Effects of transcranial direct current stimulation (tDCs) on dream recall

Abstract

Dream research has advanced considerably since the discovery of REM sleep. However, despite the progress made and the existence of various theories of dreaming, the neuroanatomical origins underlying dream recall remains a controversial topic. This may have been due to a lack of suitable tools to effectively test such theories. There is now, however, a diverse range of noninvasive brain stimulation tools available, such as transcranial magnetic stimulation (TMS) and transcranial direct current stimulation (tDCs) that may be applied to this question. Thus, this thesis aimed to take advantage of the availability of one such tool (tDCs) to investigate the underlying cortical processes in dreaming. In order to clarify the neuroanatomical regions related to dreaming, this thesis initially presents a meta-analysis of neuroimaging studies conducted during REM and NREM sleep. The most consistent finding across studies was that frontal deactivation featured prominently during REM sleep when compared to wakefulness. The experimental studies presented in this thesis employed tDCs in order to directly investigate the cortical origins of dreaming in healthy participants. Specifically, the effect of weak tDCs applied during sleep to various neocortical regions implicated in the dream recall process (prefrontal cortex, right posterior parietal cortex, occipital cortex) on visual imagery reported on awakening. In the first experiment, an attempt was made to recreate, to some extent, some of the cortical activity observed during REM sleep (prefrontal deactivation, posterior parietal activation) during stage 2 sleep. This was achieved through the application of simultaneous anodal and cathodal tDCs to the posterior parietal and prefrontal cortex, respectively. Consistent with proponents of cortical involvement (e.g. Nielsen, Solms), tDCs of posterior parietal and prefrontal cortex during stage 2 sleep was related to a higher percentage of imagery reports from the tDCs condition compared to control conditions. While the results from the stage 2 study do not discount the involvement of subcortical structures in dream generation and recall, they do however add to the growing body of research implicating the involvement of cortical mechanisms. In two further experiments, tDCs was administered during slow wave sleep (SWS) and stage 1 sleep onset (SO) in an attempt to induce the cortical activation/deactivation pattern of REM sleep. However, simultaneous anodal (posterior parietal) and cathodal (prefrontal) tDCs during SWS and simultaneous anodal (prefrontal) and cathodal (occipital) tDCs during SO failed to facilitate or inhibit dream recall from these two stages of sleep, respectively. In the final experiment, anodal (prefrontal) and cathodal (posterior parietal) tDCs was applied during REM sleep to determine whether such stimulation could inhibit dream recall from a sleep stage known for its heightened cortical activity and dream recall. TDCs had no effect on dream recall during this stage of sleep, suggesting that dreaming during this stage may be difficult to suppress. The concluding chapter explores possible methodological and theoretical explanations for the findings, and argues that it may be the unique properties of each of the sleep stage which led to the observed results.