| dc.description.abstract | Recently, studies have demonstrated that applying brain stimulation may improve the severity of anxiety symptoms. As anxiety is common in the general population as well as those with anxiety disorders, we investigated whether brain stimulation can modulate anxiety levels for the general population with the aim of understanding the underlying mechanisms of this process in a normal system. Attentional control theory (ACT) suggests that anxiety impairs cognitive control and increases the impact of external stimuli (Eysenck, Derakshan, Santos, & Calvo, 2007). In this thesis, we investigated two cognitive control function, inhibition (in Chapter 2) and updating (in Chapter 3), in relation to anxiety.
In Experiment 1, we applied a detection task to establish the relationship between anxiety levels and visual search efficiency for the general population. We found that participants with higher fear of negative evaluation were slower in performing the task. Also, we found that only males showed such a relationship, but females did not. As a previously published study recruited more females (about 81% of participants) than males, we inferred that there may be gender differences on task demands. In Experiment 2, we changed the task from detection to localization. Again, only males showed significant correlation between anxiety levels and visual search efficiency, whereas females did not. Therefore, we turned to investigating working memory which comprises another function of attentional control, updating. In Experiment 4, we administered questionnaires, presented the n-back task, and used a startle reflex as an anxiety-related measurement. As dorsolateral prefrontal cortex (DLPFC) and precuneus (PCu) have been demonstrated to show differences between those with anxiety disorders and normal people, we applied transcranial direct current stimulation (tDCS) to modulated activities of these areas in Experiment 4. However, we did not find any significant improvement or impairment on task performance, startle reflex, nor questionnaire scores as a result of tDCS stimulation. As we compared parameters of tDCS between previous studies and ours, we inferred that one possibility was that the stimulation it might not have been precise enough for the DLPFC group. However, it was unclear whether or not the electrode arrangement led to any effect of PCu tDCS on visual perception. Therefore, we investigated the effects of PCu tDCS on a visual perception task in Experiment 5, but we did not find any significant effect on task performance. Finally, we applied continuous theta burst stimulation (cTBS) over PCu and vertex in Experiment 6. We found that only the PCu group showed a change (improvement) in working memory, but the vertex group did not. Even though both PCu and vertex groups showed an improvement in the startle reflex, only the PCu group showed this improvement on the 0-back task (i.e. no memory load). This implied that applying cTBS over PCu improves the startle reflex even when task demand was low. Also, these findings supported the idea that applying brain stimulation over this area modulates anxiety levels.
In summary, Chapter 2 showed a gender difference in the relationship between anxiety levels and visual search efficiency. However, it is still unclear why only males showed such relationship. In Chapter 3, we applied both tDCS and TMS, but observed significant differences only in the TMS experiment. There are number of possibilities for this, including, for example, that TMS may more precise or efficient than tDCS. However, the findings do support a role for precuneus in anxiety and how to both diminish the differences of task performance and in the startle reflex for different task difficulties may be important when using these as anxiety-related indices. | en_US |