The relationship between hippocampal subfield volumes and autobiographical memory persistence.

Daniel N Barry,Ian A Clark,Eleanor A Maguire

doi:10.1002/hipo.23293

Abstract

Structural integrity of the human hippocampus is widely acknowledged to be necessary for the successful encoding and retrieval of autobiographical memories. However, evidence for an association between hippocampal volume and the ability to recall such memories in healthy individuals is mixed. Here we examined this issue further by combining two approaches. First, we focused on the anatomically distinct subregions of the hippocampus where more nuanced associations may be expressed compared to considering the whole hippocampal volume. A manual segmentation protocol of hippocampal subregions allowed us to separately calculate the volumes of the dentate gyrus/CA4, CA3/2, CA1, subiculum, pre/parasubiculum and uncus. Second, a critical feature of autobiographical memories is that they can span long time periods, and so we sought to consider how memory details persist over time by conducting a longitudinal study whereby participants had to recall the same autobiographical memories on two visits spaced 8 months apart. Overall, we found that there was no difference in the total number of internal (episodic) details produced at Visits 1 and 2. However, further probing of detail subcategories revealed that specifically the amount of subjective thoughts and emotions included during recall had declined significantly by the second visit. We also observed a strong correlation between left pre/parasubiculum volume and the amount of autobiographical memory internal details produced over time. This positive relationship was evident for particular facets of the memories, with remembered events, perceptual observations and thoughts and emotions benefitting from greater volume of the left pre/parasubiculum. These preliminary findings expand upon existing functional neuroimaging evidence by highlighting a potential link between left pre/parasubiculum volume and autobiographical memory. A larger pre/parasubiculum appears not only to protect against memory decay, but may possibly enhance memory persistence, inviting further scrutiny of the role of this brain region in remote autobiographical memory retrieval.

Highlights

Structural integrity of the hippocampus has long been associated with the formation and retrieval of episodic memories
This positive relationship was evident for particular facets of the memories, with remembered events, perceptual observations and thoughts and emotions benefitting from greater volume of the left pre/parasubiculum
While we did not find a significant relationship with time details, we noted that the correlation between left pre/parasubiculum volume and references to places was close to the threshold for significance (Table 4)

Summary

| INTRODUCTION

Structural integrity of the hippocampus has long been associated with the formation and retrieval of episodic memories. A focus on hippocampal subregions may be one potentially promising avenue to pursue, and their accurate delineation on structural MRI scans is best achieved using manual segmentation (Dalton, Zeidman, Barry, Williams, & Maguire, 2017; Olsen et al, 2019; Yushkevich et al, 2015) Another factor that may contribute to the relatively weak associations between hippocampal volume and memory task performance in healthy people is a lack of ecological validity. To the best of our knowledge, only one study has examined the links between hippocampal subregion volumes and autobiographical memory recall in healthy individuals, reporting a positive association between the number of internal details on the Autobiographical Interview (Levine et al, 2002) and the volume of both the left subiculum and the combined left dentate gyrus (DG)/CA2/ CA3 region (Palombo et al, 2018). We hypothesized that the amount of autobiographical memory internal details produced after a considerable delay might be related to the volume of the pre/parasubiculum, and that the positive association found between the subiculum volume and autobiographical memory reported by Palombo et al (2018) may have been driven by the pre/parasubiculum

| Participants

| RESULTS

Findings

| DISCUSSION