Consciousness is elusive, but we know it’s what vanishes when we fall into a deep sleep and reappears when we wake up. Doctors typically determine if a person is conscious by their ability to process and respond to external commands, such as “open your eyes” or “squeeze my hand.”
But these methods are superficial, as research has shown in the last decade that a brain totally disconnected from the outside world may still have some awareness. This may happen in brain-injured patients who emerge from a coma but can’t move or understand instructions, for example.
One theory is that in a conscious brain, different populations of neurons, or nerve cells, carry out their own computational roles, but can’t communicate with other neuron populations.