| dc.description.abstract | Background: Emotional facial expression plays an important role in our social interaction and communication. According to the approach and avoidance motivation, negative emotions have been generally thought to be avoidance oriented and positive emotions to be approach oriented. Facial expression in positive valence is related to approach behavior and is more difficult to inhibit than responses to neutral faces. Prior go/nogo studies found that perceived facial expression in different emotional valences could modulate motor inhibition. Intriguingly, little studies focus on how perceived emotional facial expression in difference valences modulate our motor inhibition by adopting stop signal task. Since stop signal task provides measurement of SSRT and inhibition function, the advantage of the task is that the time course of inhibition process can be assessed. Pessoa and colleagues investigated how perceived emotional facial expression modulates inhibitory control by utilizing emotional faces (i.e. fearful, happy and neutral faces) as stop signals in stop signal task. SSRT found to be shorter for emotional faces than neutral faces. No difference in SSRT between fearful and happy face proposed that emotional faces enhanced inhibitory control but there is no modulatory effect of emotional valence in inhibition. The limitation of prior study is that participants might inhibit their responses only when faces appeared without concerning the facial expressions. Current study aims to elucidate how perceived facial expression modulate motor inhibition by utilizing stimulus-selective stop signal task, because it addresses the selectivity and is more representative of real life situation. Previously, electrophysiological N2 and P3 component were proposed as important components in inhibition in some studies. However, the role of N2 is ambiguous as it is reported to be involved in conflict detection. Because SSRT is around 200 ms, the appearance of P3 is too late to account for the inhibition process. It was proposed that early component like N1 may better reflect the inhibition process. Current study aims to differentiate the role of N1 and N2 component in motor inhibition by utilizing additional continue go trials that share similar attentional characteristics with stop trials, but participants are required to respond. We argue that the role of N1 and N2 can be clarified by comparing successfully stopped trials (SST), unsuccessfully stopped trials (USST) and continue go trials (Cont. Go).
Method: Thirty participants were recruited from National Central University in this study. Stimulus-selective stop signal task with electroencephalography (EEG) measurement was employed. In order to investigate the relationship between emotional facial expression and motor inhibition, two experimental conditions were manipulated to investigate the relationships: 1) Fearful Face adopted as stop signal that the subjects are required to inhibit their pre-potent action while happy face adopted as Continue Go signal that the subjects have to ignore it and continue with their action. (i.e. Fearful Stop and Happy Continue Go Condition) 2) Happy face was adopted as stop signal and happy face adopted as Continue Go signal (i.e. Happy Stop and Fearful Continue Go Condition). Moreover, the current paradigm was modified in two following ways: 1) Fixed SSD method was utilized instead of the staircase tracking method. In this way, the non-cancelled rate can be plotted in the function of stop signal delays (SSDs) and the effect of emotional valences on the time course of the inhibition process can be elucidated. 2) Force pincher was used instead of traditional key pressing. In light of this, the peripheral stage and graded nature of inhibition process can be characterized by force measurement, we think that, compared to traditional key pressing, force pincher can provide more measures for the investigation of motor inhibition process.
Results: Larger N1 amplitudes were observed in successfully stopped trials (SST) than in unsuccessfully stopped trials (USST) but no difference between successfully stopped trials (SST) and continue go trials (Cont. Go). This pattern of results suggests that N1 component may not only represent the discrimination between the stop and continue go signals, but also the inhibition process due to the overlapping and successful inhibitory processes in SST and Cont. Go trials. No difference found in N2 amplitude between USST and Cont. Go trials. The N2 amplitude increased along with stop signal delays (SSDs) in both USST and Cont. Go trials, inferring that N2 is likely to reflect conflict detection rather than the inhibition process. In a comparison of emotional facial expression in motor inhibition, the behavioral result shows that there is a significant difference in the inhibition function to stop signals in different emotional valences. When the happy face was used as a stop signal, the non-cancelled rate was higher than that of the fearful face in early SSD but there was no difference in middle and late SSDs. For the electrophysiological result, larger N1 amplitude elicited by fearful face than happy face in SST. In the comparison of force measurement, no difference in force measurement between emotional valences in USST. Peak force rate to happy face was larger than that of the fearful face in early stop signal delay in continue go response.
Conclusion: In sum, current study provides evidence supporting the important role of the N1 component in inhibitory control. Compared to discrimination, N1 component is more likely to account for inhibition process. On the other hand, N2 is proposed to reflect conflict detection rather than inhibitory control. The lower non-cancelled rate in early SSD and larger N1 amplitude to fearful face than happy face in SST supports the idea that facial expression in positive valence is related to approach motivation, and is more difficult to be inhibited than negative valence. The effect of emotional valence is more robust at an early stage of the motor execution process. As revealed by force measurement results, no difference in peak force and peak force rate between emotional facial expressions in unsuccessfully stopped response suggesting that the modulatory effect of emotional valence on inhibition to be more robust in the central stage than in the peripheral stage of motor inhibition process. Furthermore, larger peak force rate was observed to be larger to happy face than to fearful face in early SSD in continue go response, inferring that the happy face results in higher efficiency to reinitiate the go response due to the sudden signal and the effect of emotional valence is more robust in the early stage of motor response. Taking together, the current study provides evidence to differentiate the role of N1 and N2 component in inhibitory control. From the combination of electrophysiological and peripheral pinch-force measurement, it is concluded that motor inhibition can be modulated by emotional facial expression in different valences. Moreover, emotional facial expression modulates our motor inhibition in the early stage of motor response and the modulatory effect is more robust in the central level rather than peripheral level along the motor inhibition process. | en_US |