Modulation of Corticospinal Excitability during Acquisition of Action Sequences by Observation

Excitability of the corticospinal pathway increases during observation of an action. However, how corticospinal excitability changes during observation of sequential actions in the course of acquiring novel skills (observational learning) remains unexplored. To investigate this, we used a previously unpracticed sequence of ten hand postures. Participants were asked to repeat observation and replication of the sequence. This block of observation and replication was repeated 5 times. During observation of a given hand posture (OK sign), motor-evoked potentials (MEPs) elicited by transcranial magnetic stimulation were recorded from hand muscles. In experiment 1, the OK sign appeared in the 9th position of the sequence. Almost all participants could replicate the OK sign only at the 5th block of the experiment. MEP amplitude was greater than that in the control, and decreased with the stages. This suggested that during observational learning of sequential hand postures MEP changed with the progress of the learning. To evaluate this idea, we performed two additional experiments. In experiment 2, the OK sign appeared in the 2nd position. Almost all participants replicated the OK sign even in the 1st block. The MEP amplitude did not change across stages. In experiment 3, the OK sign appeared in the 9th position, but the order of other signs was randomized in every stage. Many participants were not able to replicate the OK sign even during the 5th block of the experiment. The MEP amplitude did not change across stages. These results suggest that: (1) During observational learning modulation of corticospinal excitability is associated with the learning process. (2) Corticospinal excitability decreases as learning progresses

The Modulation of Corticospinal Excitability during Motor Imagery of Actions with Objects

We investigated whether corticospinal excitability during motor imagery of actions (the power or the pincer grip) with objects was influenced by actually touching objects (tactile input) and by the congruency of posture with the imagined action (proprioceptive input). Corticospinal excitability was assessed by monitoring motor evoked potentials (MEPs) in the first dorsal interosseous following transcranial magnetic stimulation over the motor cortex. MEPs were recorded during imagery of the power grip of a larger-sized ball (7 cm) or the pincer grip of a smaller-sized ball (3 cm)—with or without passively holding the larger-sized ball with the holding posture or the smaller-sized ball with the pinching posture. During imagery of the power grip, MEPs amplitude was increased only while the actual posture was the same as the imagined action (the holding posture). On the other hand, during imagery of the pincer grip while touching the ball, MEPs amplitude was enhanced in both postures. To examine the pure effect of touching (tactile input), we recorded MEPs during imagery of the power and pincer grip while touching various areas of an open palm with a flat foam pad. The MEPs amplitude was not affected by the palmer touching. These findings suggest that corticospinal excitability during imagery with an object is modulated by actually touching an object through the combination of tactile and proprioceptive inputs

Typical recordings of MEPs in FDI during experiment 1 (A), 2 (B) and 3 (C).

<p>These waveforms were obtained from three different participants. Ten traces were superimposed for each waveform.</p

The design of experiment 1.

<p>The video clip showed a sequence of 10 different hand postures of the right hand during an observation task. This was repeated 12 times in each block of the observation task. Transcranial magnetic stimulation was applied when participants observed the OK sign (in the 9th position) with the thumb and index finger. The observation task and the replication task were alternately repeated 5 times.</p

Mean MEP size during the observation task in experiment 2.

<p>Values on the ordinate indicate MEP size as a percentage of those obtained in the control task. Data are represented as the mean ± one SE. Daggers indicate significant differences from the control value. † p<0.05, †† p<0.01.</p

The number of participants who correctly replicated the OK sign during the replication task.

<p>The number of participants who correctly replicated the OK sign during the replication task.</p

The number of correct responses during the replication task.

<p>The number of correct responses during the replication task.</p

Mean MEP size during the observation task in experiment 3.

<p>Values on the ordinate indicate MEP size as a percentage of those obtained in the control task. Data are represented as the mean ± one SE. Daggers indicate significant differences from the control value. †† p<0.01, ††† p<0.001.</p

EMG activities during the holding conditions.

<p>Superposition of ten resting EMG in seven muscles (first dorsal interosseous muscle: FDI, abductor digiti minimi muscle: ADM, extensor carpi radialis muscle: ECR, and flexor carpi radialis muscle: FCR, Thenar, Flexor digitorum superficialis: FDS and Extensor digitorum: ED muscles) in four conditions; Pinching Posture (PP), Pinching Posture with a smaller ball (PP+Ball), Holding Posture (HP) and Holding Posture with a larger ball (HP+Ball). The leftmost records are the EMG activity during the weakest contraction of each muscle, not necessarily in the same task.</p