Daichi Nozaki

Limited transfer of learning between unimanual and bimanual skills within the same limb

Although a limbs motion appears to be similar across unimanual and bimanual movements, here we demonstrate partial, but not complete, transfer of learning across these behavioral contexts, hidden learning that remains intact (but invisible) until the original context is again encountered, and the ability to associate two conflicting force fields simultaneously, one with each context. These results suggest partial, but not complete, overlap in the learning processes involved in the acquisition of unimanual and bimanual skills.

Specific tension of elbow flexor and extensor muscles based on magnetic resonance imaging

Series cross-section images of the upper extremity were obtained for four men by magnetic resonance imaging (MRI) and anatomical cross-sectional areas (ACSA) of elbow flexor muscles [biceps brachii (BIC), brachialis (BRA), brachioradialis (BRD)] and extensor muscles [triceps brachii (TRI)] were measured. Physiological cross-sectional area (PCSA) was calculated from the muscle volume and muscle fibre length, the former from the series ACSA and the latter from the muscle length multiplied by previously reported fibre/muscle length ratios. Elbow flexion/extension torque was measured using an isokinetic dynamometer and the force at the tendons was calculated from the torque and moment arms of muscles measured by MRI. Maximal ACSA of TRI was comparable to that of total flexors, while PCSA of TRI was greater by 1.9 times. Within flexors, BRA had the greatest contribution to torque (47%), followed by BIC (34%) and BRD (19%). Specific tension related to the estimated velocity of muscle fibres were similar for elbow flexors and extensors, suggesting that the capacity of tension development is analogous between two muscle groups.

Reliability of measurement of oxygen uptake by a portable telemetric system.

SummaryThe purpose of the present study was to check the reliability of measurements of oxygen uptake (VO2) using a newly developed portable telemetry system. This system (K2) consisted of a face mask, a flow meter, a gas analyser with a transmitter, and a receiver. The total mass for the subject to carry was about 850 g. Three experiments were carried out, firstly to check the reliability and reproducibility of the flow meter and the K2 gas analyser, secondly to check the accuracy of K2 by comparing it with the Douglas bag method (DB), and thirdly to apply K2 to sports activities. In the first experiment, the flow meter was highly accurate up to 180 l·min−1 with good reproducibility. The measurement error of the gas analyser was less than 2%. In the second experiment, there was no significant difference in the calculated ventilation between K2 and DB. The VO2 showed no significant difference between K2 and DB with some exceptions. In the third experiment, we succeeded in the measurement of VO2 during rowing on water. The measurement of VO2 during running and playing soccer was also possible. It would seem that the present system could well be a powerful tool in the field measurement of VO2 during various sports activities.

The “Cutaneous Rabbit” Hopping out of the Body

Rapid sequential taps delivered first to one location and then to another on the skin create the somatosensory illusion that the tapping is occurring at intermediate locations between the actual stimulus sites, as if a small rabbit were hopping along the skin from the first site to the second (called the “cutaneous rabbit”). Previous behavioral studies have attributed this illusion to the early unimodal somatosensory body map. A functional magnetic resonance imaging study recently confirmed the association of the illusion with somatotopic activity in the primary somatosensory cortex. Thus, the cutaneous rabbit illusion has been confined to ones own body. In the present paper, however, we show that the cutaneous rabbit can “hop out of the body” onto an external object held by the subject. We delivered rapid sequential taps to the left and right index fingers. When the subjects held a stick such that it was laid across the tips of their index fingers and received the taps via the stick, they reported sensing the illusory taps in the space between the actual stimulus locations (i.e., along the stick). This suggests that the cutaneous rabbit effect involves not only the intrinsic somatotopic representation but also the representation of the extended body schema that results from body–object interactions.

Shaping Appropriate Locomotive Motor Output Through Interlimb Neural Pathway Within Spinal Cord in Humans

Direct evidence supporting the contribution of upper limb motion on the generation of locomotive motor output in humans is still limited. Here, we aimed to examine the effect of upper limb motion on locomotor-like muscle activities in the lower limb in persons with spinal cord injury (SCI). By imposing passive locomotion-like leg movements, all cervical incomplete (n = 7) and thoracic complete SCI subjects (n = 5) exhibited locomotor-like muscle activity in their paralyzed soleus muscles. Upper limb movements in thoracic complete SCI subjects did not affect the electromyographic (EMG) pattern of the muscle activities. This is quite natural since neural connections in the spinal cord between regions controlling upper and lower limbs were completely lost in these subjects. On the other hand, in cervical incomplete SCI subjects, in whom such neural connections were at least partially preserved, the locomotor-like muscle activity was significantly affected by passively imposed upper limb movements. Specifically, the upper limb movements generally increased the soleus EMG activity during the backward swing phase, which corresponds to the stance phase in normal gait. Although some subjects showed a reduction of the EMG magnitude when arm motion was imposed, this was still consistent with locomotor-like motor output because the reduction of the EMG occurred during the forward swing phase corresponding to the swing phase. The present results indicate that the neural signal induced by the upper limb movements contributes not merely to enhance but also to shape the lower limb locomotive motor output, possibly through interlimb neural pathways. Such neural interaction between upper and lower limb motions could be an underlying neural mechanism of human bipedal locomotion.

Asymmetric Transfer of Visuomotor Learning between Discrete and Rhythmic Movements

As long as we only focus on kinematics, rhythmic movement appears to be a concatenation of discrete movements or discrete movement appears to be a truncated rhythmic movement. However, whether or not the neural control processes of discrete and rhythmic movements are distinct has not yet been clearly understood. Here, we address this issue by examining the motor learning transfer between these two types of movements testing the hypothesis that distinct neural control processes should lead to distinct motor learning and transfer. First, we found that the adaptation to an altered visuomotor condition was almost fully transferred from the discrete out-and-back movements to the rhythmic out-and-back movements; however, the transfer from the rhythmic to discrete movements was very small. Second, every time a new set of rhythmic movements was started, a considerable amount of movement error reappeared in the first and the following several cycles although the error converged to a small level by the end of each set. Last, we observed that when the discrete movement training was performed with intertrial intervals longer than 4 s, a significantly larger error appeared, specifically for the second and third cycles of the subsequent rhythmic movements, despite a seemingly full transfer to the first cycle. These results provide strong behavioral evidence that different neuronal control processes are involved in the two types of movements and that discrete control processes contribute to the generation of the first cycle of the rhythmic movement.

Distinct motor plans form and retrieve distinct motor memories for physically identical movements.

We can adapt movements to a novel dynamic environment (e.g., tool use, microgravity, and perturbation) by acquiring an internal model of the dynamics. Although multiple environments can be learned simultaneously if each environment is experienced with different limb movement kinematics, it is controversial as to whether multiple internal models for a particular movement can be learned and flexibly retrieved according to behavioral contexts. Here, we address this issue by using a novel visuomotor task. While participants reached to each of two targets located at a clockwise or counter-clockwise position, a gradually increasing visual rotation was applied in the clockwise or counter-clockwise direction, respectively, to the on-screen cursor representing the unseen hand position. This procedure implicitly led participants to perform physically identical pointing movements irrespective of their intentions (i.e., movement plans) to move their hand toward two distinct visual targets. Surprisingly, if each identical movement was executed according to a distinct movement plan, participants could readily adapt these movements to two opposing force fields simultaneously. The results demonstrate that multiple motor memories can be learned and flexibly retrieved, even for physically identical movements, according to distinct motor plans in a visual space.

Noise-induced large-scale phase synchronization of human-brain activity associated with behavioural stochastic resonance

We demonstrate that both detection of weak visual signals to the right eye and phase synchronization of electro-encephalogram (EEG) signals from widely separated areas of the human brain are increased by addition of weak visual noise to the left eye. We found a close relationship between the resulting noise-induced changes in behavioural performance and the similarly resulting changes in phase synchronization between widely separated brain areas. These results imply that noise-induced large-scale neural synchronization may play a significant role in information transmission in the brain.

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.

Enhancement of stochastic resonance in a FitzHugh-Nagumo neuronal model driven by colored noise

Abstract We investigate the stochastic resonance in a FitzHugh-Nagumo neuronal model driven by colored noise with a 1 f β spectrum (0 ≤ β ≤ 2). A numerical simulation shows that the noise intensity needed to maximize the coherence between input and output signals is the smallest when β ≈ 1. We also demonstrate analytically that this phenomenon is never seen in a nondynamical threshold system.