EEG alpha and theta oscillations reflect cognitive and memory performance:a review and analysis

Evidence is presented that EEG oscillations in the alpha and theta band ref lect cognitive and memory performance in particular. Good performance is re lated to two types of EEG phenomena (i) a tonic increase in alpha but a dec rease in theta power, and (ii) a large phasic (event-related) decrease in a lpha but increase in theta, depending on the type of memory demands. Becaus e alpha frequency shows large interindividual differences which are related to age and memory performance, this double dissociation between alpha vs. theta and tonic vs. phasic changes can be observed only if fixed frequency bands are abandoned. It is suggested to adjust the frequency windows of alp ha and theta for each subject by using individual alpha frequency as an anc hor point. Based on this procedure, a consistent interpretation of a variet y of findings is made possible. As an example, in a similar way as brain vo lume does, upper alpha power increases (but theta power decreases) from ear ly childhood to adulthood, whereas the opposite holds true for the late par t of the lifespan. Alpha power is lowered and theta power enhanced in subje cts with a variety of different neurological disorders. Furthermore, after sustained wakefulness and during the transition from waking to sleeping whe n the ability to respond to external stimuli ceases, upper alpha power decr eases, whereas theta increases. Event-related changes indicate that the ext ent of upper alpha desynchronization is positively correlated with (semanti c) long-term memory performance, whereas theta synchronization is positivel y correlated with the ability to encode new information. The reviewed findi ngs are interpreted on the basis of brain oscillations. It is suggested tha t the encoding of new information is reflected by theta oscillations in hip pocampo-cortical feedback loops, whereas search and retrieval processes in (semantic) long-term memory are reflected by upper alpha oscillations in th alamo-cortical feedback loops. (C) 1999 Elsevier Science B.V. All rights re served.