Elsevier

Consciousness and Cognition

Volume 22, Issue 4, December 2013, Pages 1214-1222
Consciousness and Cognition

Testing the involvement of the prefrontal cortex in lucid dreaming: A tDCS study

https://doi.org/10.1016/j.concog.2013.08.005Get rights and content

Highlights

  • Lucid dreaming might be linked to increased frontal brain activity during REM sleep.

  • By applying tDCS, we aimed to activate the DLPFC during REM sleep.

  • According to the self-reports, tDCS increased dream lucidity.

  • The effects, however, were not strong and found only in frequent lucid dreamers.

  • Future research should control for indirect effects and probe other brain regions.

Abstract

Recent studies suggest that lucid dreaming (awareness of dreaming while dreaming) might be associated with increased brain activity over frontal regions during rapid eye movement (REM) sleep. By applying transcranial direct current stimulation (tDCS), we aimed to manipulate the activation of the dorsolateral prefrontal cortex (DLPFC) during REM sleep to increase dream lucidity. Nineteen participants spent three consecutive nights in a sleep laboratory. On the second and third nights they randomly received either 1 mA tDCS for 10 min or sham stimulation during each REM period starting with the second one. According to the participants’ self-ratings, tDCS over the DLPFC during REM sleep increased lucidity in dreams. The effects, however, were not strong and found only in frequent lucid dreamers. While this indicates some preliminary support for the involvement of the DLPFC in lucid dreaming, further research, controlling for indirect effects of stimulation and including other brain regions, is needed.

Introduction

Dreaming is often described as a state of cognitive deficiency characterized by a loss of self-reflection, bizarre, illogical situations, or a lack of control over volition and attention (Hobson, Pace-Schott, & Stickgold, 2000). On a neurophysiological level, it has been suggested that these phenomena result from hyper- or hypo-activity of specific neural networks during rapid eye movement (REM) sleep, which is the sleep stage where the most vivid dreams occur (Schwartz & Maquet, 2002). Recent neuroimaging studies have underlined that during REM sleep the brain becomes selectively deactivated as compared to waking, including the dorsolateral prefrontal cortex (DLPFC) and the precuneus, whereas other brain regions become more activated, such as the limbic and paralimbic systems (Braun et al., 1997, Maquet et al., 1996). A special kind of nocturnal dreaming is lucid where the dreamer realizes he or she is in the dream state and is often able to control dream happenings (LaBerge, 1985).

In a recent article, Hobson (2009) pointed out the relevance of lucid dreaming to the study of consciousness. Lucid dreaming has been defined as a rare but robust awareness that we are dreaming and that we are not really awake. It is considered to be mainly a REM sleep phenomenon (LaBerge, 1990), although lucidity can also occur during NREM sleep (Stumbrys & Erlacher, 2012). Hobson et al. (2000) have proposed that, during the lucid state, the previously deactivated DLPFC becomes reactivated, allowing directed thought, metacognition and awareness of being while dreaming. Preliminary empirical evidence for this hypothesis has been obtained from a recent study (Voss, Holzmann, Tuin, & Hobson, 2009) which found that when participants become lucid, there is a shift in their EEG power, especially in the 40 Hz range and in frontal brain regions. Moreover, in lucid dreaming, EEG coherence is also largest in frontolateral and frontal areas (for all frequency bands, 1–45 Hz). Another recent study, which has used fMRI to study cerebral regional activation in lucid dreams, replicated these findings and showed that, in lucid dreams, not only prefrontal but also occipito-temporal cortices, bilateral precuneus, cuneus and parietal lobes exhibit higher activation compared to what occurs during non-lucid dreams (Dresler et al., 2012). Based on this background, the hypothesis was formulated that activation of the frontolateral area of the brain during REM sleep should increase dream lucidity.

Although these findings and hypotheses concerning the neurobiology of dreams are intriguing, this research field poses several methodological challenges. One problem is the approach of activating the brain by external stimulation. In 1985, transcranial magnetic stimulation (TMS) was introduced as a neuroscience research tool able to focally and painlessly stimulate the cortex by means of a time-varying magnetic field (Barker, Jalinous, & Freeston, 1985). Although the application of TMS in sleep research is possible (Massimini et al., 2005, Massimini et al., 2007), it is complicated due to the auditory artifacts and tactile sensations on the scalp (Noreika, Windt, Lenggenhager, & Karim, 2010). Unlike TMS, transcranial direct current stimulation (tDCS) does not induce auditory artifacts, and the voltage needed to hold the current constant decreases after a short time and usually becomes subthreshold for evoking peripheral sensations. TDCS involves continuous administration of weak currents (1 mA) through a pair of surface electrodes, cathode and anode, attached to the scalp (Nitsche & Paulus, 2000).

Several studies have demonstrated that cerebral excitability was diminished by cathodal stimulation, which is thought to hyperpolarize neurons, whereas anodal stimulation results in increased cortical excitability (Nitsche et al., 2008). These tDCS induced effects have been observed in several cortical regions such as the motor (Nitsche & Paulus, 2000), visual (Antal, Kincses, Nitsche, Bartfai, & Paulus, 2004), somatosensory (Rogalewski, Breitenstein, Nitsche, Paulus, & Knecht, 2004) and prefrontal cortices (Karim et al., 2010). In 2004, it was demonstrated for the first time that tDCS can be reliably applied during sleep without awakening the participants (Marshall, Mölle, Hallschmid, & Born, 2004). Moreover, it was found that repeated application of anodal tDCS over frontocortical areas during slow wave sleep (SWS) improved declarative memory consolidation. Furthermore, another group of researchers recently explored the effects of simultaneous tDCS stimulation on the frontal and posterior parietal cortices during different stages of sleep (Jakobson et al., 2012, Jakobson et al., 2012a, Jakobson et al., 2012b). While cathodal-frontal and anodal-parietal stimulation increased reported visual dream imagery during Stage 2 sleep (Jakobson et al., 2012b), no such effects were observed during slow wave sleep (Jakobson et al., 2012a) and the reversed stimulation (i.e. cathodal-parietal, anodal-frontal) did not have an effect on visual imagery during REM sleep (Jakobson et al., 2012).

In order to go beyond the correlational data regarding the neural correlates of dream lucidity as suggested by previous EEG and fMRI studies (Dresler et al., 2012, Voss et al., 2009), we aimed in this study to experimentally manipulate the activation of the prefrontal brain cortex and test the neurobiological basis of dream lucidity. Anodal tDCS was applied during REM sleep to activate the DLPFC and – by modulating cortical excitability – should have had an effect on subjective experience of dreaming by increasing dream lucidity.

Section snippets

Participants

Twenty-three participants (7 male, 16 female) aged from 21 to 33 years (M = 25.0 ± 3.1) were recruited for the study via e-mail advertisements sent out to psychology students and known lucid dreamers. The inclusion criteria were: (1) at least average dream recall (one or more recalled dreams a week); (2) good sleeping; and (3) no serious health problems, chronic illnesses and/or medication intake. An additional criterion, which was employed towards the middle of the study, was a higher frequency of

Results

For nineteen participants, who completed the study, the median value for reported dream recall frequency was “several times a week” and the median value for reported lucid dream frequency was “about once a month”. Eleven participants could be considered frequent lucid dreamers according to the terminology of Snyder and Gackenbach (1988) (lucid dreaming frequency is once a month or higher).

A total of 109 REM awakenings were made (in average, 5.7 per participant). The comparative data on

Discussion

TDCS stimulation delivered over the DLPFC during REM sleep had an effect on the subjective experiences of dreaming. As hypothesized, it resulted in increased dream lucidity according to the self-rating of participants. This study thus provides preliminary empirical support for the causal involvement of the DLPFC in lucid dreaming. The effects, however, were not very strong and post-hoc analysis showed that they were pronounced only in frequent lucid dreamers, who reported increased awareness

Acknowledgment

This project was funded by the BIAL Foundation, Portugal (Grant 191/10).

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