Most imaging studies of the hippocampus have focussed on either the morphology of the hippocampal formation or on intra-hippocampal microcircuitry. The hippocampus is implicated in a range of diseases and normal functions, however a detailed understanding of the structural connections of the hippocampus is currently lacking. The hippocampus is considered the key component of the Papez circuit, which is generally described as the “emotional circuit” encompassing the medial temporal lobe (MTL: hippocampus, amygdala, and entorhinal cortex), hypothalamus, septum, prefrontal cortex, and the white matter (WM) circuitry connecting them. The hippocampus is one of the oldest regions of the brain (the archicortex), and has wide-reaching connections. Our findings extend the collective knowledge of hippocampal anatomy, highlighting the importance of the spinal-limbic pathway and the striking lack of hippocampal connectivity with motor and sensory cortices. Overall, our results concur with preclinical and ex-vivo data, confirming that meaningful in vivo characterisation of hippocampal connections is possible in an individual.
There was a strong age dependence of hippocampal connectivity to medial occipital regions. The strongest individual connection was to the ipsilateral thalamus. Using a connectomic approach, we identified six dominant pathways in terms of strength, length and anatomy, and characterised them by their age and gender variation. We tracked the whole-brain connections of white matter fibres from the hippocampus using ultra-high angular resolution diffusion MRI in both a single 1150-direction dataset and a large normal cohort (n = 94 391-directions). The hippocampus is a key component of emotional and memory circuits and is broadly connected throughout the brain.
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