1996/09/12 @ British Association Science Fair

Ripples in your mind

Oxford neuroscientist Susan Greenfield has a picture of how everything that we experience happens in our brains. It's all to do with groups of nerve cells -- neurons -- working together.

Simplifying wildly, it's like this. You look out of the window. Neurons in the back of your eye are activated by light. The activation spreads, starting in the optic nerve, to many parts of your brain. In one of these parts, a small group of neurons get active. The activity spreads from there out to other neurons, in much the same way that ripples spread across a pool when you drop a stone in it, or vibrations spread from an earthquake. Greenfield calls the place in the brain where the ripples start an 'epicentre', as in the 'epicentre' being the apparent centre of an earthquake.

This first epicentre is, it turns out, activated every time you look at something that's gently waving. A little later, another epicentre is activated: one that goes with things that are taller than they are wide. And another, for things that are green; and one for things that are sort-of-crinkly.

The waves spread from all of these and 'converge' on... a nearby epicentre that is activated when you see a tree! (There's always a complication: here, 'nearby' means 'closely connected', not necessarily next door in physical space.)

More 'ripples' spread from the 'tree' epicentre. They reach, and may activate, epicentres for the word 'tree'; for a certain sense of calm; or even for memories of the trees in the park near your Granny's house, and for Granny herself.

This is controversial. 'Either I'm very brave or I'm very stupid or both,' Greenfield opened her talk to the Minds, Brains and Consciousness session on Thursday, 'but I'm going to explore the transition between neuroscience and the philosophy of mind.'

Greenfield starts from two assumptions:

1: Subjectivity 'I don't have any idea what your headache is like and you'll never know about mine -- it's mine.'

2: Physicality 'Mind has not been beamed in by some cosmic ray from outer space.' In other words, it's all done by things happening in the brain.

Neither of these seems controversial. But, 'when we put these two together you run into problems,' she says: 'We [scientists] are brought up to be objective, to make measurements that other people can repeat, and that doesn't marry at all with the subjective.'

'There is no such thing as a single centre for consciousness in the mind. It seems to be a case of parallel processing -- multiple processes working together like players in a [co-operative, conductor-less] orchestra.'

'People tend to describe consciousness as an all-or-nothing thing,' she continues, 'like the lights going on in Oxford Street. What if you could have greater or lesser amounts of consciousness?'

Consciousness is, she proposes:

What's an epicentre? Studies show it's not a single neuron, nor a localised area of the brain.

In two groups of rats, equally healthy and well-fed but one brought up in a rich environment and one impoverished of stimuli, the stimulated group had many more inter-neuron connections.

So: experience creates more physical connections in the brain. And a modest increase in the density of the network of connections means an enormous increase in the number of ways those connections can be activated -- and thus in the number of epicentres which can form in that network.

Furthermore, the number of connections between neurons increases with age.

Both of these pieces of research provide direct evidence that learning is associated with an increase in the connection density of the brain -- and thus, less directly, support Greenfield's picture.

How do 'epicentres' form and spread in the physical network?

Recent studies in Israel show directly, by dyeing the cells which are actively connected to a central neuron, that in some cases the spread looks exactly like ripples spreading in a pool. (Unfortunately, the picture wouldn't reproduce well on the Web.)

'Recruitment' to epicentres -- the 'power of the ripples' -- depends on generalised levels of arousal in the brain. Greenfield suggests that it is very low when you're dreaming. Events in the brain will then not be highly correlated -- another way of saying (very loosely) that we have random thoughts.

Newborn humans, interestingly, spend almost all their time dreaming -- in the jargon, in Rapid Eye Movement or R.E.M. sleep.

Time did not allow Greenfield to elaborate the whole of her theories about how epicentres and groups of epicentres may represent perceptions, and be the seat of thoughts and emotions.

But it seems, for example, that very large and stable epicentres are connected with depression. Indeed, Greenfield has proposed an intriguing classification of different mental states according to connectivity in the brain, the 'strength' of epicentres, level of mental arousal, turnover or fluidity of connections, and the size of neuronal assemblies.

Much more research remains, as they say, to be done in this fascinating field.