On MRI Scans, Scientists Find What Could Explain Altered States of Consciousness : ScienceAlert

At the base of the brain is the brainstem, a thick cord of nerve cells that new research suggests may keep human consciousness sharp.

The findings, carried out by a team in the United States and France, build on previous work and could lead researchers to new understandings of altered states of consciousness, such as comas and vegetative states.

It could also help them understand why some people have vague awareness or remain attentive to their surroundings under anesthesia while others “wake up” completely during surgery.

Although scientists and philosophers have stimulated the brain and probed human consciousness for centuries, researchers have only come to understand it in rather vague terms.

Current thinking is that consciousness, the ability to sense the world and our own existence, can be divided into two dimensions: awareness (or awakening) and awareness.

In 2016, researchers at Harvard Medical School first discovered a connection between the brainstem, thought to regulate wakefulness, and parts of the brain involved in consciousness.

In hospitalized patients with brainstem damage, most unconscious patients had damage to a particular part of their brainstem called the rostral dorsolateral pontine tegmentum, while only one conscious patient had damage. The new connection between the brainstem and two cortical regions was also disrupted in comatose patients and those in a vegetative state, brain scans showed.

Neurologist Brian Edlow was part of that team, and he embarked on this new study with colleagues at the Massachusetts Institute of Technology (MIT) and the University of Tours in France, because mapping neural networks in the brainstem has stalled compared to mapping efforts. the cortex, the outer layer of the brain.

This is partly because neuroimaging techniques could not tease out the activity of individual neurons clustered in the brainstem or networks emanating from it, upward in the brain.

In this new study, Edlow and his colleagues focused on looking for traces of resting wakefulness: when the brain is in “sleep mode,” capable of processing information but not actively engaged in a task or particularly attentive.

They named this proposed network extending out of the brainstem the default ascending arousal network (dAAN) – default for the way in which no active input is required – and hoped to find more evidence of its connections with centers of consciousness of the cortex.

Such connections have been sketched using a variety of techniques in several animal studies, “but previous evidence for such interconnectivity is limited in the human brain,” Edlow and colleagues write in their published paper.

The team used MRI to scan three postmortem brains donated for research and examined MRI data from 84 other healthy people whose brains were scanned as part of the Human Connectome Project, an effort led by the United States to map neuronal connections across the brain.

In the midbrain, they found a broadly connected hub in the ventral tegmental area (VTA) that exhibited extensive connectivity to other newly mapped nodes of the dAAN. The VTA was also well connected to another network in the cortex known for its role in consciousness.

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Prior to this, VTA was thought to primarily modulate behavior and cognition, two highly active processes, but this study adds to growing evidence that VTA may also help maintain arousal and, by extension, consciousness.

Two other direct connections connecting the brainstem and cortex, and linking together arousal and consciousness, have been found in the lateral forebrain fasciculus and the medial forebrain fasciculus, which lie nestled beneath the anterior lobe of the brain .

The researchers acknowledge that their study is limited; other brains might reveal something different. With something as slippery and fluid as consciousness, there are also many other interpretations of where human consciousness might be located in the brain or where it might arise from.

The study was published in Scientific translational medicine.

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