Background

Previous research suggests that the size of the hippocampus can vary in response to intensive training (e.g., during the acquisition of expert knowledge). However, the role of the hippocampus in maintenance of skilled performance is not well understood. The Stanford/Veterans Affairs Aviation MRI Study offers a unique opportunity to observe the interaction of brain structure and multiple levels of expertise on longitudinal flight simulator performance.

Methods

The current study examined the relationship between hippocampal volume and three levels of aviation expertise, defined by pilot proficiency ratings issued by the U.S. Federal Aviation Administration (11). At 3 annual time points, 60 pilots who varied in their level of aviation expertise (ages ranging from 45 to 69 yr) were tested.

Results

At baseline, higher expertise was associated with better flight simulator performance, but not with hippocampal volume. Longitudinally, there was an Expertise x Hippocampal volume interaction, in the direction that a larger hippocampus was associated with better performance at higher levels of expertise.

Discussion

These results are consistent with the notion that expertise in a cognitively demanding domain involves the interplay of acquired knowledge ('mental schemas') and basic hippocampal-dependent processes.

The presence of an ApoE ε4 allele (ε4+) increases the risk of developing Alzheimer's disease (AD). Previous studies support an adverse relationship between ε4+ status and brain structure and function in mild cognitive impairment and AD; in contrast, the presence of an ε2 allele may be protective. Whether these findings reflect disease-related effects or pre-existing endophenotypes, however, remains unclear. The present study examined the influence of ApoE allele status on brain structure solely during middle-age in a large, national sample. Participants were 482 men, ages 51-59, from the Vietnam Era Twin Study of Aging (VETSA). T1-weighted images were used in volumetric segmentation and cortical surface reconstruction methods to measure regional volume and thickness. Primary linear mixed effects models predicted structural measures with ApoE status (ε3/3, ε2/3, ε3/4) and control variables for effects of site, non-independence of twin data, age, and average cranial vault or cortical thickness. Relative to the ε3/3 group, the ε3/4 group demonstrated significantly thinner cortex in superior frontal and left rostral and right caudal midfrontal regions; there were no significant effects of ε4 status on any temporal lobe measures. The ε2/3 group demonstrated significantly thicker right parahippocampal cortex relative to the ε3/3 group. The ApoE ε4 allele may influence cortical thickness in frontal areas, which are later developing regions thought to be more susceptible to the natural aging process. Previous conflicting findings for mesial temporal regions may be driven by the inclusion of older individuals, who may evidence preclinical manifestations of disease, and by unexamined moderators of ε4-related effects. The presence of the ε2 allele was related to thicker cortex, supporting a protective role. Ongoing follow-up of the VETSA sample may shed light on the potential for age- and disease-related mediation of the influence of ApoE allele status.