Does recalling an older spatial memory activate less of the cortex?

Details

The neocortex plays a critical role in the gradual formation and storage of remote declarative memories. But the circuitry mechanisms of systems-level consolidation are poorly understood, and the precise cortical storage sites and nature of enduring memory correlates, or mnemonic plasticity, remain largely unknown. Detailed maps of neuronal activity during recent and remote recall can highlight regions involved in consolidation and potential storage sites, potentially helping researchers detect mnemonic plasticity.

To locate cortical regions involved in recalling a spatial memory task, researchers trained rats in a water maze. They mapped mRNA expression of the neuronal activity marker Arc/Arg3.1 (Arc) during recall of recent memories, 24 hours after training, or remote memories, one month after training, and compared the patterns with swimming and naive controls.

Arc gene expression was significantly stronger at 24 hours than at one month. Expression diminished in parietal, cingulate and visual areas, but select segments of the prefrontal, retrosplenial, somatosensory and motor cortex showed similarly robust increases in Arc expression. Across select segments of sensory, motor and associative regions within the recent and remote memory groups, the overall magnitude and cortical layer patterns of task-specific Arc expression were similar, or stereotypical. The fractions of Arc mRNA expressed in upper cortical layers 2/3 and 4 increased after both recent and remote recall, while the fraction in layer 6 decreased only after recent recall.

The data suggest that robust recall of remote memory requires a smaller overall increase in neuronal activity across fewer cortical segments. This activity trend highlights the difficulty of detecting storage sites and plasticity underlying remote memory. Applying Arc maps may help overcome that difficulty.