Gray matter

Brain tissue is divided into two types: gray matter and white matter. These names derive very simply from their appearance to the naked eye. Gray matter is made up of the cell bodies of nerve cells. White matter is made up of the long filaments that extend from the cell bodies - the "telephone wires" of the neuronal network, transmitting the electrical signals that carry the messages between neurons.

The volume of gray matter tissue - a measure you will see cited in various reports - is a measure of the density of brain cells in a particular region.

Recently, the most comprehensive structural brain-scan study of intelligence to date has supported an association between general intelligence and the volume of gray matter tissue in specific regions of the brain (you can see a picture of these areas here). These structures are the same ones implicated in memory, attention and language.

Previous research has shown the regional distribution of gray matter in humans is highly heritable. But it also clearly has a strong environmental influence. Recent studies have found:

• an increased volume of gray matter in Broca's area of professional musicians, apparently reflecting, at least in part, the number of years devoted to musical training
• an increased volume of gray matter in the posterior hippocampus of experienced London taxi drivers (a brain region involved in spatial navigation), with volume correlated with length of taxi-driving experience
• an increase in the development of new brain cells in older adults who underwent an aerobic training program compared with those who did not

The brain-scan study also found age differences: in middle age, more of the frontal and parietal lobes were related to IQ; less frontal and more temporal areas were related to IQ in the younger adults.

Age differences have already been found to exist in gray matter volume and distribution.

Mapping of the progressive maturation of the human brain in childhood and adolescence has found an initial overproduction of synapses in the gray matter after birth, which is followed, for the most part just before puberty, with their systematic pruning. This process occurs in different regions at different times, with gray matter loss beginning first in the motor and sensory parts of the brain, and then slowly spreading downwards and forwards, to areas involved in spatial orientation, speech and language development, and attention (upper and lower parietal lobes), then to the areas involved in executive functioning, attention or motor coordination (frontal lobes), and finally to the areas that integrate these functions (temporal lobe). The sequence appears to agree with regionally relevant milestones in cognitive development.

Various learning and memory problems have been associated with decreased gray matter in particular regions of the brain:

• children with selective problems in short term phonological memory and others diagnosed with specific language impairment had less gray matter in both sides of the cerebellum compared to controls
• adolescents had less gray matter in an area in the left parietal lobe if they had a deficit in calculation ability, compared to those who had no such deficit

Gray matter is not the sole arbiter of ability and knowledge, of course. The number of neurons is clearly important, but so is the connectivity of the neuronal network. Interestingly, although gray matter declines steadily from adolescence, white matter keeps growing until our late forties. This is consistent with a large-scale study of mental abilities in adults, that found that mental faculties were unchanged until the mid-40s, when a marked decline began and continued at a constant rate. Accuracy did not seem to be affected, only speed. White matter governs the speed with which signals travel in the brain.