This comparative neuroanatomy study examined how direct cortical input to the hippocampal region changed across six mammalian species — tenrec, rat, cat, marmoset, macaque, and human — spanning >100 million years of evolution, using tract-tracing data for non-human animals and fMRI connectivity as a proxy for humans.
Direct unimodal (sensory) cortical input to the hippocampal region was selectively eliminated as brain size increased (Pearson r = −0.94, p < 0.01 for total cortical input vs. cortical surface area), while transmodal input was relatively preserved (repeated measures ANCOVA, F(1,4) = 22.84, p < 0.01). Primary sensory inputs were lost before non-primary sensory inputs; in humans, no direct non-primary sensory connections remain, except possibly piriform cortex.
- Only six species were studied (the only ones with known distributed hippocampal connectivity), limiting broad cross-taxa generalization. - Human connectivity relied on fMRI, an indirect proxy; absence of functional connectivity cannot confirm absence of anatomical connections. - Defining unimodal vs. transmodal areas is inherently imprecise given evidence for multisensory properties in early sensory cortices.
This study is basic/translational science with no direct clinical action steps, but it offers context for interpreting cross-species animal models of hippocampus-dependent memory: rodent hippocampi receive rich direct sensory input that human hippocampi do not, so findings from rodent memory tasks may not translate straightforwardly to human episodic or abstract memory.
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