Theoretical and practical implications of dual-task performance in Alzheimer's disease

The ability of human beings to perform more than one thing (e.g. Logie et al., 2000). Other studies have shown that the size of the dual-task decrement increases as the disease at a time has long been a focus of study in the literature on human attention and memory. Evidence from studies of progresses and this progressive decrement is much greater than that seen for individual tasks (Baddeley et al., 1991). healthy volunteers has suggested that there may be an identifiable cognitive function responsible for dualor multiSeveral other cognitive tasks, such as episodic memory and response inhibition, have been shown to be sensitive to task coordination in overall task performance. The failure of this coordination function is a characteristic impairment of the effects of Alzheimer’s disease (e.g. Perry et al., 2000). However, these tasks are also sensitive to the effects of mild Alzheimer’s disease patients both in a laboratory setting (Baddeley et al., 1986; Della Sala et al., 1995; Greene et al., normal ageing. The specific nature of the dual-task deficit in Alzheimer’s disease, in the absence of an impact of normal 1995; Collette et al., 1999; see also the paper by Baddeley et al. in this issue), and in everyday tasks such as holding ageing, offers considerable potential for developing noninvasive instruments to aid diagnosis, to track the progression conversations (Alberoni et al., 1992), or walking while talking (Camicioli et al., 1997). There are published reports of the disease and to evaluate the effectiveness of any treatments that might become available. of dual-task deficits in the healthy elderly (see review in McDowd and Shaw, 1999). However, these studies do not From a theoretical point of view, the findings contribute to the understanding of the organization of cognitive resources take account of the lower initial levels of performance on each task performed on its own (Hartley and Little, 1999). in the human brain. One view is that there is available a single, general purpose pool of attentional resources that can Indeed, healthy volunteers can perform under demanding dual-task conditions with very little performance degradation. be allocated on demand until the demand exceeds the resource available. Therefore, in the case of Alzheimer’s disease these Moreover, there is no evidence that dual-task performance is affected by normal ageing, provided that tasks are chosen to resources would be reduced resulting in poorer cognitive performance overall. Such a view would have some difficulty rely on different parts of the cognitive system, and the level of difficulty of the two individual tasks is adjusted so as to with the finding that dual-task performance appears to be impaired in Alzheimer’s disease, while the patients are not equate single-task performance across groups (see Fig. 1). However, mild Alzheimer’s disease patients show substantial differentially affected by increasing the demands of a single task. A second view is that cognition is limited by the speed performance decrements under such dual-task conditions (see Fig. 1). with which the cognitive system can operate, and that normal ageing results in a slowing of processing. Baddeley and Baddeley and colleagues (this issue) have, in addition, reported that increasing the demands of a single task has no colleagues extend this argument to investigate whether cognitive slowing might offer a possible interpretation of the greater impact on Alzheimer’s disease patients than it does on healthy controls, and that the dual-task deficit in Alzheimer’s cognitive impairments in Alzheimer’s disease (Baddeley et al., 2001). However, the speed of processing view faces a disease can be demonstrated using different combinations of tasks (Baddeley et al., 2001). This lends further support to significant challenge from their findings that speed of responding in a focal attention task does not result in the suggestion that Alzheimer’s disease patients have a specific difficulty with dual-task performance rather than differential impairments for Alzheimer’s disease. The results are more consistent with the alternative interpretation general cognitive demand. Some of our own recent research has shown that the effect (Baddeley et al., 1986, 1991) that cognition is supported by several, specialized functions, coupled with a coordination is robust across practice, appears regardless of the relative cognitive demand of each task and can be demonstrated function. Baddeley and colleagues add to the gamut of studies showing that thorough analyses of Alzheimer’s disease when combining two memory-loaded tasks providing that each task employs theoretically distinct cognitive resources patients’ performances contribute to our understanding of