Neural Correlates of Dreaming Research
Consciousness is a constant companion during our waking hours. However, the experiences we have during sleep are more elusive. Upon waking, we sometimes recall vivid dreams, while other times, we remember nothing at all. Traditionally, dreaming has been closely linked to rapid eye-movement (REM) sleep. REM sleep is characterized by brain activity that resembles our waking state, with globally 'activated', high-frequency electroencephalographic activity.
However, this view has been challenged by the fact that dreaming also occurs in non-REM (NREM) sleep, which is typically associated with prominent low-frequency brain activity. This discovery forces us to rethink our understanding of the neural basis of conscious experiences during sleep.
To investigate the neural correlates of dreaming, researchers have employed high-density electroencephalography (EEG) to compare brain activity when dreams are present and absent in both NREM and REM sleep.
In both NREM and REM sleep, reports of dream experience were associated with local decreases in low-frequency activity in posterior cortical regions. High-frequency activity in these regions correlated with specific dream contents.
Dreaming experience vs. NREM sleep
Cortical distribution of t-values for the contrast between DEs and NEs at the source level for low-frequency power (1-4 Hz) in NREM sleep (20s before the awakening). p<0.05, after correction for multiple comparisons (two-tailed, paired t-tests, 32 subjects, t(31) > 2.04).
Dreaming experience vs. REM sleep
Cortical distribution of t-values for the contrast between DEs and NEs at the source level for low-frequency (1-4 Hz) power in REM sleep (20s before the awakening). p<0.05 after correction for multiple comparisons (two-tailed, paired t-tests, 10 subjects, t(9)>2.26).
Dreaming experience vs. NREM sleep
Cortical distribution of t-values for the contrast between DEs and NEs at the source level for high-frequency power (20-50 Hz) in NREM sleep (20s before the awakening). p<0.05 after correction for multiple comparisons (two-tailed, paired t-tests, 32 subjects, t(31) > 2.04). Same as A for the contrast between DE and DEWR in NREM sleep (two-tailed, paired t-tests, 20 subjects, t(19) > 2.09)
Correlation between dream content and brain activity
The relationship between specific dream content and brain activity has also been investigated.
Correlation between the thinking/perceiving score and 25-50 Hz power (last 8s, 7 subjects). Left: mean Spearman rank correlation coefficients (7 subjects). Right: significant voxels p<0.05 (one-tailed permutation test, r>0.14). Left: 25-50 Hz power differences (DE with face minus DE without face). ROI contrast R FFA p=0.023; one-tailed paired t-test (7 subjects, t(6) = 2.52). Right: fusiform face area (red). Upper row: 25-50 Hz average power differences between DEs with and without a spatial setting (6 subjects, t(5) > 2.57). Right: right posterior parietal cortex. Middle row: movement vs. no movement (7 subjects, t(6) > 2.45). Right: superior temporal sulcus. Bottom row: speech vs. no speech (7 subjects, t(6) > 2.45). Right: Wernicke’s area. Two-tailed paired t-tests.
Awakenings were performed in NREM when neural activity surpassed a bispectral threshold in low- frequency (LF; 0.5-4.5 Hz) and high-frequency (HF; 18-25 Hz) power over the posterior hot zone.
DE trials had significantly lower LF activity and significantly higher HF activity in DE compared to NE prediction trials. The bottom and top of the boxes show the 25th and 75th percentile (the lower and upper quartiles), the inner band shows the median, and the whiskers show the upper and lower quartiles +/− 1.5 * IQR (inter quartile range). Asterisks indicate significant differences (p=0.001; two-tailed t-test) between CE and NCE trials.
Prediction accuracy for DE (55) and NE (27) trials. Asterisks indicate significant differences (p<0.001; one sample t-test two-tailed) between CE and NCE prediction accuracy and chance (50%).
The following table summarizes the brain activity associated with different dream elements:
| Dream Element | Brain Region | Activity |
|---|---|---|
| Faces | Fusiform Face Area (FFA) | Increased 25-50 Hz power |
| Spatial Setting | Right Posterior Parietal Cortex | Increased 25-50 Hz power |
| Movement | Superior Temporal Sulcus | Increased 25-50 Hz power |
| Speech | Wernicke’s Area | Increased 25-50 Hz power |
References
- Aserinsky E, Kleitman N. Regularly occuring periods of eye motility, and concominant phenomena, during sleep. Science.
- Moruzzi G, Magoun HW. Brain stem reticular formation and activation of the EEG. Electroencephalogr Clin Neurophysiol.
- Nir Y, Tononi G. Dreaming and the brain: from phenomenology to neurophysiology. Trends Cogn Sci.
- Schwartz S, Maquet P. Sleep imaging and the neuro-psychological assessment of dreams. Trends Cogn Sci.
- Stickgold R, Malia A, Fosse R, Propper R, Hobson JA. Brain-mind states: I. Longitudinal field study of sleep/wake factors influencing mentation report length. Sleep.