The Impact of Context on Age Related Changes to Pattern Separation in the Medial Temporal Lobe

Abstract

Pattern separation is a hypothesized neural function by which two highly overlapping neural representations are made distinct from one another. Pattern separation is believed to allow for behavioral recognition discrimination among highly overlapping stimuli, such as objects. Impaired pattern separation may underlie changes to memory with age (Burke and Barnes, 2006; Burke et al., 2010; Holden & Gilbert, 2012). Older adults tend to perform worse than younger adults at correctly identifying similar but not previously seen objects as “similar” Older adults also tend to make more errors falsely identifying similar objects as one they have seen before (Holden & Gilbert, 2012; Holden, Hoebel, Loftis, & Gilbert, 2012; Stark, Stevenson, Wu, Rutledge, & Stark, 2015). A recent process model by Burke et al. (2018) describes age related changes to medial temporal lobe circuitry which processes detail information. This may lead older adults to over-rely on global or holistic information due to poor detail processing. These age related changes to detail processing may make it more difficult for older adults to distinguish between highly overlapping similar representations which requires an analysis of fine detail. The goal of this study is to test whether contextual information provided by repeated background scenes contributes to poor pattern separation performance in older adults. In this study, I used a task modelled after the classic object mnemonic discrimination task (Kirwan & Stark, 2007; Stark, Yassa, Lacy, & Stark, 2013a) but which manipulates contextual information by placing objects in naturalistic scenes that are either repeated or novel between the first and second presentation. We used region of interest (ROI) analyses to identify peak cluster activation for each condition in the hippocampus, parahippocampal gyrus, and perirhinal cortex. In addition, multivoxel pattern similarity analysis were performed within each of the ROIs to examine similarity of fMRI activation patterns in each ROI for each condition. We observed poorer recognition discrimination for similar objects for older adults than younger adults. Older adults were also more biased by repeated context information to falsely recognize a similar object. Consistent with a cooperative, process-based view of the medial temporal lobe, we observed similar patterns of response across all three medial temporal lobe regions. The perirhinal cortex, parahippocampal cortex, and hippocampus exhibited greater pattern similarity for repeated objects than similar objects or novel objects presented on a white background. Once objects were presented in a repeated scene, we observed no difference in fMRI pattern similarity across object types. Furthermore, the repeated context scene increased fMRI pattern similarity for novel objects across all three MTL regions. This project integrates findings from both object recognition literature and pattern separation literature which have typically operated independently. We find evidence that contextual information plays an important role in pattern separation for both younger and older adults.