Memory consists of interdependent but dissociable processes, such as encoding, storage, consolidation and retrieval. Lesion studies have shown that the hippocampus is necessary for declarative memory, but pinning down its exact contribution to these individual stages has proved tricky. Because brain lesions are permanent, they cannot unambiguously dissociate the stages of memory.

Richard Morris and his colleagues (University of Edinburgh Medical School) have now developed a technique that could overcome this problem. On page 898, they report reversible, temporary inactivation of the hippocampus with LY326325, a selective water soluble antagonist of AMPA/kainate glutamate receptors. The authors infused the dorsal hippocampus (inclusive of areas CA1-CA3 and the dentate gyrus) of rats with this drug during training and/or retention on a version of the water maze test. The authors verified physiologically that they could selectively 'switch off' the dorsal hippocampus for varying periods and then switch it on again and have it work normally afterward. This technique revealed that the hippocampus is critical for both encoding and retrieval of spatial memory. This approach represents a potentially powerful way to test whether the functional integrity of various brain areas is necessary for specific memory processes.