Do total hippocampus and hippocampal subfield volumes relate to navigation ability? A call towards methodological consistency.

Despite the need for successful navigation, humans vary greatly in their ability to navigate, and these individual differences may relate to variation in brain structure. While prior research provides support for a correlation between hippocampal volume and navigation ability in both navigation experts and in older individuals, this relationship is under scrutiny for healthy, young adults. We assessed 99 healthy young adults' ability to navigate in a virtual, desktop maze and correlated their performance with total hippocampal gray matter volume. For a subset of these individuals, we further segmented the medial temporal lobe-including regions of the hippocampus-into anatomically-distinct subregions to uniquely examine the association between volumes of hippocampal subfields and navigation. Given the need to distinguish between similar-looking maze hallways and partially overlapping routes, young adults with stronger pattern separation ability may perform better in this task. Thus, we theorized that successful navigation would positively correlate with hippocampal CA3 and dentate gyrus (DG) subfield volumes due to these regions' role in pattern separation. CA1 and entorhinal cortex (ERC) are both associated with rodent spatial memory, too, suggesting a possible relationship between their volumes and navigation performance. Consistent with our hypotheses, we observed a positive relationship between volumes of hippocampal subfields and wayfinding accuracy, while ERC and parahippocampal cortex volumes correlated with navigation efficiency. However, when analyzing total hippocampal volume, a nuanced interpretation is warranted. We found evidence of Simpson's Paradox, where total hippocampal volume and navigation accuracy displayed no correlation in males, a negative correlation in females, yet a positive correlation when considering the full sample of males and females combined. Furthermore, no significant relationship was observed between total hippocampal volume and path efficiency. Given these findings, we urge caution in interpreting the results because these associations differ by analysis techniques (including voxel-based morphometry), after sex stratification, and with anterior and posterior hippocampal subdivisions. Overall, this study enhances our understanding of the relationship between brain volume and navigation ability for young adults but also emphasizes the need for methodological consistency across studies with respect to boundary definitions, neuroimaging techniques, statistical methods, and factors that give rise to individual differences.