indicate that eye movement certainly affects the temporal control of arm movement, but may not affect corticospinal excitability in the arm muscles during arm movement.

Long latency electromyographic response induced by transcranial magnetic stimulation over the cerebellum preferentially appears during continuous visually guided manual tracking task.

We investigated whether long latency motor response induced by transcranial magnetic stimulation over the cerebellum (C-TMS) preferentially appears during a continuous visually guided manual tracking task, and whether it originates in a concomitantly evoked neck twitch. C-TMS or magnetic stimulation over the neck (N-MS) was delivered during one of four tasks: a continuous or discrete visually guided manual tracking task, or phasic or tonic contraction of the first dorsal interosseous muscle. The probability of long latency fluctuation of index finger movement induced by C-TMS was not significantly different from that induced by N-MS, but the probability of long latency fluctuation induced by C-TMS and that induced by N-MS was significantly higher than that induced by sham TMS during all the tasks. The probability of long latency electromyographic response in the first dorsal interosseous muscle induced by C-TMS was significantly higher than that induced by N-MS and that induced by sham TMS during the continuous visually guided manual tracking task. Such significant differences were not present during the other tasks. Long latency electromyographic response induced by C-TMS preferentially appears during the continuous visually guided manual tracking task and is not explained by a concomitantly evoked neck twitch.

Immediate effect of visual attention on corticospinal excitability in the upper trapezius muscle.

This study investigated the immediate effect of visual attention on corticospinal excitability in the upper trapezius muscle. 10 healthy males gazed at a horizontal bar. A warning tone, indicating that a line would be slightly displaced 7,400 msec. later, was delivered in the high-attention-demand condition. A different warning tone, indicating the line would not be displaced, was delivered in the low-attention-demand condition. Participants verbally declared the direction of line displacement in the high-attention-demand condition, or stated that the warning tone meant that the line would not be displaced in the low-attention-demand condition. The amplitude of the motor-evoked potential in the upper trapezius muscle increased 7,000 msec. after the warning tone in the high-attention-demand condition. Apparently, visual attention immediately increases corticospinal excitability in the upper trapezius muscle.