REHEARSAL PROCESSES IN WORKING MEMORY AND SYNCHRONIZATION OF BRAIN AREAS

In this study we investigated brain activity characteristics of rehearsal processes in working memory using analysis of EEG coherence as a measure of synchronization of brain areas. In a delayed serial recall paradigm we enabled subjects to carry out rehearsal in one condition and disrupted it by irrelevant speech in another condition. Results show that rehearsalspecific changes of coherence duration are located at central electrode sites (C3, Cz, C4) in the Gamma frequency range (35-47 Hz). These changes in synchronization follow our behavioural data that show the classical irrelevant speech effect. Additionally, we found significant coherence changes between frontal and parietal electrodes in the Theta band (47.5 Hz) suggesting that rehearsal components can be dissociated from processes reflecting mental effort for retention. Describing psychological processes by the investigation of quantitative external measures has a long tradition and goes back to the beginnings of psychophysics in the 19 century. During the last few years, however, one can notice an endeavour to find internal characteristics of psychological processes by analyzing brain activity as part of neuropsychological research. This trend results from the great development of imaging techniques on the one hand and on the other hand from the difficulty of describing more complex cognitive processes by traditional behavioural methods such as reaction time or error rate measures. Thus, analyzing brain activity online provides a new and exciting method to investigate temporal and spatial characteristics of so-called higher cognitive processes like memory. In this study we examined a special part of working memory, namely the subvocal rehearsal process. It comprises active maintenance of memory traces and renewal of decaying representations in a short-term store. As one part of the phonological loop in the influential working memory model of Baddeley (1999) rehearsal was proposed on the basis of experiments that examined the disruption of the phonological loop by articulatory suppression or irrelevant speech. In the 1980s and 1990s numerous findings were reported showing which materials disrupt the phonological loop (and hence rehearsal processes) under which conditions. One outcome which is relevant for our experiment showed that presentation of irrelevant speech considerably interrupted phonological rehearsal whereas presentation of unstructured noise did not (Salamé & Baddeley, 1987). Neuropsychological studies using fMRI or PET gave evidence for the activation of frontal and parietal brain areas in working memory tasks (e.g. Smith & Jonides, 1997). Other activation studies proved the participation of several brain areas in rehearsal: especially premotor and prefrontal regions (consistent with the hypothesis that during rehearsal, motor programs are executed like in a recall situation but without overt articulation) (e.g., Fujimaki et al., 1999). However, these structures have to cooperate functionally in order to carry out psychological processes. One special mechanism of this interaction has recently been discussed above all, that is the coupling by synchronization of electrical activity. In our experiment we investigated the synchronization of rehearsal-related brain areas using EEG coherence as an indicator. Coherence gives evidence of the degree of interrelatedness with respect to frequency (this can vary between 0 and 1). As for our experiment, we hypothesized that during rehearsal the duration of high coherence should be long in the prefrontal and premotor regions, and between those two regions. Additionally, disruption of rehearsal is predicted to lead to smaller coherence durations in these areas. For the behavioural data we predicted the classical irrelevant speech effect.