title: Neural Substrates of Semantic Prospection - Evidence from the Dementias.
abstract: The ability to envisage personally relevant events at a future time point represents an incredibly sophisticated cognitive endeavor and one that appears to be intimately linked to episodic memory integrity. Far less is known regarding the neurocognitive mechanisms underpinning the capacity to envisage non-personal future occurrences, known as semantic future thinking. Moreover the degree of overlap between the neural substrates supporting episodic and semantic forms of prospection remains unclear. To this end, we sought to investigate the capacity for episodic and semantic future thinking in Alzheimer's disease (n = 15) and disease-matched behavioral-variant frontotemporal dementia (n = 15), neurodegenerative disorders characterized by significant medial temporal lobe (MTL) and frontal pathology. Participants completed an assessment of past and future thinking across personal (episodic) and non-personal (semantic) domains, as part of a larger neuropsychological battery investigating episodic and semantic processing, and their performance was contrasted with 20 age- and education-matched healthy older Controls. Participants underwent whole-brain T1-weighted structural imaging and voxel-based morphometry analysis was conducted to determine the relationship between gray matter integrity and episodic and semantic future thinking. Relative to Controls, both patient groups displayed marked future thinking impairments, extending across episodic and semantic domains. Analyses of covariance revealed that while episodic future thinking deficits could be explained solely in terms of episodic memory proficiency, semantic prospection deficits reflected the interplay between episodic and semantic processing. Distinct neural correlates emerged for each form of future simulation with differential involvement of prefrontal, lateral temporal, and medial temporal regions. Notably, the hippocampus was implicated irrespective of future thinking domain, with the suggestion of lateralization effects depending on the type of information being simulated. Whereas episodic future thinking related to right hippocampal integrity, semantic future thinking was found to relate to left hippocampal integrity. Our findings support previous observations of significant MTL involvement for semantic forms of prospection and point to distinct neurocognitive mechanisms which must be functional to support future-oriented forms of thought across personal and non-personal contexts.
authors.affiliation: Neuroscience Research Australia, Sydney, NSWAustralia; School of Psychology, The University of New South Wales, Sydney, NSWAustralia; Australian Research Council Centre of Excellence in Cognition and its Disorders, Sydney, NSWAustralia.
authors.forename: Muireann
authors.initials: M
authors.lastname: Irish
authors.affiliation: Neuroscience Research Australia, Sydney, NSW Australia.
authors.forename: Nadine
authors.initials: N
authors.lastname: Eyre
authors.affiliation: Neuroscience Research Australia, Sydney, NSWAustralia; School of Psychology, The University of New South Wales, Sydney, NSWAustralia.
authors.forename: Nadene
authors.initials: N
authors.lastname: Dermody
authors.affiliation: Behavioural and Clinical Neuroscience Institute, University of Cambridge, CambridgeUK; Brain and Mind Centre, The University of Sydney, Sydney, NSWAustralia.
authors.forename: Claire
authors.initials: C
authors.lastname: O'Callaghan
authors.affiliation: Neuroscience Research Australia, Sydney, NSWAustralia; Australian Research Council Centre of Excellence in Cognition and its Disorders, Sydney, NSWAustralia; School of Medical Sciences, The University of New South Wales, Sydney, NSWAustralia.
authors.forename: John R
authors.initials: JR
authors.lastname: Hodges
authors.affiliation: Australian Research Council Centre of Excellence in Cognition and its Disorders, Sydney, NSWAustralia; Norwich Medical School, University of East Anglia, NorwichUK.
authors.forename: Michael
authors.initials: M
authors.lastname: Hornberger
authors.affiliation: Neuroscience Research Australia, Sydney, NSWAustralia; Australian Research Council Centre of Excellence in Cognition and its Disorders, Sydney, NSWAustralia; School of Medical Sciences, The University of New South Wales, Sydney, NSWAustralia.
authors.forename: Olivier
authors.initials: O
authors.lastname: Piguet
keywords: Alzheimer’s disease; episodic memory; frontotemporal dementia; future thinking; hippocampus; imagination; prefrontal cortex; semantic memory