Figure 1.
(A) Change in primary motor cortex (M1) excitability for agonist and antagonist muscles during probabilistic reward tasks was investigated. Subjects were seated comfortably in a chair. The right arm hung to the side in a relaxed posture, with the palm and forearm placed on the equipment. (B) Schematic of a head with a grid showing the stimulated scalp sites. Cz represents the intersection of nasion-inion and the interaural lines. (C) Experimental design in probabilistic reward task. Probabilistic reward tasks comprised 3 conditions of 30 trials: 30 trials contained 10% reward stimulus and the remaining trials contained a non-target stimulus, 30 trials contained 50% reward stimulus, and 30 trials contained 90% reward stimulus. The inter-trial interval was randomized between 7–8 s. Single-pulse transcranial magnetic stimulation (TMS) was delivered at 2 s after appearance of the red fixation cross and 1 s after appearance of the reward/non-reward stimuli.
Figure 2.
The color code of each map of FCR (A) and ECR (B) muscles ranges from gray (0 mV) to white (0.5 mV or over). The map areas of the FCR and ECR muscles clearly overlapped, although they were spread differently. The center of gravity (black circle) of the FCR muscle was located at x (anteroposterior) = 6.5±2.6 mm and y (mediolateral) = 56.5±2.3 mm and that of the ECR muscle was located at x = 4.5±3.6 mm and y = 56.4±2.7 mm. FCR: flexor carpi radialis; ECR: extensor carpi radialis.
Figure 3.
Electromyography traces of the right FCR and ECR muscles in one representative subject.
MEP amplitude of the FCR muscle at 2 s before response was the highest for 10% reward probability during the task, whereas that of the ECR muscle was the lowest for 10% reward probability. However, MEP amplitude of the FCR muscle at 1 s before response was the highest for 90% reward probability during the task, whereas that of the ECR muscle was the lowest for 90% reward probability. MEP, motor-evoked potential; FCR, flexor carpi radialis; ECR, extensor carpi radialis.
Figure 4.
Bar graphs of relative MEP amplitudes for FCR and ECR muscles.
Relative MEP amplitude at 2(A) and at 1 s after reward/non-reward stimuli (B) during the task. Relative MEP amplitude at 2 s after the red fixation cross was significantly higher for 10% reward probability than for 90% reward probability (p = 0.008) during the task, whereas relative MEP amplitude at 1 s after reward/non-reward stimuli was significantly higher for 90% reward probability than for 10% (p = 0.001) and 50% (p = 0.001) reward probabilities. Bar graphs of relative MEP amplitudes for FCR and ECR muscles at 1 s after only reward stimuli presentation (C) and only non-reward stimuli presentation (D) during the task. Relative MEP amplitude at 1 s after only reward stimuli presentation was significantly higher for 90% reward probability than for 10% (p<0.0001) and 50% (p = 0.006) reward probabilities. However, relative MEP amplitudes for FCR and ECR muscles at 1 s after only non-reward stimuli presentation were not significantly changed. MEP, motor-evoked potential; FCR, flexor carpi radialis; ECR, extensor carpi radialis.
Table 1.
Peak-to-peak MEP amplitudes obtained for the ECR and FCR muscles during probabilistic reward tasks.
Table 2.
Peak-to-peak MEP amplitudes obtained for the ECR and FCR muscles after reward and non-reward stimuli presentations during probabilistic reward tasks.
Figure 5.
Bar graphs of RMT, SICI, and SAI before and after probabilistic reward tasks.
RMT of FCR (A) and ECR (B) for 10% reward probability, RMT of FCR (C) and ECR (D) for 50% reward probability, RMT of FCR (E) and ECR (F) for 90% reward probability, SICI of FCR (G) and ECR (H) for 10% reward probability, SICI of FCR (I) and ECR (J) for 50% reward probability, SICI of FCR (K) and ECR (L) for 90% reward probability, SAI of FCR (M) and ECR (N) for 10% reward probability, SAI of FCR (O) and ECR (P) for 50% reward probability, and SAI of FCR (Q) and ECR (R) for 90% reward probability. Only the SICI of the FCR was significantly decreased after 10% probabilistic reward tasks (p = 0.0008). RMT, resting motor threshold; SICI, short-interval intracortical inhibition; SAI, short-latency afferent inhibition; FCR, flexor carpi radialis; ECR, extensor carpi radialis.
Table 3.
MEP amplitudes and values of RMT, SICI, and SAI obtained for the ECR and FCR muscles before and after probabilistic reward tasks.