Generally, the early studies have reported that a low intelligence quotient (IQ) was related to slow alpha frequencies and the existence of delta and theta rhythms, while higher levels of intelligence was related to faster alpha frequencies and a lack of delta and theta rhythms (Vogel & Broverman, 1964), but later studies indicate that the relationship is more complex. If the EEG of the mature brain of young healthy adults is compared either with the developing brain, the aging brain or the brain which is affected by neurological diseases of various kinds, the conclusion is that:
• alpha frequency is positively related to cognitive performance, and
• large power in the range of the upper alpha band but small power in the theta frequency range indicate good cognitive peformance.
These conclusions are based on findings which show that:
• alpha frequency increases from early childhood to adulthood but then decrease with increasing age or age related neurological diseases
• alpha frequency is lowered in demented subjects (as well as in patients with other types of neurological disorders),
• alpha frequency is significantly higher in subjects with good memory performance as compared to age matched controls with bad memory performance
• alpha frequency is positively correlated with speed of processing information
• theta power decreases and alpha power increases from early childhood to adulthood,
• theta power increases and upper alpha power decreases during the late part of the lifespan
• theta power is enhanced and alpha power lowered in subjects with a variety of different neurological disorders as compared to age matched controls.
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