title: Mediating effects of processing speed and executive functions in age-related differences in episodic memory performance: a cross-validation study.
abstract: OBJECTIVE
  Age differences in episodic memory (memory) have been attributed to a general reduction in processing speed (the "speed mediation hypothesis"), but also to declines in the efficiency of executive functions operations ("executive decline hypothesis"). To test predictions from these competing models, we examined the mediating effects of processing speed (speed) and executive functions (executive) on age and episodic memory in three older adult cohorts.
  
  
  METHODS
  The first sample comprised 842 individuals from the Sydney Memory and Aging Study (MAS). The second and third samples included 476 individuals from the Older Australian Twins Study (OATS), with each twin from a pair randomly assigned to form two samples. A series of regression analyses was performed on each of the three samples independently, so as to obtain the sizes and statistical significances of the indirect effects of age on each of the memory variables, mediated by each of the Executive and Speed composites. Sex was a control variable for all analyses. Analyses were repeated with current depression as an additional control variable.
  
  
  RESULTS
  Data from the MAS sample suggested that both Speed and Executive composites were significant mediators, with the former having a stronger mediation effect. A similar pattern was found in the two OATS samples.
  
  
  CONCLUSIONS
  These findings are consistent with those of previous studies in which speed had a stronger mediating effect than executive on age-related variation in memory. They provide further support for the speed mediation hypothesis, although not negating the executive decline hypothesis.
authors.affiliation: Centre for Healthy Brain Ageing, School of Psychiatry, Faculty of Medicine, University of New South Wales.
authors.forename: Teresa
authors.initials: T
authors.lastname: Lee
authors.affiliation: Centre for Healthy Brain Ageing, School of Psychiatry, Faculty of Medicine, University of New South Wales.
authors.forename: John D
authors.initials: JD
authors.lastname: Crawford
authors.affiliation: School of Psychology, University of Queensland.
authors.forename: Julie D
authors.identifier: 0000-0002-2081-3717
authors.initials: JD
authors.lastname: Henry
authors.affiliation: Centre for Healthy Brain Ageing, School of Psychiatry, Faculty of Medicine, University of New South Wales.
authors.forename: Julian N
authors.identifier: 0000-0002-7685-2977
authors.initials: JN
authors.lastname: Trollor
authors.affiliation: Centre for Healthy Brain Ageing, School of Psychiatry, Faculty of Medicine, University of New South Wales.
authors.forename: Nicole A
authors.initials: NA
authors.lastname: Kochan
authors.affiliation: School of Psychology, University of Queensland.
authors.forename: Margaret J
authors.identifier: 0000-0001-7133-4970
authors.initials: MJ
authors.lastname: Wright
authors.affiliation: National Ageing Research Institute & Academic Unit for Psychiatry of Old Age, Department of Psychiatry, University of Melbourne.
authors.forename: David
authors.initials: D
authors.lastname: Ames
authors.affiliation: Centre for Healthy Brain Ageing, School of Psychiatry, Faculty of Medicine, University of New South Wales.
authors.forename: Henry
authors.initials: H
authors.lastname: Brodaty
authors.affiliation: Centre for Healthy Brain Ageing, School of Psychiatry, Faculty of Medicine, University of New South Wales.
authors.forename: Perminder S
authors.identifier: 0000-0002-9595-3220
authors.initials: PS
authors.lastname: Sachdev
mesh_terms: D000367:Age Factors; D000368:Aged; D000369:Aged, 80 and over; D000375:Aging / Q000502:physiology* / Q000523:psychology; D001315:Australia / Q000453:epidemiology; D003071:Cognition / Q000502:physiology*; D015331:Cohort Studies; D003863:Depression / Q000503:physiopathology; D056344:Executive Function / Q000502:physiology*; D005260:Female; D006306:Health Surveys; D006801:Humans; D008297:Male; D061212:Memory, Episodic; D009483:Neuropsychological Tests; D012042:Registries; D018433:Twin Studies as Topic