Imaging and neural modelling in episodic and working memory processes

Neuroimaging studies using positron emission tomography (PET) and functiona l magnetic resonance imaging (fMRI) have revealed the involvement of distri buted brain regions in memory processes mainly by the use of subtraction st rategy based data analyses. Covariance analysis based data analysis strateg ies have been introduced more recently which allow functional interactions between brain regions of a neuronal network to be assessed. This contribution focuses on studies aiming to (1) establish the functional topography of episodic and working memory processes in young and old norma l volunteers. (2) to assess functional interactions between modules of netw orks of brain regions by means of covariance based analyses and systems lev el modelling, (3) to characterise the temporal dynamics by the use of magne toencephalography (MEG) and (4) to relate neuroimaging data to the underpin ning neural networks. Male normal young and old volunteers without neurological or psychiatric il lness participated in neuroimaging studies (PET, fMRI, MEG). Studies were a pproved by the ethical committee and federal authorities. Our results in young volunteers show distributed brain areas that are invol ved in memory processes (episodic and working memory) and show much of an o verlap with respect to the network components. Systems level modelling analyses support the hypothesis of bihemispheric, a symmetric networks subserving memory processes and revealed both similariti es in general and differences in the interactions between brain regions dur ing episodic encoding and retrieval as well as working memory. Changes in memory function with ageing are evident from functional topograp hic studies in old volunteers activating more brain regions as compared to young volunteers. There are more and stronger influences of prefrontal regi ons in elderly volunteers comparing the functional models between old and y oung subjects. We discuss the way that the systems level models of the PET and fMRI result s have implications for the underlying neural network functioning of the br ain. This is done by developing simplifying assumptions, which lead from th e equations describing the activities of the coupled neural modules to the systems level model equations. The resulting implications for the neural in teractions are then discussed, in terms of a set of synaptically coupled ne ural modules. Finally, we consider how a similar analysis could be extended from the spat ial to the temporal domain thus including the EEG and MEG results. The impl ication of preliminary MEG results presented here for the temporality arisi ng in the interaction between the coupled neural modules in a working memor y paradigm is discussed in terms of the previously developed neural network models arising from the PET and fMRI data. (C) 2000 Elsevier Science Ltd. All rights reserved.