Genes —> Myelin —> Intelligence
The volumes of grey matter and white matter contribute to intelligence, and both are heritable. A form of MRI (called HARDI) can be used to measure the amount of water diffusing through white matter, and thus assess the integrity of myelination of neurons. Remember that myelination contributes heavily to neural conduction speed, and speed of conduction is correlated with intelligence. Using HARDI with fraternal and identical twins, a group of U.S. and Australian neuroscientists has determined the heritability of myelin integrity in different parts of the brain. The heritability varies to a surprising degree from one area to another, for example 45% in the temporal lobes and 100% in the corpus callosum. Journal of Neuroscience, Vol 29, 2212-2224. You can see a fascinating graphic of connections in the brain that also displays the heritability of white matter integrity in various areas at New Scientist.
Why the Elderly Have That "In a Fog" Feeling
The text emphasizes the importance of integrated activity among distant parts of the brain, and there is new evidence this is an area showing decline as a person ages and loses cognitive ability. Researchers at Harvard used fMRI to monitor activity in structures that are active when the person is not engaged in a task. In young adults activity tended to occur simultaneously in two areas, sometimes as distant as the anterior and posterior parts of the brain; activity was poorly coordinated in the brains of the elderly volunteers. In addition, this poor coordination was associated with deterioration in white matter pathways and with reduced performance on cognitive tests. Nine of the subjects screened earlier and shown to be free of amyloid plaques had severe reductions in correlated activity, indicating that the deterioration could not be attributed to pre-Alzheimer's pathology. Neuron, Vol 56, 924-935.
Intelligent Brains Have Thicker Cortex
Several brain correlates of intelligence have been identified, and improvements in MRI techniques have have enabled neuroscientists at McGill University to add yet another. In a study of 6- to 18-year olds, scores on a test of general intelligence were correlated with cortical thickness. The correlation was found in all four lobes of the brain; the relationship was strongest, as might be expected, in association areas. The authors point out that the result is consistent with a distributed model of intelligence. Intelligence, Vol 37, 145-155.
Talk about animal intelligence . . . !
First we learned that we are not the only animals that use tools, then we observed chimps and some birds actually making the tools they need to obtain food. Then three years ago researchers showed that bonobos and orangutans could learn to choose and save the right tool to get foot out of an apparatus as much as 14 hours in advance (Science, Vol 312, 1038-1040). Now enter Santino, a chimp in the Swedish Furuvik Zoo. When zoo visitors start arriving he stomps around the compound, his hair standing on end, and he hurls stones at the humans. But what is impressive about Santino is that each morning before the zoo opens he calmly goes about collecting stones and arranging them in neat piles. In other words, Santino anticipates a future emotional state. Thus the gap between us and the other members of the animal kingdom has narrowed once again. Current Biology, Vol 19, R190-R191.
The Complexity of Autism
Recent research is increasing our appreciation of just how complex autism is. Many genes contribute to widespread developmental anomalies, which in turn lead to the varied symptoms of autism spectrum disorders. In addition, researchers believe that more than 10% of autism cases can be explained by copy number variations—duplication or, more especially, deletion of a stretch of chromosomal material. In some cases these mutations are heritable, but more often they are unseen in the parents; in fact, in one study most of the mutations were unique to the individual. Obviously there are numerous pathways to autism, and understanding and effective treatment are a long way off. Cell, Vol 135, 391-395.
Is the Increase in Autism Environmental?
According to your text (p 412) it is unclear whether the reported increase in autism is real; for example, there is some evidence the increase is due to more liberal diagnosis. A new study attempting to explain the 600-700% increase in California has arrived at a different conclusion. Using data over a 17-year period, researchers at UC Davis ruled out the migration of people into the state as a factor. Earlier diagnosis accounted for a 24% increase, diagnosis of milder cases another 56%, and changes in state reporting an additional 120%. That leaves more than half of the increase unaccounted for. The researchers believe there has been too much emphasis on the genetic causes of autism and too little on environmental causes, with research on the former receiving 10-20 times the funding. Epidemiology, Vol 20, 84-90.
For Further Reading: Born on a Blue Day
Born on a Blue Day is a high-functioning autistic's fascinating description of how his life differs from everybody else's. Daniel Tammet has Asperger's syndrome, but he performs feats like memorizing the first 22,000 digits in the number Pi. Tammet is also a synesthete, and experiences numbers and letters as having shapes, color, and sounds. The title comes from the fact that he was born on a Wednesday; he sees Wednesdays as blue. Free Press, 2006. See excerpts here.
