UNDER THE MICROSCOPE; The dawning light of consciousness

The recent cases of the aborted twin and the potential octuplets have revived in the public eye the moral issue of termination: but there is a scientific question on which it is far harder to take sides. Is a foetus actually conscious?

Imagine Scenario A: the foetus is not conscious. Rather, it is as sentient as a plant that turns to the light, a cross between a vegetable and a robot. When it is born, then, one of two further possibilities must occur. First, the foetus remains unconscious. This is hard to swallow. Not only would we have to accept the counter-intuitive idea that a new mother might as well be cooing and clucking over a rather sophisticated, battery-powered doll, but it becomes increasingly implausible that consciousness sets in suddenly, but of course imperceptibly, in later life.

The second possibility in Scenario A is that consciousness descends bright and sudden, at birth, like the Christmas lights in Oxford Street. The passage through the birth canal could then be viewed as our own individual rite of passage. But again this is not an easy idea. Would it be the mere age of the foetus that threw the switch? If so, premature babies would spend their first two months in the world as robots and thus, for example, not need pain killers. Alternatively, the birth process itself might be the switch-thrower. But it would have to be some factor other than the squeezing, squashing, pulling and pushing of labour; otherwise those born by Caesarean section would remain unconscious for the rest of their days.

The simple fact is that there is no conspicuous and sudden change in a baby's body when it is born that could be identified as the monitor light twinkling red for consciousness. True, at birth we are suddenly breathing on our own, but the brain, that questionably conscious object, is indifferent to the source of the oxygen molecules it receives, be they second-hand from mum, or direct through the nose.

Every way you look at it, Scenario A is really a non-starter. So what of the only other scenario, Scenario B, that the foetus is conscious in the womb? A conscious foetus is certainly plausible, but the questions that ensue are far from straightforward. When might a foetus become conscious? Surely not as a fertilised egg, or even a few days later, as a ball of cells: after all, it would have no nervous system to be conscious with.

So what about when the brain appears? The problem here is that Nature builds the brain gradually. From a primitive plate to a sealed tube, neurons- to-be proliferate, before setting off on a one-way journey on a monorail of guiding cells to reach their final home in the new brain. Here each new neuron eventually sets down roots and slowly starts establishing contacts with its neighbours and ultimately networking with those further afield. Layer is built upon layer in the outer reaches of the brain. There are certain definite times that mark the different stages of this development, but any of these stages - from as early as four weeks, when a primitive brain has formed, or a little later at eight weeks, when there are recognisable brain regions, to as late as 30 weeks, when rudimentary brain waves can be recorded in a scan - might be claimed to herald the onset of consciousness. Which brings us to the second question: what would a foetus be conscious of? Even if it weren't sealed in its warm, wet, dark world, a half-baked brain is surely not capable of the same consciousness as us sophisticates.

Both these problems posed by Scenario B can be circumvented by jettisoning the Oxford Street model. If instead consciousness were like a dimmer switch, then we could think of it as growing slowly, as the brain did. A foetus would indeed be conscious, but appreciably less so than a child, and certainly a lot less than an adult. There is still the problem of working out when foetal consciousness is sufficiently extensive to require consideration medically and ethically: but the problem is perhaps not black and white, but rather increasing shades of grey.

! Susan Greenfield is a neuroscientist at the University of Oxford, and Gresham Professor of Physics, London.