Dentate Gyrus activation during visual but not auditory pattern separation: an investigation of perceptual modality differences in young and old individuals using fMRI.

Introduction: Pattern separation (PS) is a neurocognitive process that is essential for distinguishing highly similar objects from one another. The dentate gyrus has been previously considered the primary anatomical and neurofunctional focus of this central memory process. PS has traditionally been studied in the context of visual discrimination of highly similar everyday objects. To date, the neurofunctional substrates of auditory PS have not been investigated, and only a handful of studies have examined the behavioural age effects on auditory PS. Methods: The present study adapted the prominent mnemonic similarity task from Stark et al. (2019) to assess and directly compare both visual and auditory PS using fMRI. A sample of 46 subjects, 23 per group, aged between 19 and 79 years of age, completed alternating trials of visual and auditory pattern separation using non-semantic visual shapes and two-syllabic pseudowords. Results: Contrary to our expectations, no hippocampal activation was found during auditory PS. Instead, left-lateralized cortical regions were identified, including the superior temporal gyrus, the inferior frontal gyrus, and the pre-supplementary motor area. Furthermore, the older subjects demonstrated age-related deficits in lure discrimination and recognition memory performance; however, these deficits were not consistently observed at the neurofunctional level. Discussion: In sum, the present findings point to a double dissociation between visual and auditory PS. Furthermore, the results on ageing, both behavioural and functional, could reflect age-related changes in neurovascular coupling and declines in visual and auditory perception, as well as methodological challenges in examining PS across different sensory modalities and age.