A study of EEG hemispheric asymmetry during REM sleep in young healthy adults

Christianne Bolduc,^1,2 Anne-Marie Daoust,^1,3 Élyse Limoges,^1,2  Marc-André Gingras,^1,3
Claude M. J. Braun,³ Roger Godbout^1,4
1Laboratoire du sommeil, Hôpital Rivière-des-Prairies, 7070 boul. Perras, Montréal, Québec, Canada, H1E 1A4
²Département de Psychologie, Université de Montréal
³Département de Psychologie, Université du Québec à Montréal
⁴Département de Psychiatrie, Université de Montréal

Abstract: The EEG of 18 right-handed participants was recorded during REM sleep using a 14-electrode monopolar montage and submitted to spectral analysis (0.75-19.75 Hz). Power amplitude (µV) was extracted for Delta (0.75-3.75 Hz), Theta (4.0-7.75 Hz), Alpha (8.0-13.75Hz) and Beta (14.0-19.75 Hz) activity bands. A right lateralization was found for the following electrode pairs and activity bands: prefrontal (Theta), frontal (Theta, Beta and total power) and occipital (Delta and total power). A left lateralization was found for the two temporal electrode pairs and activity bands (Delta, Theta, Alpha, Beta and total power). The activity of the central and parietal electrodes was not asymmetric. These results suggest that spontaneous EEG activities generated during REM sleep are supported by topographically specific thalamo-cortical influences.

INTRODUCTION

Quantified analysis of the EEG can be used to characterize specific cortical areas that are activated under experimentally controlled conditions (for example, see [1]). Moreover, measures of left-right EEG asymmetry have been found to reflect cognitive task hemisphericity [2]. Few studies have investigated EEG lateralization during sleep, most of which being performed during REM sleep. Indeed, REM sleep is a state characterized by major endogenous activation of the central nervous system and thus constitutes an appropriate model to characterize spontaneous organization of neural networks such as the thalamo-cortical loop, on which EEG generation is based [3,4]. An early study has suggested that the right hemisphere is more activated than the left during REM sleep, as indexed by higher EEG power in healthy participants [5]. Thus, the aim of the present study was to investigate whether there exists an EEG asymmetry in REM sleep using lateralization coefficients, full EEG montage, and carefully selected experimental conditions.

METHODS

Subjects were 18 healthy right-handed adults (10M, 8F; mean age 21.25 ± 2.05 years). Bilateral frontal (Fp1, Fp2, F7, F8), central (C3, C4), temporal (T3, T4, T5, T6), parietal (P3, P4) and occipital (O1, O2) EEG was recorded for two consecutive nights using monopolar montage referred to linked ears with serial 10 KOhms resistance for signal equilibrium purposes. REM sleep EEG samples were made of 15 four-second segments, taken in equal proportions from the first three REM sleep periods of night two. These 60 seconds of EEG were extracted from artifact-free, quiescent REM sleep. EEG samples were Fast Fourier-transformed using cosine window smoothing, with a frequency resolution of 0.25 Hz. Spectral analysis was performed on total frequencies and four frequency bands, namely: Delta (0.75-3.75 Hz), Theta (4.0-7.75 Hz), Alpha (8.0-13.75 Hz), Beta (14.0-19.75 Hz). EEG lateralization was determined by computing a coefficient using the formula: ((Right - Left) / (Right + Left)) x 100, applied on spectral amplitude values (mV) from homologous right and left electrodes. The null hypothesis was tested independently for EEG samples taken from REM sleep using paired t-tests.

RESULTS

During REM sleep, a right lateralization on prefrontal (Theta: T = 2.19, p < .05), frontal (Theta: T = 2.19, p < .05, Beta: T = 0.01, p < .01, total power activity: T = 0.03, p < .04) and occipital (Delta: T = 0.01, p < .01, total power activity: T = 0.04, p < .04) areas was observed. In contrast, a left temporal activity on Delta (T = 0.03, p < .03), Theta (T = 0.03, p < .04), Alpha (T  = 0.00, p < .01), Beta (T  = 0.00, p < .01) and total power activity (T = 0.00, p < .01) was observed. (see Table 1).

DISCUSSION

The fact that right prefrontal, frontal and occipital dominance was found in REM sleep suggests that these areas could be supported, at least in part, by a common thalamo-cortical neural network. Indeed it has been claimed that REM sleep dreaming is associated with the right hemisphere in right handers [6, 7, but see 8], a proposal that is consonant with the right frontal EEG dominance we found during REM sleep in our right-handed participants. On the other hand, the left temporal dominance is interesting. It could reflect controlateralized visuo-spatial and/or limbic activities related to REM sleep cognition. We conclude that spontaneous EEG activities generated during REM sleep are supported by topographically specific thalamo-cortical influences.

TABLE 1

Electrode pairs (1)	Frequency bands (2)	Lateralization coefficient
Fp1-Fp2	Delta	-0.01±1.78
	Theta	1.26±2.43*
	Alpha	1.30±3.83
	Beta	1.28±4.08
	Total	0.78±2.69
F7-F8	Delta	1.43±4.72
	Theta	2.00±3.88*
	Alpha	1.62±4.02
	Beta	3.02±4.17*
	Total	1.92±3.54*
C3-C4	Delta	0.49±3.65
	Theta	0.43±4.07
	Alpha	0.32±3.03
	Beta	0.14±3.24
	Total	0.31±3.22
T3-T4	Delta	3.06±19.16
	Theta	2.83±21.64
	Alpha	2.83±20.03
	Beta	1.36±19.16
	Total	2.35±20.23
T5-T6	Delta	-3.57±4.96*
	Theta	-3.03±4.30*
	Alpha	-5.17±4.71*
	Beta	-5.35±4.28*
	Total	-4.25±3.88*
P3-P4	Delta	0.69±0.78
	Theta	0.01±2.73
	Alpha	3.18±3.38
	Beta	1.39±2.29
	Total	1.10±1.15
O1-O2	Delta	2.78±3.78*
	Theta	1.59±3.48
	Alpha	1.58±5.23
	Beta	0.78±3.10
	Total	1.68±3.12*

* = Significantly different from zero.                    
(1) EEG electrode derivations according to [9]
(2) Beta (13.0-19.75 Hz), Alpha (8.0-12.75 Hz), 
Theta (4.0-7.75 Hz) and Delta (0.75-3.75 Hz)     

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Supported by the Canadian Institutes for Health Research.