A collaborative study conducted by scientists from five different countries determined that change in one’s IQ is correlated with the thickness of their cortex. The researchers noted that large changes in IQ are often observed at different periods in the same person. While small changes are common, large changes are less common, and widely associated with errors in measurement. However, one of the researchers on the team argues that IQ can profoundly increase or decrease and that there is a real neurobiological correlate for this change, rather than simply a change in measurement. That objective correlate is the thickness of the brain’s cortex.
But what is the cortex? “The cortex is the thin, outermost layer of nerve cell tissue of the brain, typically measuring a few millimeters in thickness. The cortex contains nerve cell bodies and is critical for cognitive functions such as perception, language, memory and consciousness” (McGill University, 2014). The study went as follows:
The cortex begins to thin after the age of five or six as part of the normal aging process. This study by Professor Karama and his colleagues involved 188 children and adolescents over a period of two years. MRIs of the study participants were taken at six sites across the US. This study is the first to show the association between cortical thickness and development in full scale IQ. They found that within a relatively short period of 2 years:
people with a significant increase in IQ did not have the expected cortical thinning,
people whose IQ stayed the same had the normal expected cortical thinning,
people with a significant decrease in IQ had exaggerated cortical thinning (McGill University, 2014)
Another important study determined a genetic predisposition to cortical thickness:
The researchers looked at over 54,000 genetic variants possibly involved in brain development. They found that, on average, teenagers carrying a particular gene variant had a thinner cortex in the left cerebral hemisphere, particularly in the frontal and temporal lobes, and performed less well on tests for intellectual ability. The genetic variation affects the expression of the NPTN gene, which encodes a protein acting at neuronal synapses and therefore affects how brain cells communicate.
To confirm their findings, the researchers studied the NPTN gene in mouse and human brain cells. The researchers found that the NPTN gene had a different activity in the left and right hemispheres of the brain, which may cause the left hemisphere to be more sensitive to the effects of NPTN mutations. Their findings suggest that some differences in intellectual abilities can result from the decreased function of the NPTN gene in particular regions of the left brain hemisphere.
The genetic variation identified in this study only accounts for an estimated 0.5% of the total variation in intelligence. However, the findings may have important implications for the understanding of biological mechanisms underlying several psychiatric disorders, such as schizophrenia, autism, where impaired cognitive ability is a key feature of the disorder (King’s College London, 2014)
McGill University. (2014, March 4). New evidence confirms link between IQ, brain cortex. ScienceDaily. Retrieved June 10, 2014 from www.sciencedaily.com/releases/2014/03/140304141734.htm
King’s College London. (2014, February 11). Scientists identify gene linking brain structure to intelligence. ScienceDaily. Retrieved June 11, 2014 from www.sciencedaily.com/releases/2014/02/140211084051.htm