Effects of Aging on the Encoding of Spatial Direction in the Human Brain

Abstract

1Human aging is characterized by impaired spatial cognition and reductions in the distinc-2 tiveness of category-specific fMRI activation patterns. Yet, little is know about age-related 3 decline in neural distinctiveness of spatial information. Here, we asked whether neural tun-4 ing functions of walking direction are broadened in older versus younger adults. To test this 5 idea, we developed a novel method that allowed us to investigate changes in fMRI-measured 6 pattern similarity while participants navigated in different directions in a virtual spatial 7 navigation task. We expected that directional tuning functions would be broader in older 8 adults, and thus activation patterns that reflect neighboring directions would be less dis-9 tinct as compared to non-adjacent directions. Because loss of distinctiveness leads to more 10 confusions when information is read out by downstream areas, we analyzed predictions of a 11 decoder trained on these representations and asked (1) whether decoder confusions between 12 two directions increase proportionally to their angular similarity, (2) and how this effect 13 may differ between age groups. Evidence for tuning-function-like signals was found in the 14 retrosplenial complex and primary visual cortex. Significant age differences in tuning width, 15 however, were only found in the primary visual cortex, suggesting that less precise visual 16 information could lead to worse directional signals in older adults. Yet, age differences in 17 visual tuning were not related to behavior. Instead, directional information encoded in RSC 18 correlated with memory on task. These results shed new light on neural mechanisms under-19 ling age-related spatial navigation impairments and introduce a novel approach to measure 20 tuning specificity using fMRI. 21 1 1 Introduction 24 A central goal of aging research is to understand how aging-related neurobiological changes 25affect computational functions of the brain. One important approach has been to investigate 26 how aging changes the representation of sensory information in the brain (Voss et al., 2008;