New Study Maps Brainstem Network That May Sustain Human Consciousness

At the bottom of the brain sits the brainstem, a dense cord of nerve cells which new research suggests may help maintain wakefulness in human consciousness.

The results, produced by researchers in the United States and France, reinforce earlier findings and may give scientists fresh insight into altered states of consciousness, including coma and vegetative states.

They could also help explain why certain people retain a hazy awareness or stay alert to their surroundings during anaesthesia, while others fully 'wake up' while undergoing surgery.

For centuries, scientists and philosophers have investigated the brain and examined human consciousness, yet it remains understood only in relatively broad terms.

Current theory divides consciousness - the capacity to perceive the outside world and our own existence - into two dimensions: arousal, or wakefulness, and awareness.

Brainstem links to conscious awareness

In 2016, researchers at Harvard Medical School identified, for the first time, a link between the brainstem, believed to control arousal, and brain areas associated with awareness.

Among hospital patients with brainstem lesions, most unconscious individuals had damage in a specific brainstem region called the rostral dorsolateral pontine tegmentum; just one conscious patient had such damage. Brain scans also showed that the newly identified connection between the brainstem and two cortical regions was disrupted in patients in comas and vegetative states.

Neurologist Brian Edlow took part in that earlier research. He began the new study alongside colleagues at the Massachusetts Institute of Technology (MIT) and France's University of Tours because efforts to map neural networks in the brainstem have lagged behind attempts to chart the cortex, the brain's outer layer.

One reason is that neuroimaging methods have not been able to separate the activity of individual neurons packed within the brainstem, or of networks extending upwards from it into the rest of the brain.

Mapping the default ascending arousal network

For the latest study, Edlow and his colleagues looked for signs of resting wakefulness, in which the brain is in 'standby mode', able to process information without actively performing a task or paying close attention.

They called the proposed network spreading from the brainstem the default ascending arousal network (dAAN). The term default reflects that no active input is needed. The team hoped to uncover further evidence of its links with cortical centres responsible for awareness.

Various methods and several animal studies have broadly outlined these connections, "but prior evidence of such interconnectivity is limited in the human brain," Edlow and colleagues write in their published paper.

The researchers used MRI to scan three post-mortem brains donated for research. They also analysed MRI data from 84 healthy people whose brains had been scanned through the Human Connectome Project, a US-led initiative to map neural connections throughout the brain.

In the midbrain, the researchers identified a highly connected hub in the ventral tegmental area (VTA), which had extensive links with other newly mapped dAAN nodes. The VTA was likewise strongly connected to a cortical network known to be involved in conscious awareness.

Previously, the VTA was considered mainly to influence behaviour and cognition - both highly active processes. However, the study adds to increasing evidence that it may also contribute to sustaining wakefulness and, consequently, awareness.

Further brainstem-cortex connections

The team also found two direct pathways between the brainstem and cortex, interweaving arousal with awareness. These were located in the lateral forebrain bundle and medial forebrain bundle, which lie beneath the brain's frontal lobe.

The researchers stress that the study is small, and other brains could produce different results. Given that consciousness is so elusive and fluid, numerous other explanations also remain for where human consciousness is located in the brain or how it emerges.

The study was published in Science Translational Medicine.