Abstract
The hippocampal longitudinal axis has been linked to dissociated functional networks relevant to episodic memory. However, the organization of axis-dependent networks and their relation to episodic memory in aging remains less explored. Moreover, age-related deterioration of the dopamine (DA) system, affecting memory and functional network properties, might constitute a source of reduced specificity of hippocampal networks in aging. Here, we characterized axis-dependent large-scale hippocampal resting-state networks, their relevance to episodic memory, and links to DA in older individuals (n = 170, 64–68 years). Partial least squares identified 2 dissociated networks differentially connected to the anterior and posterior hippocampus. These overlapped with anterior–temporal/posterior–medial networks in young adults, indicating preserved organization of axis-dependent connectivity in old age. However, axis-specific networks were overall unrelated to memory and hippocampal DA D2 receptor availability (D2DR) measured with [11C]-raclopride positron emission tomography. Further analyses identified a memory-related network modulated by hippocampal D2DR, equally connected to anterior–posterior regions. This network included medial frontal, posterior parietal, and striatal areas. The results add to the current understanding of large-scale hippocampal connectivity in aging, demonstrating axis-dependent connectivity with dissociated anterior and posterior networks, as well as a primary role in episodic memory of connectivity shared by regions along the hippocampalaxis.
Highlights
Evidence indicates that the organization of cortical input to the hippocampus is heterogeneous along its longitudinal axis, shaping the role of anterior and posterior hippocampal regions in declarative memory (Ranganath and Ritchey 2012; Poppenk et al 2013; Strange et al 2014; Grady 2019)
The results add to the current understanding of large-scale hippocampal connectivity in aging, demonstrating axis-dependent connectivity with dissociated anterior and posterior networks, as well as a primary role in episodic memory of connectivity shared by regions along the hippocampal axis
Participants performed at similar levels on all 3 subtests of episodic memory: word recall M = 49.98 ± 9.98; number–word recall M = 49.99 ± 9.83; object–location recall M = 49.85 ± 9.66, with the composite episodic memory score showing a mean of 49.87 ± 7.51
Summary
Evidence indicates that the organization of cortical input to the hippocampus is heterogeneous along its longitudinal axis, shaping the role of anterior and posterior hippocampal regions in declarative memory (Ranganath and Ritchey 2012; Poppenk et al 2013; Strange et al 2014; Grady 2019). Primary cross-talk is commonly reported between the anterior hippocampus and anterior temporal and ventromedial prefrontal areas, whereas the posterior hippocampus is more functionally connected to medial frontal and posterior parietal areas (Kahn et al 2008; Qin et al 2016; Robinson et al 2016; Wagner et al 2016) This type of axis-specific connectivity has been linked to episodic memory in younger adults (Poppenk and Moscovitch 2011; Adnan et al 2016; Persson et al 2018), there are several examples of hippocampal connectivity deviating from this type of anterior–temporal/posterior–medial framework (Ranganath and Ritchey 2012). Adding to the overall discrepancy among findings, some studies find greater overlap than separation of anterior and posterior networks (Blessing et al 2016; Wang et al 2016)
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