The memory has separate pathways for various different types of information. This has been demonstrated by means of anatomical and electrical measurements of the hippocampus and the adjacent cerebral cortex of laboratory rats. The signal pathways ultimately meet in the so-called subiculum of the brain, from which information from the hippocampus is passed on to other areas of the brain, such as the cerebral cortex. The research project which produced these findings was financed by the NWO's Council for Medical and Health Research and involved researchers from the Vrije Universiteit Amsterdam and Amsterdam University.
The researchers studied two signals from different sensory parts of the brain, one of which arrived at the perirhinal and the other at the postrhinal cerebral cortex. These parts of the brain are located close to the sulcus and receive information from areas of the brain which process different types of sensory information. The information enters the hippocampal memory system, which plays an important role in learning and memory processes, via the perirhinal and postrhinal cortex. The signal pathways would appear to remain separate throughout the whole memory system and only to meet in the subiculum.
The researchers also found that signal pathways are used differently in the case of stimuli in the visual area than in the case of stimuli in the area of the brain where information from the whiskers enters. This suggests that the pathways are used differently for different types of information. Rats primarily use their whiskers to recognise objects and their eyes for orientation.
The way the information pathways are organised shows that the subiculum is a junction for various signals resulting from the same stimulus. These signals follow different pathways, meaning that "associations" can be made between various different signals so as to build up an overall perception of a bird, for example. Perceiving a bird involves observation of its position and proximity, its speed, and its colour. This information meets in the subiculum. It is probably the subiculum which determines where and how memory traces are formed.
In order to determine whether the organisation of the hippocampal area of the brain is similar in rats and in humans, further research will be carried out on human test subjects to establish what different types of information correspond to the various pathways. This is of medical relevance because Alzheimer's disease and other learning and memory disorders associated with ageing are linked to disorders in the hippocampal memory system.
Further information:
Riet Naber (Vrije Universiteit Amsterdam)
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