Could a brain ever exist on its own, divorced from or independent of a body? For a long time, philosophers have pondered such "brain-in-a-vat" scenarios, asking whether isolated brains could maintain consciousness when separated from their bodies and senses.
Typically, a person's experiences are characterized by a web of interactions between the human brain, body and environment.
But recent developments in neuroscience mean this conversation has moved from the realm of hypothetical speculation and science fiction, to isolated examples where consciousness could be sealed off from the rest of the world.
In a 2020 study, detailed in the journal Trends in Neuroscience, philosopher Tim Bayne, of Monash University in Melbourne, and neuroscientists Anil Seth, of the University of Sussex in England, and Marcello Massimini, of the University of Milan in Italy, describe contexts in which such "islands of awareness" could exist.
In one possible situation, a brain that has been removed from its host is able to sustain consciousness using the oxygen and nutrients necessary for function delivered via some kind of apparatus. This is called the ex cranio brain.
In a study that sounds like something out of a horror movie, researchers were able to successfully restore blood flow to brain cells, cellular functions of neurons, and spontaneous synaptic activity in pigs' brains that were removed after death and connected to a system called BrainEx. The system, which is designed to slow the degeneration of brain tissue after death, can be connected to the base of a postmortem brain, delivering warm artificial oxygenated blood.
In people who suffer from severe refractory epilepsy, one treatment called a hemispherotomy involves completely disconnecting the damaged half of the brain from the other hemisphere, brainstem and thalamus. In these cases, the damaged half remains inside the skull, and connected to the vascular system. While the disconnected hemisphere continues to receive the nutrients and oxygen needed for function, some have wondered whether this isolated hemisphere supports an adjacent consciousness to the opposing, connected hemisphere.
And scientists have created lab-based mini-brains, 3D structures developed from stem cells that display various features of the developing human brain. Some of these brains-in-a-dish have brainwaves similar to those seen in preterm babies.
But do any of these "brains" actually possess consciousness?
Scientists can't deduce consciousness from behavior in these cases, nor can they ask these brains if they are experiencing consciousness. This conundrum has led neuroscientists to devise a potential "objective" measure of consciousness.
For instance, scientists could use the so-called perturbational complexity index (PCI), which is based on the level of interactions between neurons within these "brains." Using this index, scientists would electrically stimulate a part of the brain and then measure the resulting patterns of neural activity to gauge the complexity of brain-cell interactions. If the resulting measurement of these interactions carries lots of information, then the system can be said to be more conscious.
It's kind of like tossing a rock into a pond and measuring the resulting ripples. If the ripples interact with other objects in the pond, setting off more ripples, then the more conscious the system.
In states where people have not been fully conscious, PCI has been a reliable indicator of their level of consciousness. For instance, being in a coma, or sleeping, would be considered a "lower" level of consciousness or awareness.
"PCI has proven effective in detecting disconnected awareness during dreaming, ketamine anesthesia, and has also been fruitfully applied to patients who are non-responsive following severe brain injury," Bayne told Live Science.
It could be the case that consciousness is tightly coupled to dynamics of the brain that are relatively easy to measure, such as the case with the PCI.
But even if consciousness doesn't turn out to be reducible to any neural signal in the brain, Bayne believes the task of developing an "objective" measure of consciousness is still a valid one.
While these techniques might not be able to definitively answer the question of whether consciousness is present in these contexts, they will provide answers to some fundamental questions, such as whether islands of awareness have the same levels of neural complexity as the brains of conscious subjects. Or do these brains slowly go offline once disconnected from the external world?
Understanding what the contents of consciousness could look like in such cases offers an even trickier problem.
Originally published on Live Science.
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Conor Feehly is a New Zealand-based science writer. He has earned a master's in science communication from the University of Otago, Dunedin. His writing has appeared in Cosmos Magazine, Discover Magazine and ScienceAlert. His writing largely covers topics relating to neuroscience and psychology, although he also enjoys writing about a number of scientific subjects ranging from astrophysics to archaeology.