A groundbreaking study conducted by researchers at Massachusetts General Hospital and Boston Children’s Hospital has mapped out a brain network crucial to human consciousness. Utilizing advanced high-resolution imaging, the team discovered previously uncharted pathways connecting various brain regions, forming what they call the “default ascending arousal network” (DAAN). This network plays a pivotal role in maintaining wakefulness and integrating arousal with awareness in the resting human brain.
The findings have been published in Science Translational Medicine.
Understanding human consciousness, especially its disruptions in conditions like coma, vegetative states, and minimally conscious states, has been a long-standing challenge. Consciousness consists of two main components: arousal, which is the state of being awake, and awareness, which involves the content of consciousness. These components can be dissociated, as seen in patients who exhibit wakefulness without awareness, such as those in a vegetative state.
The researchers aimed to fill gaps in knowledge about the subcortical pathways that contribute to arousal and how these integrate with cortical pathways related to awareness. By doing so, they hoped to provide clinicians with better tools to detect, predict, and facilitate the recovery of consciousness in patients with severe brain injuries.
Data for the study was derived from ex vivo (post-mortem) brain specimens from three neurologically normal individuals and in vivo (living) 7-Tesla MRI scans from 84 healthy controls, sourced from the Human Connectome Project. The ex vivo brain specimens provided a detailed anatomical map, while the in vivo data helped verify the functional connectivity of these anatomical pathways.
A key finding of the study was the identification of 18 nodes within the DAAN that are interconnected through specific projection pathways. These pathways facilitate communication between the brainstem and higher brain regions, forming a structural basis for sustaining wakefulness.
The ventral tegmental area (VTA), in particular, emerged as a significant hub within this network, showing extensive connectivity with the cortical default mode network (DMN), which is involved in self-awareness and other higher cognitive functions. This suggests that the VTA’s dopaminergic pathways are vital for modulating wakefulness and integrating it with awareness, providing a neuroanatomical foundation for the conscious state.
“Our goal was to map a human brain network that is critical to consciousness and to provide clinicians with better tools to detect, predict, and promote recovery of consciousness in patients with severe brain injuries,” explained lead author Brian Edlow, the co-director of Mass General Neuroscience, associate director of the Center for Neurotechnology and Neurorecovery (CNTR) at Mass General, an associate professor of Neurology at Harvard Medical School and a Chen Institute MGH Research Scholar 2023-2028.
Edlow continued, “Our connectivity results suggest that stimulation of the ventral tegmental area’s dopaminergic pathways has the potential to help patients recover from coma because this hub node is connected to many regions of the brain that are critical to consciousness.”
Senior author Hannah Kinney, Professor Emerita at Boston Children’s Hospital and Harvard Medical School, added that “the human brain connections that we identified can be used as a roadmap to better understand a broad range of neurological disorders associated with altered consciousness, from coma, to seizures, to sudden infant death syndrome (SIDS).”
While the study offers groundbreaking insights, it also has limitations. The small sample size of three ex vivo brain specimens means that further research with larger samples is needed to confirm these findings. Additionally, the study’s imaging resolution, though high, is still not sufficient to map individual axons accurately. Therefore, the results should be considered inferential rather than definitive.
The study, “Multimodal MRI reveals brainstem connections that sustain wakefulness in human consciousness,” was authored by Brian L. Edlow, Mark Olchanyi, Holly J. Freeman, Jian Li, Chiara Maffei, Samuel B. Snider, Lilla Zöllei, J. Eugenio Iglesias, Jean Augustinack, Yelena G. Bodien, Robin L. Haynes, Douglas N. Greve, Bram R. Diamond, Allison Stevens, Joseph T. Giacino, Christophe Destrieux, Andre van der Kouwe, Emery N. Brown, Rebecca D. Folkerth, Bruce Fischl, and Hannah C. Kinney.