Differential Role of Prefrontal and Parietal Cortices in Controlling the Level of Consciousness

Pal, D., Dean, J. G., Liu, T., Li, D., Watson, C. J., Hudetz, A. G., & Mashour, G. A. (2018). Differential role of prefrontal and parietal cortices in controlling the level of consciousness.  Current Biology ,  28 (13), 2145-2152. https://doi.org/10.1016/j.cub.2018.05.025 

Summary 

Past literature and research have shown that determination of consciousness is by both content and level. Also, subcortical regions in the brain have been known to play a causal role in regulating the level of consciousness. Nonetheless, little is known about the role of the cortex despite the employment of clinical and correlative data in refuting or supporting the role of the prefrontal and posterior cortices in consciousness levels. Moreover, causal evidence regarding the influence or role of posterior cortical sites is lacking. Thus, the researchers in this study sought to investigate and test the hypothesis that prefrontal cortex is essential in its role in regulating the level of consciousness. Further, the researchers also sought to reverse sevoflurane anesthesia cholinergic or noradrenergic stimulation of the prelimbic prefrontal cortex as well as both areas of the parietal cortex. However, since human clinical trials are unsanctioned due to ethical implications, a sample of sixty rats (n=60) served as the subjects of the study with the results set to be generalized for humans.

In the bid to ascertain the site specificity of the effects observed in prefrontal cortex and subsequently test the alternative hypothesis –that the posterior cortical areas including parietal cortex are critical in consciousness, cholinergic stimulation with 5mM carbachol in two distinct regions within the parietal cortex of two separate groups of rats was conducted. The first group consisted of a sample of eleven rats (n=11) which was to undergo posterior parietal cortex examination while the second group included of a sample of eight rats (n=8) which were to undergo medial parietal association cortex (linked to attention and cognition) examination. Similarly, another group of eleven rats (n=11) was prepped for dialysis delivery of noradrenaline into the posterior parietal cortex.

Upon cholinergic stimulation of the prelimbic prefrontal cortex in anesthetized rats, a level of consciousness and wake-like behavior was observed. Elaborately, of the total eleven rats exposed too 5mM carbachol during the sevoflurane induction, four regained full consciousness while continuously breathing between 1.9% and 2.4% of clinically relevant concentrations of sevoflurane. Further, as the authors ascertain, the change in behavior in the four mice was preceded by a low-voltage electric activity in the electroencephalogram, a tenet of wakefulness.

Moreover, inferring from the results of the spectral density analysis, it was evident that the delta or theta frequency ratio, a conventional metric for electroencephalogram stimulation increased significantly during carbachol delivery as opposed to sevoflurane anesthesia. Further investigations as well showed that the stimulation of electroencephalogram was accompanied by an increased rate of respiration as well as a minor but significant spike in heart rate. Comparatively, delivery of carbachol into the posterior parietal cortex during sevoflurane anesthesia induced neither wake-like behavior nor righting (uncoordinated spastic muscle twitches, occasional movement in whiskers, limb, or tail by the rats). Thus, the researchers concluded that cholinergic stimulation of the prefrontal cortex can restore the level of consciousness despite persistent anesthetic administration as opposed to noradrenergic stimulation. As such, the prefrontal cortex serves as a potential target for regulating the level of awareness.

Questions/Extrapolations 

Indeed, cholinergic stimulation has a substantial influence on the prefrontal cortex as well as the parietal cortex in determining conscious level. However, do other areas of the brain such as the posterior cortical region important in influencing conscious level? Indeed functional connectivity and network theory as proposed by Shafi et al. (2012) posits that interregional correlations in the brain are essential in neural variability and thus consciousness. Therefore, it is possible that other regions of the brain in addition to those mentioned in the study form a network that jointly regulates consciousness.

This study also impacts patients with consciousness disorders. Thus, how do consciousness disorders affect the level of consciousness of patients under anesthetic stimulants? Perhaps the best way of investigating this postulation is through neuroscience that employs neuroimaging. This way, a correlation between level and contents of consciousness and consciousness disorders can be determined.

References

Pal, D., Dean, J. G., Liu, T., Li, D., Watson, C. J., Hudetz, A. G., & Mashour, G. A. (2018). Differential role of prefrontal and parietal cortices in controlling the level of consciousness.  Current Biology ,  28 (13), 2145-2152. https://doi.org/10.1016/j.cub.2018.05.025 

Shafi, M. M., Westover, M. B., Fox, M. D., & Pascual‐Leone, A. (2012). Exploration and modulation of brain network interactions with noninvasive brain stimulation in combination with neuroimaging.  European Journal of Neuroscience ,  35 (6), 805-825. Doi:10.1111/j.1460-9568.2012.08035.x.