New insights into anatomical connectivity along the anterior–posterior axis of the human hippocampus using in vivo quantitative fibre tracking

Abstract

The hippocampus supports multiple cognitive functions including episodic memory. Recent work has highlighted functional differences along the anterior–posterior axis of the human hippocampus, but the neuroanatomical underpinnings of these differences remain unclear. We leveraged track-density imaging to systematically examine anatomical connectivity between the cortical mantle and the anterior–posterior axis of the in vivo human hippocampus. We first identified the most highly connected cortical areas and detailed the degree to which they preferentially connect along the anterior–posterior axis of the hippocampus. Then, using a tractography pipeline specifically tailored to measure the location and density of streamline endpoints within the hippocampus, we characterised where these cortical areas preferentially connect within the hippocampus. Our results provide new and detailed insights into how specific regions along the anterior–posterior axis of the hippocampus are associated with different cortical inputs/outputs and provide evidence that both gradients and circumscribed areas of dense extrinsic anatomical connectivity exist within the human hippocampus. These findings inform conceptual debates in the field and emphasise the importance of considering the hippocampus as a heterogeneous structure. Overall, our results represent a major advance in our ability to map the anatomical connectivity of the human hippocampus in vivo and inform our understanding of the neural architecture of hippocampal-dependent memory systems in the human brain.

Reconstructing Spatio-Temporal Trajectories of Visual Object Memories in the Human Brain

“…Once pattern completion is initiated, most information is relayed via the subiculum to the entorhinal cortex which in turn has back-projections to other cortical areas (Chrobak et al, 2000;Rolls, 2013;Staresina & Wimber, 2019). A majority of direct hippocampal and entorhinal efferents terminate in late areas of the visual processing hierarchy where representations cluster in abstract-semantic categories, while there are few connections to low-level sensory areas (Dalton et al, 2022;Insausti & Muñoz, 2001).…”

“…Once pattern completion is initiated, most information is relayed via the subiculum to the entorhinal cortex which in turn has back-projections to other cortical areas (Chrobak et al, 2000; Rolls, 2013; Staresina & Wimber, 2019). A majority of direct hippocampal and entorhinal efferents terminate in late areas of the visual processing hierarchy where representations cluster in abstract-semantic categories, while there are few connections to low-level sensory areas (Dalton et al, 2022; Insausti & Muñoz, 2001). We therefore expect that memories are initially reinstated on a conceptual level, and information coded in lower-level visual areas can only be reached at later stages of memory reconstruction (Insausti & Muñoz, 2001; Linde-Domingo et al, 2019).…”

Two long-axis dimensions of hippocampal cortical integration support memory function across the adult lifespan

“…While the principal anteroposterior gradient is well-established in the literature (Genon et al, 2021; Poppenk et al, 2013), demonstrated not only in function (Przeździk et al, 2019; Tian et al, 2020; vos de Wael et al, 2018), but also in structural covariance (Plachti et al, 2019), gene expression (Vogel et al, 2020), and in graded anteroposterior transitions of anatomical connections (Beaujoin et al, 2018; Dalton et al, 2022), the secondary long-axis gradient remains more elusive. Here, we present multiple lines of evidence supporting an interpretation of the second-order gradient as a local representation of the principal gradient of macroscale cortical organization (Huntenburg et al, 2018; Margulies et al, 2016): the unimodal-transmodal distribution of a) large-scale cortical networks in gradient space (Figure 2); b) meta-analytic terms in Neurosynth (Figure 3B); and the spatial correspondence with a functional unimodal-transmodal cortical gradient (Figure 2).…”

An axis-referenced morphometric model reshaping the hippocampus following its longitudinal organization

“…The human hippocampus is a crucial brain structure responsible for memory processing, learning, spatial navigation and emotions (Chauhan P et al, 2021) and is vulnerable to most neurocognitive diseases (Baazaoui N et al, 2022). Recent researches have shown that the hippocampal longitudinal organization is tightly integrated into large-scale functional/structural networks (Dalton MA et al, 2022;Kharabian Masouleh S et al, 2020; Vogel JW et al, 2020). (Plachti A et al, 2020) discovers different longitudinal structural subdivisions of the hippocampus that relate to normal aging and dementias.…”