The consciousness brain network has been the target of much scientific interest, which has grown as technology transfered the seat of consciousness from the heart to the head. Although arousal and wakefulness are parts of consciousness, their precise anatomy is surprisingly poorly understood, as emphasized by Fischer and colleagues in a paper in Neurology 2016, 87: 2427-2434. The classical idea throughout my professional life, as they write in their introduction, is that the brainstem reticular formation, which some authors write would extend through the basal ganglia to the thalamus, is the hallmark of the 3 functions, consciousness, arousal and wakefulness. But no hard data indicated where exactly these functions rested.
So, now they looked at a series of 36 patients with brainstem lesions which were or were not in coma; then they delineated the lesions by CT, MRI or autopsy; then they used resting-state functional connectivity MRI to evaluate the regions associated with the regions that caused coma in 98 healthy individuals; finally, they went back to 51 patients in coma and 21 controls and investigated the functional connectivity of the pathway they had established.
This was carried out at various departments at Harvard Medical School and Mass General Hospital in Boston, Massachussets. They found a region in the upper brainstem, 2 cubic mm “coma-specific region”, in the left pontine tegmentum, near the medial parabrachial nucleus. This region worked in functional connectivity with another on the left ventral anterior insula, extending into the claustrum. And with another in the pregenual cingulate cortex. then the investigators found that there was poor connectivity of the insula with the cingulate cortex.
The authors argue that although a little surprising, the left sided findings are not totally unexpected. It has been predicted from intracarotid amobarbital tests; from seizures and penetrating trauma. It may be that other functions, like attention, remain on the right. The insula-cingulate network described by the researchers is strongly linked to Von Economo neurons, large spindle shaped neurons found in all layers of the cortex of the most advanced mammals, specifically those with the most advanced cognition, such as recognition of themselves in a mirror.
Dr Paulo Bittencourt
