Abstract
The hippocampus is one of the most phylogenetically preserved structures in the mammalian brain. Engaged in a host of diverse cognitive processes, there has been increasing interest in understanding how the hippocampus dynamically supports these functions. One of the lingering questions is how to reconcile the seemingly disparate cytoarchitectonic organization, which favors a dorsal-ventral layering, with the neurofunctional topography, which has strong support for longitudinal axis (anterior-posterior) and medial-lateral orientation. More recently, meta-analytically driven (e.g., big data) approaches have been employed, however, the question remains whether they are sensitive to important task-specific features such as context, cognitive processes recruited, or the type of stimulus being presented. Here, we used hierarchical clustering on functional magnetic resonance imaging (fMRI) data acquired from healthy individuals at 7T using a battery of tasks that engage the hippocampus to determine whether stimulus or task features influence cluster profiles in the left and right hippocampus. Our data suggest that resting state clustering appears to favor the cytoarchitectonic organization, while task-based clustering favors the neurofunctional clustering. Furthermore, encoding tasks were more sensitive to stimulus type than were recognition tasks. Interestingly, a face-name paired associate task had nearly identical clustering profiles for both the encoding and recognition conditions of the task, which were qualitatively morphometrically different than simple encoding of words or faces. Finally, corroborating previous research, the left hippocampus had more stable cluster profiles compared to the right hippocampus. Together, our data suggest that task-based and resting state cluster profiles are different and may account for the disparity or inconsistency in results across studies.
Highlights
One of the most preserved neural structures across species, the hippocampus has been a prime target for evolution theorists and cognitive neuroscientists alike
We examined submillimeter functional magnetic resonance imaging (fMRI) datasets collected at Auburn University on a selection of tasks that had similar neurocognitive constituents to the neurofunctional topography identified in prior meta-analytic studies (Robinson et al, 2015; Plachti et al, 2019), including a face-name paired associate task, and encoding/recognition of words, faces, and scenes
Hierarchical Clustering Stability Collapsing across tasks, hierarchical clustering resulted in an average similarity index of 0.60, with a standard deviation of 0.06 and a range of 0.46–0.70 (Figure 3A)
Summary
One of the most preserved neural structures across species, the hippocampus has been a prime target for evolution theorists and cognitive neuroscientists alike. Most studies of hippocampal sub-specialization limit their investigations to a single behavioral domain or a single paradigm that compares specific neurocognitive processes (i.e., encoding vs retrieval) (Giovanello et al, 2009; Nadel et al, 2012; Duarte et al, 2014; Nakamura and Sauvage, 2015; Przezdzik et al, 2019) This led to a series of studies that used meta-analytic techniques to better characterize the functional and anatomical subfields of the hippocampus (Chase et al, 2015; Persson and Söderlund, 2015; Robinson et al, 2015, 2016; Plachti et al, 2019). Understanding the topography of the hippocampus could yield transformative insights into neurocognitive processes, as well as disease pathology
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