Short-term memory and the EEG: effects of instructions to rehearse

Transoccipital EEG was analyzed during the performance of a task involving the presentation and recall of auditorily presented nine-digit strings. Three strategies of processing were imposed by instruction: (i) passive listening without recall; (ii) passive listening followed by attempted recall; (iii) active rehersal followed by attempted recall. Phasic EEG arousal (8.5-12.5 Hz), inferred from the decline in abundance during string presentation, was found to be a function of the level of 'engagement' in the task. This finding was viewed as supporting an earlier phenomenological description of the task demands involved in recall. Tonic levels of arousal were found to be frequency dependent. At low frequencies (8.5-10.5 Hz) EEG abundance was inversely related to engagement but at higher frequencies (10.5-12.5 Hz) abundance was directly related to the level of engagement. The results are discussed in terms of a possible functional dissociation of EEG arousal within the alpha frequency band.