Size isn't everything ...it's all in the mind

Published on Saturday 5 August 2006 00:24

IT is arguably nature's most complex invention, capable of everything from formulating Aristotelian logic and solving sudoku puzzles to breathing and blinking. The human brain has emerged from tens of thousands of years of evolution to reign supreme as the most intelligent device in the known universe, still leaving all man-made computers in its wake.

Early scientists had something of an obsession with size, and indeed brains have grown larger throughout evolutionary history.

But increasingly there has been a realisation that size isn't everything and the way the brain is "wired" is as important as a large brain to support intelligent thinking. As science advances, more and more is being learned about the nature of our minds, but given the brain's astounding complexity, not to mention numerous interactions with its surroundings, can we ever hope to comprehend how the different parts of the brain and its components work?

"If the human brain were so simple that we could understand it, we would be so simple that we couldn't", was the judgment of author Emerson Pugh. He may well have a point, but that has not stopped people trying, and progress is being made in understanding this extraordinary organ.

And much of it suggests that brain development depends crucially on external influences from even before birth.

Pregnant women who drink alcohol, smoke or catch rubella may give birth to children with learning difficulties. Most expectant mothers are aware that smoking or drinking is not the best thing for baby, but the environment continues to have a significant impact on the brain in the early years of childhood.

For instance, ignoring or failing to stimulate infants can even prevent the development of certain brain areas. In contrast, providing youngsters with a stimulating environment with plenty of social interactions has been shown to boost intelligence.

Scientists are beginning to unravel the reasons behind all this. The heavily wrinkled cerebrum, or cortex, is the largest part of the human brain and is responsible for higher brain functions, including emotions, memory, reasoning, speech and movement.

People are born with more than 100 billion brain cells or "neurons", each of which forms thousands of contacts with other neurons via long spindly "wires" called axons that grow to connect with their appropriate partners in a structure similar to a large plate of tangled spaghetti.

In the developing embryo, there is an over-proliferation of connections, which are then pruned and refined after birth depending on how heavily a circuit is used, conforming to the "use it or lose it" idea.

This explains why London taxi drivers have a larger hippocampus, which is a region of the brain that deals with spatial memory, since they form lots of new connections to deal with the enormous number of routes they have to recall.

Most neuroscientists describe the brain as being like a computer with a set of components that take information in, process it and produce actions, but this is merely a simple way to describe something they don't really understand.

"Modern brain research is really a bit like physics or astronomy in the 16th century," comments Colin Blakemore, the chief executive of the Medical Research Council. "At the moment we are overwhelmed by data ... without a clear theory to hold it all together.

"The problem is that we don't know what the essential computing elements are. Are they the individual connections on to nerve cells - more than 10,000 per cell - are they nerve impulses or sets of nerve impulses in a coded form, or groups of nerve cells working together?"

Dr Thomas Pratt, of the genes and development group at Edinburgh University, is one of those trying to make sense of this "spaghetti".

He says: "This type of research is important since many brain disorders caused by accident, illness or genetic disorders involve damage to axon tracts .

"The infant brain develops according to a strict timetable, and problems early on, for example being cross-eyed, can have disastrous effects on later vision. By understanding how the brain develops normally we can better understand what has gone wrong and, although this may be some way ahead, put it right."

How these wires and cells interact can be visualised to a certain extent using MRI scans. Images can be obtained that show which parts of the brain are active during specific tasks, from visual perception to recalling memories.

Using this approach, coupled with animal studies, a key discovery has been that there are short time-windows called "critical periods" in brain development during which key senses, such as vision, are established.

For example, if a child is deprived of light during the critical period for sight, this can result in impaired vision.

These critical periods are also responsible for the fact that between the ages of three and ten, the brain is most adept at learning new skills such as music and language as the neural connections can be remodelled more easily than in later life.

A research team at Edinburgh headed by Dr Peter Kind is investigating this type of brain-environment interaction, which is known as "cortical plasticity".

This deals with how brain connections are altered in response to environmental experience, a key process in learning and memory.

Dr Kind says scientists are making good progress towards improving learning for individuals suffering from mental disabilities such as Fragile X Syndrome, a family of heritable diseases, which includes autism.

It might be possible to treat some of these conditions with some effect, if this is done early enough in the brain's development while the cortex is still plastic and able to rewire itself.

Scientists are also attempting to induce local plasticity in a specific region of the cortex to repair spinal-cord injuries and other forms of nerve damage.

However, Dr Kind warns against any dreams of some future ability to keep the brain plastic beyond its critical period to help adults to learn new languages or become the next Mozart.

"If your brain was permanently plastic, you would have to relearn everything from scratch every morning, as you would constantly forget information that you had learned in the past," Dr Kind says.

But the size of the task in trying to solve Emerson Pugh's conundrum has driven some scientists to seek understanding of the human brain from a rather unlikely source: the humble nematode worm. Despite having just 120 brain cells, it has unbelievable complexity in the workings of its mind.

"If we can't understand these [simple organisms], what chance have we got with understanding the workings of 100 billion nerve cells in the human brain?" says Prof Blakemore.