During PR performance, task-related cortical activation was induced principally in both right and left sensorimotor cortices. The magnitude of oxy-Hb signals was larger in the left sensorimotor area than in the right hemisphere. This activation pattern was essentially the same as during right hand movements in healthy subjects in accordance with previous fNIRS studies [22
]. This is consistent with primary motor cortex activity, reflecting the use of the contralateral hand [7
]. Therefore, we suggest that the involvement of the primary motor cortex in motor skill learning occurs predominantly at the movement execution level of the contralateral hand.
The magnitude of the oxy-Hb responses was significantly decreased around the left sensorimotor cortex as learning progressed, irrespective of the outcome. This pattern of change was the same as observed in the Kendama task with the right hand in accordance with a previous fNIRS study [14
]. This previous study suggested that the reduction in cortical activation of the sensorimotor cortex reflects changes in motor commands for a multi-joint discrete motor task during the course of learning. Likewise, the subjects in the present study used multiple joints on the right side to keep a stylus on a round target.
In this study, the magnitudes of oxy-Hb responses were significantly decreased as learning progressed around the left sensorimotor cortex, irrespective of the outcome of the cycles. In previous studies using PR tasks, activation of the sensorimotor cortex showed no learning-related change during the PR task [12
]. The difference between our results and the previous studies might be attributable to different subjects age (average age = 24 in this study, 39 [12
], and 53 [13
]) and different cycle repetitions (number of repetition = 18 in our study, 8 [12
], and 8 [13
]). We guessed that the reduction in brain activation during motor skill learning in the younger subjects was occurred in early stage compared to the older subjects.
Across such studies, practice may result in an increase or decrease in activation of the brain areas involved in task performance, or it may induce functional reorganization of brain activity, which is a combined pattern of increases and decreases of activation across a number of brain areas [9
]. For example, learning-related increases of cortical activation during velocity-dependent motor skill learning were shown in the contralateral primary motor area [40
]. When speed of movement was held constant, cortical activity did not change [42
] or show a reduction [43
] in the primary motor cortex. Changes in neurological and psychological processes, which involved factors such as variation of the investigated tasks, methodological differences, or motor learning stages, were reflected in practice-related changes of cortical activation [5
When learning a new motor sequence, we must execute the correct order of movements while simultaneously optimizing sensorimotor parameters such as trajectory, timing, velocity, and force [44
]. Neuroimaging studies have reported that changes in cortical activation in the sensorimotor cortex are associated with performance changes in relation to the rate of movement [40
], force production [45
], movement distance [46
], motor velocity [47
], and integrated muscle torque [14
]. The establishment of a novel arbitrary sensorimotor association is closely related to attention, decision and selection of movements, sensory feedback processing, and working memory [6
]. We suggest that the reduction in cortical activation of the contralateral sensorimotor cortex would be primarily attributed to changes in a number of factors including sensory feedback processing, correct motor commands, and perceptual or cognitive functions, in order to practice the task efficiently.
There is now sufficient evidence to suggest that the changes in cortical response monitored longitudinally by fNIRS evolve as a function of motor skill learning such as during the PR task [12
], Kendama task [14
], and knot-tying task [15
]. In these studies, the magnitude of the change in cortical oxygenation attenuates in line with behavioral improvements in the task (e.g., time taken, number of movements, trajectory pattern, and muscle kinematics) [39
]. Learning-related attenuation of cortical hemodynamic responses have now been observed with fNIRS in the prefrontal cortex [16
], presupplementary motor cortex [12
], and sensorimotor cortex [14
]. These fNIRS studies may be considered a result of practice-related “pruning” of functional activation [9
], which refers to the pattern of activation change observed when practice is associated with the attainment of automatic or asymptotic performance, and therefore a reduced demand on control or attentional processes and an increased demand upon storage and processing in task-specific areas [9
Previous studies have proposed that more efficient use of specific neural circuits for an identical task over a number of cycles is accompanied by a reduction in learning-related activation [9
]. The efficient use of specific neural circuits has a close relationship with neural models of repetitive suppression [15
]. As the present task required the simple repetition of identical shoulder or elbow joint movements, the reduction of activation in the sensorimotor cortex might be partially accounted for by mechanisms related to repetitive suppression [14
Furthermore, it appeared that both PR task performance and the magnitude of oxy-Hb around the left sensorimotor cortex reached a plateau level during the late cycles. We found significant correlations between PR task performance and the magnitude of oxy-Hb responses in the left and right sensorimotor areas to each cycle. There is evidence to suggest that hemodynamic
response can be modulated by the frequency [48
], intensity [49
], or complexity [50
] of a motor task. Stronger sensorimotor activation is reflective of task complexity and does not necessarily depend upon the number of muscles required [51
]. Task complexity-related modulation is further indexed by the attenuated sensorimotor cortex response that accompanies motor imagery compared to that accompanying motor execution [52
There was a reduction in the oxy-Hb signal only in the left sensorimotor area with repetition of the PR task. However, a correlation between performance gain and oxy-Hb signals was found in both hemispheres. During PR task performance, task-related cortical activation was induced principally in both the right and left sensorimotor cortices. Therefore, we hypothesized that PR task performance with the right hand would correlate with sensorimotor cortical activation in both hemispheres. Specially, because primary motor cortex activity reflects the use of the contralateral hand [7
], a reduction in the oxy-Hb signal was found only in the left sensorimotor area with repetition of the PR task.
ADLs, plays, or works require motor sequence learning [13
], which requires postural control, even during motor performance using the upper extremity. During rehabilitation
, patients relearn motor skills for the ADLs, plays, or works by repeating sequential movements. The PR task requires motor control of the proximal parts of the upper extremity including the shoulder and elbow, as well as postural control for sitting or hand-eye coordination [12
]. Because the PR task is a robust and compact tool with which to evaluate motor skills in a specific patient, medical doctors, occupational therapists, or physical therapists can use the PR task easily in the hospitals or institutions. Assessing the performance in the PR task can be used useful in screening for the levels of disabilities in the ADLs, plays, or works. In our study, each 15-s task period was alternated with a 30-s rest period for a total of 18 repetitions to measure cortical activation during the early phase of PR task learning and after PR task performance reached a plateau level. This protocol enables us to record the behavioral and brain activation changes from starting point of motor sequence learning to acquisition of motor sequence skill. Therefore, changes in sensorimotor activation during a PR task may serve as a motor sequence learning biomarker for the evaluation or development of motor coordination, the judging rehabilitation outcomes, or the prediction of recovery.
In motor sequence learning, the early phase of learning depends on both the cortico-striatal and cortico-cerebellar networks, whereas late motor learning is attributed to the cortico-striatal system [2
]. We recorded cortical activation during the early phase of learning; therefore, we needed to examine both the cortico-striatal and cortico-cerebellar networks. By using fNIRS, we could only measure cortical activity in a limited area associated with motor learning. Future neuroimaging studies are required to investigate activation changes over a variety of wider brain regions.