Executive function : a comparison between normal ageing adults and older adults suffering traumatic brain injury

This thesis contrasted the performance of normal ageing adults with older TBI patients on one of the most elusive neuropsychological constructs, executive function. Executive function purportedly controls and integrates cognitive activity, reflecting conscious, strategic goal-directed operations (Stuss & Levine, 2002). However, debate rages as to whether executive function is actually a distinct construct, or merely represents g (Salthouse, 2005; Wood & Liossi, 2007). With respect to normally ageing, it is promulgated that executive functions are among the first to be impacted given that the 'seat' of executive function, the frontal lobes, decline at a faster rate than other brain regions (West, 1996). The pathophysiology of traumatic brain injury (TBI) also renders the frontal lobes and thus executive function vulnerable (McDonald, Flashman & Saykin, 2002), and older adults have the second higher incidence of TBI. As such, both groups represent logical choices to study and a research paucity exists (Garden, Phillips & MacPherson, 2001; Rappoport et al. 2006). This thesis aims to reduce this paucity by further elucidating the executive function of both these populations. A secondary aim is to further examine the utility of the Alternate Uses (AU) Test (Guilford, Christensen, Merrifield & Wilson, 1978) as a measure of executive function. Study 1 examines the performance of a normal ageing cohort (n= 100, age range 50-79 years) on measures of executive function, memory and processing speed. An age related decline was hypothesised to differentially impact executive measures. The results however ran contrary to predictions; while there was some impact of age on executive function, memory and information processing speed also suffered. Study 2 recruited a group of older TBI sufferers (n = 20, age range 50-79 years) at 6-12 months post injury. Patients were tested against age and education matched controls from Study 1. Executive function was expected to be preferentially impacted by TBI and this hypothesis was supported. The Alternate Uses paradigm showed sensitivity to both normal ageing and TBI. Ultimately, the author postulates that executive function may not be a particularly useful concept among normal populations. The equivocal state of the literature, historical problems with defining and measuring executive function and doubt as to whether executive function merely represents g, coupled with the lack of a differential age decrement in the current study all contribute to this viewpoint. Executive dysfunction on the other hand has long held relevance in clinical settings.