Not a flash in the dark but a dimmer switch

WE TEND to take consciousness for granted. But when, mentally, do all the lights actually go on? As an adult one has to admit that, for some of the time at least, one is oneself conscious, whereas it is hard to attribute sentience to a newly fertilised egg. What Rubicon then did we all cross as our brains grew? And when? There are certainly a range of candidate indices as events unfold in pregnancy, each occurring at a certain stage of foetal development, but often at very different times, and in each case of arguable significance.

One of the earliest markers might be simple movement, which is detectable from eight weeks onwards; on the other hand, the nascent nervous system might be merely displaying reflexes, akin to the headless chicken. The appearance of sensory detectors and sensory responses from nine weeks, through possibly of some relevance, have in some cases none the less been mimicked on a definitively unconscious computer.

Instead of these brain inputs and outputs, the status of that secretive grey box itself might be the deciding factor. Take for example the outer layer of the brain, the "cortex", named after the Latin for bark since it wraps around the brain like its arboreal namesake wraps around a tree. In mammals, at least, the cortex has been identified by some as a requisite factor for consciousness. However it is difficult to decide precisely which aspect of cortical development would be critical. Cortical cells can be present in their correct position in the brain from six weeks, become insulated with the fatty covering myelin in certain selective regions from about 20 weeks, develop local connections from 25 weeks, and generate an electrical wave pattern from about 30 weeks. The development of the cortex thus spans a very wide time frame; moreover all the changes are gradual; they start to take place at certain times, but are by no means complete until well after birth.

Another feature of the brain that has also been regarded as central to the conscious state are the connections from a central relay station for the sensors deep inside the head, the thalamus, and the outer reaches of the brain. There is no evidence as yet, however, to prove that the integrity of these connections might be anything other than a necessary, rather than sufficient, criterion for the creation of the rich inner world that we each claim to enjoy and that no one else can hack into. Another problem with attributing this kind of monopoly on importance to the actual circuitry of the mammalian brain is well exemplified in the case of the common octopus. This invertebrate, which has featured in many experiments on learning and memory, may well be indulging in some molluscan inner life, and with a brain configured differently from that of the human. The deciding proof, therefore, for the presence of consciousness, however crude, basic or raw, is not necessarily the appearance of the relatively detailed physical features of a developing foetal mammalian brain.

Rather than singling out a one-off criterion by which to judge the consciousness of octopi or Einstein, perhaps consciousness should be considered as not all or none but rather as a continuum; it is not a flash in the dark but rather a dimmer switch, growing in depth as the brain grows. If different degrees of consciousness are indeed accommodated in brains of different species and at different stages of development, then we will have to look for alternative clues within the brain if we still wish to pinpoint the birth of consciousness - the most spectacular and frustrating of riddles.

Professor Susan Greenfield is the author of `The Human Brain: a guided tour' (Phoenix, pounds 6.99)