What Is Consciousness?

The 'hard problem', the search for neural correlates of consciousness, blindsight and split brains, and the difference between being conscious and being conscious of something.

Two meanings

Consciousness has two common senses. Levels of consciousness — being awake versus asleep, anaesthetised or in a coma — depend on the brainstem, thalamus and widespread cortical networks. Contents of consciousness — what you are aware of at a moment — depend on specific representations becoming part of experience. You can be fully awake yet unaware of most of what your brain is processing.

The hard problem

In 1995 the philosopher David Chalmers distinguished the "easy problems" of consciousness — explaining how the brain discriminates stimuli, integrates information, reports mental states and controls behaviour — from the "hard problem": why any of this processing is accompanied by subjective experience, why there is "something it is like" to see red. (The "easy" problems are not easy — just tractable with standard methods.) Some philosophers argue the hard problem is a confusion that will dissolve as science progresses; others think it requires new concepts.

Neural correlates of consciousness

In 1990 Francis Crick — co-discoverer of DNA's structure — and Christof Koch proposed a practical strategy: look for the neural correlates of consciousness (NCC), the minimal brain activity sufficient for a specific conscious experience. A key method compares conditions where the stimulus stays the same but awareness changes. In binocular rivalry, each eye sees a different image, and awareness alternates between them; neurons in higher visual areas tend to follow what is seen, not just what is shown. In masking, a briefly flashed image goes unseen when followed by another image.

Clues from patients

Blindsight patients with damage to primary visual cortex can point to objects they insist they cannot see — showing processing without awareness. Split-brain patients raise questions about whether one brain can host two streams of experience. Hemispatial neglect shows awareness can be lost for half of space while the eyes see it.

Measuring consciousness

Clinically, assessing consciousness matters enormously for patients who cannot respond. Researchers developed the perturbational complexity index, which stimulates the cortex with TMS and measures how complex the EEG response is; it distinguishes conscious from unconscious states across sleep, anaesthesia and brain injury with good accuracy.