arousal thresholds may explain the loss of consciousness characteristic of sleep, particularly NREM sleep [23]. With respect to dreaming, all this is likely to suppress “the experience of perception and mentation during NREM sleep” ([2], p. 824).

However, the results of the current meta-analysis did not reveal any significant increase or decrease in thalamic activity in REM sleep when compared to wakefulness. Such a result is not altogether surprising since the imaging techniques employed in these studies may not effectively capture brief, sporadic ponto-geniculo-occipital activity in structures such as the lateral geniculate nucleus (LGN) which, to date, has only been detected using depth electrodes in animals. Such activity (generated by the pontine brainstem and relayed to the cortex via the LGN) conveys pseudosensory information that is subsequently elaborated by the cortex and experienced as dreams [2].

4.2. Frontal Cortex Activation Patterns during Sleep

In addition, compared to wakefulness there was a decrease in activity in frontal regions during NREM (inferior frontal gyrus) and REM sleep (middle, inferior, and superior frontal gyrus). These regions correspond to the dorsolateral [27] and orbitofrontal cortex [28]. This has lead theorists to postulate that such decreased activity may provide an essential role in dream recall [2,23]. Specifically, as these frontal cortical areas are predominantly involved in executive functions during wakefulness (such as directed thought and decision making, memory and attention) [28-31], such decreased activity during NREM and REM sleep has been linked to the observed disruptions in such processes during sleep [23, 32,33]. For instance, working memory is deficient from REM sleep dream reports, with scene shifts often experienced without conscious reflection by the dreamer [34]. Hence, such disruptions in memory and other executive processes may explain the unique qualities of dreams from REM sleep like their characteristic bizarreness, disorientation, and loss of self-reflective awareness [2,23,35].

4.3. Anterior Cingulate Activation Patterns during Sleep

Finally, the meta-analyses also revealed significant patterns of regional activity in the anterior cingulate. The anterior cingulate gyrus is involved in the processing of emotional information, particularly the emotions and memories evoked in response to fearful stimuli as well as the fear potentiated startle response [36]. This information processing in turn facilitates the behavioral and autonomic responses to such stimuli [36]. In addition to its role in processing emotion, the anterior cingulate gyrus is also involved in executive functions such as attention [37], including response monitoring [38] and error detection [39]. These various functions may be due to the location of the anterior cingulate gyrus, situated between the frontal cortex (executive functions) and limbic regions (emotion) [20].

In the current study, two areas of the anterior cingulate were found to be involved in sleep, exhibiting selective activation/deactivation patterns across REM and NREM. Specifically, decreases were observed in the left anterior cingulate gyrus (BA 24 and 32) during NREM. These regions of the anterior cingulate roughly correspond to the perigenual anterior cingulate cortex and midcingulate cortex, which are implicated in the aforementioned processes involving affect and response selection, respectively [40]. Thus, the decreases observed are consistent with the known deficits in such processes and overall brain deactivation inherent during this stage of sleep. However, there was also a circumscribed area of increased activity identified in the left anterior cingulate (BA 24). It is possible that such activity may reflect the transient appearance of certain sleep rhythms. For example, increased activity in the anterior cingulate cortex was found to coincide with periods of spindle activity [15] and slow waves [41]. These findings fit well with the results of the current meta-analysis, and suggest that the pattern of activity observed may reflect the activity of such NREM rhythms. Alternatively, the activation observed in our meta-analysis may reflect increased activity, which coincides with periods of dreaming in NREM. However, not all of the included studies systematically investigated dreaming, so such an explanation is purely speculative.

Interestingly, during REM sleep increases comprised areas of the right anterior cingulate cortex (i.e. BA 32). Such activity is not surprising since the anterior cingulate cortex has substantial connections to other structures in the forebrain limbic system such as the amygdala, a major centre involved in emotion [20,21,42,43]. Thus, such activation during REM sleep (combined with the aforementioned frontal deactivation during this stage of sleep) is consistent with the exaggerated affect and bizarre content, disorientation, as well as a lack of reflection and awareness characteristic of dreams from REM [44].

4.4. Methodological Considerations

The results of the aforementioned meta-analyses as well as the subsequent discussion and conclusions drawn must be interpreted in light of various limitations. Specifically, the major limitation concerns the type of studies included in the current review. Attempts were made to obtain a relatively homogenous sample of studies for inclusion in the analyses. As a result, a number of studies that may otherwise have provided valuable information regarding brain activity during sleep were excluded. For instance, studies that administered an external stimulus or pharmacological agent prior to or during sleep were excluded as such manipulation may produce different profiles of brain activity when compared to spontaneous sleep. However, despite such attempts the included studies were not strictly homogenous due to variability in the methods employed across studies. For example, although only PET studies were analyzed, the two PET techniques (as well as methods used in their analyses) differ. Furthermore, some studies included in the meta-analyses employed sleep deprivation procedures, a procedure which is likely to alter global as well as regional brain activity. Thus, the inclusion of such studies may have skewed the observed results. Finally, some of the included studies involved awakening participants in order to collect dream reports. These awakenings may have altered sleep integrity and architecture, and potentially the brain activity observed in subsequent scans. These issues highlight how the inclusion of such studies may influence the final result. However, had these studies been excluded, this would have further reduced the already limited sample size. These issues aside, significant clusters were revealed using the updated ALE technique.

4.5. Summary

The current study revealed a network of brain regions implicated in the neurophysiology of REM and NREM sleep. Specifically, in REM sleep this network included the anterior cingulate as well as the dorsolateral and orbitofrontal cortex. In NREM sleep, the network comprised these same areas as well as the thalamus. Further research is needed to elucidate the role of these regions in sleep and whether such activity coincides with periods of dreaming.

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NOTES

*Corresponding author.

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