Modulating dream experience: Noninvasive brain stimulation over the sensorimotor cortex reduces dream movement.

Recently, cortical correlates of specific dream contents have been reported, such as the activation of the sensorimotor cortex during dreamed hand clenching. Yet, despite a close resemblance of such activation patterns to those seen during the corresponding wakeful behaviour, the causal mechanisms underlying specific dream contents remain largely elusive. Here, we aimed to investigate the causal role of the sensorimotor cortex in generating movement and bodily sensations during REM sleep dreaming. Following bihemispheric transcranial direct current stimulation (tDCS) or sham stimulation, guided by functional mapping of the primary motor cortex, naive participants were awakened from REM sleep and responded to a questionnaire on bodily sensations in dreams. Electromyographic (EMG) and electroencephalographic (EEG) recordings were used to quantify physiological changes during the preceding REM period. We found that tDCS, compared to sham stimulation, significantly decreased reports of dream movement, especially of repetitive actions. Other types of bodily experiences, such as tactile or vestibular sensations, were not affected by tDCS, confirming the specificity of stimulation effects to movement sensations. In addition, tDCS reduced EEG interhemispheric coherence in parietal areas and affected the phasic EMG correlation between both arms. These findings show that a complex temporal reorganization of the motor network co-occurred with the reduction of dream movement, revealing a link between central and peripheral motor processes and movement sensations of the dream self. tDCS over the sensorimotor cortex interferes with dream movement during REM sleep, which is consistent with a causal contribution to dream experience and has broader implications for understanding the neural basis of self-experience in dreams.


Supplementary content analysis of movement sensations in verbal dream reports
Movements were reported in 49.8% (SEM=10) of dreams following sham-stimulation and 54.9% (SEM=10.9) of dreams following tDCS. Repetitive actions were the most common type of movement, followed by single actions, with passive movements being the least common (see Tables 2 and S1), replicating the pattern observed in the BED Questionnaire data. However, there were no significant differences between the sham-stimulation and tDCS conditions (see Table S1), in contrast to the effects observed in the questionnaire data (see Table 1). The discrepancy could be due to a considerably smaller proportion of explicitly expressed movements in free dream reports compared to the BED Questionnaire answers, i.e. participants tended to omit explicit descriptions of movements from the spontaneous verbal reports.
Nevertheless, the proportion of repetitive actions correlated strongly between the free dream reports and the BED Questionnaire answers in the sham-stimulation condition (Spearman rank order correlation: rho=0.81, p B-8 =0.033), indicating a strong convergence between these two types of measurement. Interestingly, this association did not hold in the tDCS condition (rho=-0.19, p B-8 =1). No other correlations were significant.

Control analysis of the time of night effect on the presence of bodily experiences in dreams
To control the possible effect of the time of night on the frequency of bodily experiences reported in the BED Questionnaire upon awakening, we compared the answers to the five main questions regarding the presence of different types of bodily experiences (tactile-somatosensory, vestibular-balance, movement, movement alterations, body scheme alterations) between the first and the last reports of each night, separately for the sham-stimulation and tDCS conditions. Cases in which a given experience was present in the first but not in the last set of questionnaire responses from the same night were coded as -1. Cases in which a given experience was either present or absent in both the first and the last set of questionnaire responses from the same night were coded as 0, indicating no change during night. Finally, cases in which a given experience was absent in the first but present in the last set of questionnaire responses from the same night were coded as 1. We observed no systematic difference between the first and the last set of questionnaire responses from the same night, excluding cumulative effects over night (see Supplementary   Table S1).   Fig. S1).

5) Did you experience any tactile or somatosensory sensations in your dream? *
(for instance touching something, being touched, self-touch, cold, heat, pleasant or unpleasant bodily sensations, numbness, vibration, tickling, pain, pressure, or itching)

6) Please specify the duration, body part localization and intensity of tactile and somatosensory sensations in your dream! *
For each sensation, mark the duration, whole body / body part and intensity. If you mark "never", jump to the next sensation.

7) Did you experience any of the following sensations? *
For each sensation, mark the duration, whole body / body part and intensity. If you mark "never", jump to the next sensation.

Appendix 1
For each sensation, mark the duration, whole body / body part and intensity. If you mark "never", jump to the next sensation.  (for instance enhanced or reduced precision, enhanced or reduced coordination, physically impossible movement, enhanced effort, effortlessness, immobility, paralysis, forced or prevented movement, repetitive, slowed or hurried movement, acceleration)

20) Did you experience any of the following types of altered or impaired movements in your dream? *
For each movement, mark the duration and body part. If you mark "never", jump to the next movement.

21) Did you experience any of the following types of altered or impaired movements in your dream? *
For each movement, mark the duration and body part. If you mark "never", jump to the next movement.

22) Did you experience any of the following types of altered or impaired movements in your dream? *
For each movement, mark the duration and body part. If you mark "never", jump to the next movement. (such alterations may include having supernumerary limbs or a doppelgänger (that is, the presence of a double in addition to your dream self), absent limbs or the complete lack of a body, heavier, lighter, bigger, smaller or animal body parts or whole body, being a different sex, being a different person of the same sex, being older, younger, being an object etc, loss of ownership for whole body or certain body parts)

27) Please specify the duration and body part localization of alterations to your dream body! *
For each type of alteration, mark the duration and whole body / body part. If you mark "never", jump to the next sensation. A SUPERNUMERARY LIMB refers to the presence of an additional (physically impossible) body part. A DOPPELGÄNGER refers to the presence of a double in addition to the dream self (i.e., the character you are identifying with in your dream).

28) In what way was your dream body altered compared to your waking body? *
For each type of alteration, mark the duration and whole body / body part. If you mark "never", jump to the next sensation.

29) In what way was your dream body altered compared to your waking body? *
For each type of alteration, mark the duration and whole body / body part. If you mark "never", jump to the next sensation.