Read more: “Instant Expert 31: The human brain“
Almost 2500 years since Hippocrates first saw the brain as the centre of our thoughts, we can now interrogate its inner world with some of the most advanced technology known to humanity. The ultimate aim is to work out how the brain generates our conscious mind. Such understanding is still a long way off, but we are beginning to get to grips with many previously intractable problems.
Painting the big picture
With brain-imaging techniques becoming ever more sophisticated, some neuroscientists want to draw a map of the brain’s connections. The Human Connectome Project aims to detail all the long axonal connections in the wiring of the healthy human brain, with the help of 1200 volunteers. By comparing the connections in sets of identical twins with those in fraternal twins, we should be able to reveal the relative contributions of genes and environment to shaping brain circuitry. The resultant “connectome” could help research into conditions such as autism and schizophrenia, the symptoms of which may be down to differences in brain wiring.
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A second, equally ambitious project will trace gene expression in both the developing and adult human brain. The importance of gene expression in the brain is hard to over emphasise. Differences between neurons are determined by differences in patterns of gene expression, and as the properties of a neuron change during development, ageing, memory formation or in disease, gene expression also changes. For this reason, the Human Brain Transcriptome project will be central to our understanding of the finer details of the brain’s workings.
Decoding the senses
How do neurons encode an idea of something so that we can immediately recognise a familiar face, our house or a favourite book? Most neuroscientists believe that the brain stores our concept of an object over many neurons, with all these cells having to work together for you to recognise something. According to this theory, the activity of any one neuron is not representative of a particular object – it could respond to similar features in other objects. Instead, it is the behaviour of the group that determines what meaning comes to mind.
But some neuroscientists claim that we may encode concepts using smaller, more selective, networks of neurons. According to this view, a neuron may sometimes specialise in a single idea. For instance, in one study volunteers were shown pictures of movie stars and famous buildings while the researchers recorded the resulting activity from a selection of single neurons. The results were surprising, showing, for example, that one of the neurons studied responded to many different pictures of the actor Jennifer Aniston ().
In some cases it wasn’t just pictures that triggered the neuron’s activity; some also responded to a word representing the object or person. It’s almost as if the neuron being studied somehow encoded the essence of the person or object, which may explain why we can recognise things from different perspectives or in unfamiliar surroundings.
Reflecting on each other
Some neuroscientists believe that the discovery of “mirror neurons” will transform our understanding of the human mind and brain just as DNA transformed evolutionary biology. They could potentially help to de-mystify the most human of our qualities, such as empathy.
So what are mirror neurons? The defining characteristic is that they fire both when we perform an action such as reaching for a coffee cup, and when we see someone else doing the same. This suggests that they embody an understanding of the meaning or intentions of the actions of others, and through a similar mechanism allow us to grasp their emotions too.
It has also been suggested that they lie behind language. According to one theory, human language originated with physical gestures – and mirror neurons were instrumental in helping us to translate the meaning of these gestures. Although the idea is controversial, evidence is accumulating. For example, functional MRI studies show that a mirror neuron system lies close to a language centre called Broca’s area.
A conscious computer?
“Consciousness is a fascinating but elusive phenomenon,” wrote the late Stuart Sutherland of the University of Sussex in Brighton, UK. “It is impossible to specify what it is, what it does, or why it evolved. Nothing worth reading has been written on it.”
The problem arises because although consciousness must come from a physical structure, no one has been able to work out how. A potential breakthrough may lie in attempts to create robots with artificial brains capable of conscious thought and understanding.
One approach is to build an accurate, large-scale model or simulation of the neuronal networks of the human cortex in the hope that this will capture the signatures of human cognition. The Semantic Pointer Architecture Unified Network project is a promising example. To date, the SPAUN model consists of 2.5 million artificial neurons and recent reports suggest it can perform tasks of the kind that contribute to human cognition (). For example, it can recognise visual images, perform well in a variety of memory tasks, and has even passed an IQ test.
“The SPAUN brain simulation performs well in memory tasks – it even passed a human IQ test”
By using more advanced models of this kind, it may be possible to test out the prerequisites of consciousness in a way that would not be feasible with a human or animal brain.
This article appeared in print under the headline “The quest for the human mind”
