Changes in the EEG as the subject learns to recall.

The EEG was monitored from posterior leads located above the left and right hemispheres, while the subject learned to recall visually presented 9-digit strings. Processing of material to be recalled led to systematic and progressive activation of the left hemisphere together with an increasing ratio in the EEG activity of the two hemispheres. As performance improved with practice, the initial activation (during the first six digits) was followed by deactivation just prior to recall. This is interpreted as reflecting a shift from active processing (rehearsal) to passive processing (echoic buffer storage). Subjects who recalled well more activated in the left hemisphere than in the right and the level of EEG activity in the left hemisphere during early trials predicted overall recall performance throughout the task. Activity in the right hemisphere was unresponsive to the presentation of material and unrelated to performance. This study shows therefore that: (i) rehearsal for short term recall is a function of left hemisphere activity, (ii) individual differences in left hemisphere activity predict performance, and finally (iii) dynamic changes in brain organisation accompany an overall improvement in performance.