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07Consciousness and Its Reversible Loss

Anaesthesia gives the study of consciousness something it otherwise lacks: a way of removing awareness and restoring it, repeatedly and on schedule. That is a powerful experimental handle, and it settles less than it appears to.

Why the state is scientifically useful

Consciousness is difficult to study because it cannot ordinarily be manipulated. Sleep is not fully under control, comes in stages, and cannot be started at a chosen moment. Injury removes awareness but also removes the tissue one wanted to study, and cannot be reversed. Anaesthesia is different: the state can be produced deliberately, deepened and lightened in steps, held constant, and reversed, in a person who was awake ten minutes before and will be awake ten minutes after. Nothing else in the biological repertoire offers that.

Describing depth

The first attempts to describe the state were purely observational. A graded scheme published in the 1930s divided the progression under ether into successive stages, each identified by the pattern of breathing, the movement of the eyes, the response of the pupil and the presence or absence of particular reflexes. It was a genuine achievement, giving observers a shared vocabulary and making the state something that could be discussed precisely between people who had not been in the same room.

Its limits became apparent when practice changed. The scheme described the progression under a single agent given alone. Once relaxation was produced separately, most of the observable signs on which the scheme depended were abolished by something other than depth, and the whole descriptive apparatus stopped working. This is a recurring pattern in the subject: a description that is valid for one technique is silently carried over to another where its foundations no longer hold.

Electrical signatures

Recording of electrical activity from the scalp, published in 1929, eventually supplied a different approach. As the state deepens the electroencephalogram changes in characteristic ways: activity slows and becomes larger and more synchronised, and at greater depths the trace becomes intermittent, with periods of activity separated by periods of near-silence. From the 1990s these changes were reduced to single processed numbers intended to summarise depth.

The interpretive difficulty is fundamental rather than technical. Such an index is calibrated against a population, and it reports a correlate of the state rather than the state. Different agents produce different electrical signatures at equivalent depths, because they act at different targets, so a single number cannot mean the same thing across them. What the electrical work has established solidly is not a measure of consciousness but a description of what happens to cortical communication when awareness goes: long-range integration between regions breaks down while local activity continues, which is why the loss looks less like a switch and more like a disconnection.

The awareness problem

The clearest evidence that the state is not unitary comes from the observation that awareness and the ability to signal it can be separated. If the muscles are relaxed, a person may be conscious and unable to indicate it, and studies using a technique that preserves movement in one arm while the rest of the body is relaxed have demonstrated that patients can sometimes respond to instruction while showing no other sign of being present.

Two conclusions follow, and both matter for the study of consciousness generally. Behavioural unresponsiveness is not the same thing as unconsciousness, and the two are routinely confused because in ordinary life they coincide. And the components of the state, awareness, memory, movement and response to injury, are dissociable, since a person may have been briefly aware with no memory afterwards, or may show responses without awareness. Whatever consciousness turns out to be, it is not a single quantity that anaesthesia turns down.

What it does not settle

It is tempting to conclude that if the state can be switched off by a molecule then consciousness is straightforwardly a physical process and the philosophical problem dissolves. That does not follow. Knowing what abolishes a phenomenon is not the same as knowing what constitutes it, and a light switch tells you nothing about optics. What anaesthesia genuinely contributes is a set of constraints: any adequate account of consciousness must explain why it is disrupted by these particular molecular actions, why long-range cortical integration tracks its presence, and why its components come apart in the specific ways that they do. Those constraints are worth a great deal, and they are not an answer.