The meaning of alpha phase synchronization for memory

Our general aim is to apply different measures of brain oscillatory phase synchronization to allow for a more sophisticated analysis of memory processes than is possible with analyses based on amplitude estimates. With respect to the involvement of different frequencies for memory processes it could be demonstrated that theta plays a specific role for working memory that appears quite well understood. But in sharp contrast to theta, less is known about the functional meaning of alpha in memory processes. We have suggested a new theory, the inhibition- timing hypothesis which essentially states that alpha plays an important role for the precise timing during the top- down controlled access to information stored in a complex knowledge system that allows us to be oriented in space, time and with respect to the semantic meaning of objects. The interesting and crucial prediction is that alpha does not reflect any type of top-down processes, but those top-down processes that are related to the access to and/or to the processing of meaningful information. We propose several experiments to investigate this question. Specific aims: (i) We suggest a series of experiments that will allow us to dissociate more general processes of top- down control from those specifically related to the access to and processing of semantic information in different modalities. (ii) Previous research is largely based on event-related task designs. In these tasks, however, evoked activity (due to on- off effects of stimulation) may have a strong and potentially biasing effect on phase synchronization measures. We, suggest using continuous` task designs, which lack evoked activity and at the same time are more related to `real world` information processing. (iii) In event-related tasks, the influence of oscillatory phase and evoked activity on the generation of the event-related potential (ERP) are difficult to dissociate and there are only a few studies on the relationship between memory-related oscillations and ERP components. We introduce a new analysing method to investigate the functional relationship between ERP components and alpha phase synchronisation during the event-related access to memory. The innovative aspect of the proposed research is seen in the possibility to establish a theory of alpha oscillations that will enable us to understand basic principles of human memory which are associated with the use of knowledge, stored in long-term memory.

The general project aim was the application and development of phase sensitive measures to investigate the functional meaning of alpha oscillations. In the 4 years of the project work it was possible to publish 8 articles and to perform several experiments in which the electrical brain activity was recorded with the EEG. This work enabled a more comprehensive understanding of alpha oscillations regarding divergent issues such as inhibition, attention, and access to a specific kind of memory, which is termed knowledge system. The basic interpretation is that alpha provides a temporal structure that underlies controlled knowledge access and semantic orientation, an ability to be consciously oriented in time, space, and context. It was also possible to develop a formal theory that allows to predict the numerical values and frequency bandwidths of all brain and body oscillations. The algorithm underlying this theory also allows one to predict the distribution of white matter connectivity.