Two competing theories of the operating mechanisms of the hippocampus in memory ? the cognitive map theory and the relational theory ? have been the subject of debate for many years. Writing in The Journal of Neuroscience, Kumaran and Maguire describe a functional MRI (fMRI) study during which participants carried out a novel cognitive task designed to resolve this issue, and report evidence in favour of the cognitive map theory.

According to the cognitive map theory, the hippocampus is specifically involved in creating and maintaining spatial maps of the environment. By contrast, the relational theory suggests that the hippocampus more generally processes associations and event sequences that are incorporated into a relational framework, with spatial navigation representing just one form of relational processing.

To test the relative strengths of these two theories, Kumaran and Maguire designed two parallel tasks to highlight brain activation in response to spatial and non-spatial (social) forms of relational processing. Participants were required to determine an optimal route either between friends' homes or between the friends themselves using social connections as navigational markers. These processes are conceptually similar in that, mathematically, they can be represented with nodes and edges, and form an interconnected network through which we can navigate and choose preferable routes.

The two behaviourally matched tasks elicited distinctly different patterns of fMRI activation. Importantly, the hippocampus was active in response to the spatial, but not the non-spatial, relational processing task ? findings that are consistent with the cognitive map theory of hippocampal operations.

This work helps to settle the long-standing debate between two competing theories of hippocampal functions in memory, and indicates that the role of the hippocampus in relational memory is specific to the domain of space. However, the relationship between hippocampal spatial memory operations and other forms of hippocampal-dependent memory, such as episodic memory, remains to be determined.