EEG power changes during a multiple level memory retention task.

EEG changes related to the amount of information held in memory during a 4-s retention period were studied. The predictability of the amount of information held in memory was varied. In the weighted condition, 60% of the trials contained only one item and the remaining 40% of the trials were evenly distributed between trials containing 3, 5, 7, or 8 items. In the random condition, the levels were equally represented and randomly presented. In the blocked condition the levels were equally represented but presented in five blocks containing only items from one of the levels. Initial widespread decreases in alpha band power were followed by increased activity in all three conditions. The more difficult of the five levels produced decreased alpha activity in more localized posterior left hemisphere sites. This suggests two alpha mechanisms, one associated with task engagement and the other related to the cognitive demands regardless of the presentation context. Theta band power increased over frontal scalp, and to a lesser extent over left parietal and temporal areas and bilateral occipital sites, during only the weighted condition. These changes were uniform over the entire retention period. Beta 2 activity was also influenced by the task difficulty and the time course of the retention period in the two conditions. Beta 2 activity resembled both alpha and theta in that in levels 1, 2 and 3 it acted like alpha with increasing power over time at numerous widespread sites while the higher difficulty levels showed higher power at the beginning of the retention period and then decreased.