Neuroscience of Intelligence

Studies to Locate Intelligence

Considerable research efforts have sought to identify whether single brain areas are related to intelligence. It has long been observed that significant brain damage to humans often does not result in a dramatic lowering of IQ scores. Even “psychosurgery” to sever the connections between the frontal lobes and the rest of the brain practiced in earlier decades (rarely used today) to treat schizophrenia and other mental conditions, produced little impairment in tests of general intelligence. Retrospective studies of humans after brain injury do not provide definitive maps of “intelligence areas,” although specific areas for language and other cognitive abilities have been identified, and there is evidence that a network of these brain areas also underlies intelligence. Similarly, early lesion experiments in laboratory rats found that the severity of impaired performance during learning experiments was more related to the size rather than to the location of a brain injury, and more recent rat studies show discrete brain networks throughout the brain are related to general problem solving and that different networks are related to specific problems. The existence of a general cognitive factor underlying diverse problem solving in mice also is now well established. Together, both clinical lesion studies in humans and experimental lesion studies in animals indicate that intelligence may be represented throughout the brain rather than reside in a single specific center.

Brain Waves and Intelligence

The brain is constantly active as billions of neurons create and react to chemical and electrical interactions. One noninvasive technique to measure the electrical activity produced as neurons fire on and off is the electroencephalogram (EEG). Because the brain is always engaged in many simultaneous activities, all of which contribute to the overall EEG, spontaneous EEG is a noisy mixture. It is no surprise that attempts to correlate spontaneous EEG to measures of intelligence have been disappointing overall. However, there are a number of EEG techniques that separate specific brain responses to specific stimuli from the noise of the totality of all the brain's activity at any one moment. The most widely used technique is based simply on repeating the same stimulus many times and averaging a half-second block of the spontaneous EEG that occurs just after each stimulus presentation. With averaging, only the specific EEG response to the stimulus will be left because it is the same each time. This technique is called the average evoked potential (AEP), also referred to as the event-related potential (ERP). In general, modest correlations have been reported between some AEP parameters and intelligence measures. A number of detailed reviews of this literature are available.

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