# Hippocampal microstructure as a measure of cognitive resilience to tau PET burden in older adults

## TL;DR

This study shows that better hippocampal microstructure helps older adults maintain cognitive function despite early tau buildup linked to Alzheimer's.

## Contribution

The study identifies hippocampal microstructure, measured by mean diffusivity, as a novel marker of cognitive resilience to early tau pathology.

## Key findings

- Lower hippocampal mean diffusivity (MD) reduces the negative impact of tau burden on global cognition and memory.
- Lower MD is linked to a weaker association between tau burden and mild cognitive impairment diagnosis.
- Hippocampal volume does not moderate the relationship between tau and cognitive outcomes.

## Abstract

Cognitive resilience, the ability to maintain better than expected cognitive function despite neuropathological burden, is a key contributor to clinical outcomes in Alzheimer’s disease (AD), though the underlying neurobiological mechanisms remain poorly understood.

To determine whether hippocampal volume and microstructure moderate the relationship between early tau pathology and cognitive performance, thereby serving as potential markers of cognitive resilience.

Cross-sectional observational study.

Participant data was obtained from the longitudinal BIOCARD Study, a volunteer-based research cohort.

The sample included 190 dementia-free adults (mean age = 68 years), comprising 176 cognitively unimpaired individuals and 14 with mild cognitive impairment (MCI).

Hippocampal volume and microstructure (mean diffusivity (MD)) were measured using structural magnetic resonance imaging (MRI) and diffusion-weighted imaging (DWI), respectively. Tau pathology was measured using FMK-6240 tau PET imaging across Braak stages I–III. Cognitive performance was indexed using global and domain-specific composite scores. Regression models tested the interactions between hippocampal volume or MD and tau burden, adjusting for demographics, APOE genotype, amyloid status, and diagnostic status.

Lower hippocampal MD (indicative of better microstructural integrity) attenuated the negative association between tau burden in Braak stages II–III and both global cognition and episodic memory (ps < 0.010). Logistic regression models indicated that lower hippocampal MD was associated with a weaker relationship between tau burden in Braak stages II–III and the likelihood of MCI diagnosis (ps < 0.050). In contrast, hippocampal volume did not moderate the relationship between tau and any cognitive outcome (ps > 0.250).

Hippocampal MD may serve as a promising imaging marker of cognitive resilience to early tau pathology, with potential utility for risk stratification and as a target for preventive interventions in AD.