In this post, I want to discuss a paper entitled the “Breakdown of Cortical Effective Connectivity During Sleep”. In plain English, this paper discusses the theoretical possibility that consciousness fades during the night because the cortex essentially doesn’t talk to itself as much. More specifically, this study focused on NREM sleep, which accounts for roughly 75-80% of our total sleep time. During NREM sleep, people often report no dream experiences, and it is this lack conscious activity that the researchers wanted to investigate. What goes on in our brains during this period of non-consciousness?
In order to answer this question, the researchers used a combination of transcranial magnetic stimulation(TMS) and electroencephalography(EEG). Using TMS was advantageous for the researchers because they could stimulate the cortex directly without activating the subcortical reticular formation and the thalamo gating relay.
The researchers used TMS to stimulate the rostral portion of the right premotor cortex, which has dense connections with the rest of the cortex area, which is heavily correlated to typical wakeful consciousness. Now for the results:
During wakefulness, TMS induced a sustained response made of recurrent waves of activity…With the onset of NREM sleep, the brain response to TMS changed markedly. After [the initial] large wave, no further TMS-locked activity could be detected.
Thus, during wakefulness, the perturbation of the rostral premotor cortex was followed by spatially and temporally differentiated patterns of activation that appeared to propagate along its anatomical connections. In striking contrast, during NREM sleep the location of maximum current density remained confined to the stimulated area.
During wakefulness, the site of maximum activation moved back and forth among premotor and prefrontal areas in both hemispheres and, in some subjects, it also involved the motor and posterior parietal cortex. During NREM sleep, by contrast, the activity evoked by TMS did not propagate in space and time in any of the subjects.
Thus, an impairment in the ability to integrate information among specialized thalamocortical modules—a proposed theoretical requirement for consciousness—may underlie the fading of consciousness in NREM sleep early in the night.
The researcher’s speculation on the potential neural mechanisms behind this decreased cortical activity during NREM sleep is a little beyond the scope of this blog, but it could have something to do with “down states” of depolarization being triggered more easily. Regardless,
Whatever the precise mechanisms, they are most likely engaged by the progressive reduction of the firing of diffuse neuromodulatory systems that occurs when we fall asleep.
For those interested, a good summary article of this research can be found here