Why are dual-task costs reduced with ideomotor (IM) compatible tasks (see e.g. Greenwald & Shulman, 1973)? In a series of experiments, I tested the way in which task structure affects dual-task performance (Halvorson et al., 2012). The results suggest that in some cases, typical dual-task costs arise from task structure rather than response limitations. Further examination of this question has shown that dual-task costs cannot be predicted solely on the basis of the relationship between the stimuli and the responses; the relationship between the tasks, or the task pairing, plays a critical role in whether the tasks overlap and performance is impaired. A series of experiments using novel task pairings showed that when one task uses a spatial central code and the other uses a verbal central code, dual-task costs are eliminated.

Yes / Objective: This study investigated whether dual tasks make disproportionately high demands in Huntington’s disease (HD) compared with controls, and also tested the Multiple Resources Theory. Method: Thirteen HD participants and 13 controls completed 2 dual task sets that varied in difficulty and complexity: Set 1 paired simple choice reaction time (RT) with digit forward, and Set 2 paired complex choice RT with digit backward. Results: We found that HD participants were overall slower; however, although they maintained similar levels of accuracy in the simple choice RT tasks with controls, their accuracy decreased in the complex choice RT tasks. In addition, we found that HD participants were more susceptible to speed-accuracy trade-offs. Despite that, they did not show greater dual task costs than controls. Conclusions: Overall, our findings do not support the Multiple Resources Theory, but they do provide some support for the Unitary Resource Theory and the attentional impairment hypothesis.

Falls and their consequences among older people are a serious medical and public health problem. Identifying individuals at risk of falling is therefore a major concern. The purpose of this thesis was to evaluate measurement tools of balance control and their predictive value when screening for fall risk in physically dependent individuals ≥65 years old living in residential care facilities, and physically independent individuals ≥75 years old living in the community. Following baseline assessments falls were monitored during six months in physically dependent individuals based on staff reports, and during one year in physically independent individuals based on self reports. In physically dependent individuals test-retest reliability of the Timed Up&Go test (TUG) was established in relation to cognitive impairment. Absolute reliability measures exposed substantial day-to-day variability in mobility performance at an individual level despite excellent relative reliability (ICC 1.1 >0.90) regardless of cognitive function (MMSE ≥10). Fifty-three percent of the participants fell at least once during follow-up. Staff judgement of their residents’ fall risk had the best prognostic value for ruling in a fall risk in individuals judged with ‘high risk’ (positive Likelihood ratio, LR+ 2.8). Timed, and subjective rating of fall risk (modified Get Up&Go test, GUG-m) were useful for ruling out a high fall risk in individuals with TUG scores <15 seconds (negative LR, LR- 0.1) and GUG-m scores of ‘no fall risk’ (LR- 0.4), however few participants achieved such scores. In physically independent individuals balance control was challenged by dual-task performances. Subsequent dual-task costs in gait (DTC), i.e. the difference between single walking and walking with a simultaneous second task, were registered using an electronic mat. Forty-eight percent of the participants fell at least once during follow-up. A small prognostic guidance for ruling in a high fall risk was found for DTC in mean step width of ≤3.7 mm with a manual task (LR+ 2.3), and a small guidance for ruling out a high fall risk with DTC in mean step width of ≤3.6 mm with a cognitive task (LR- 0.5). In cross-sectional evaluations DTC related to an increased fall risk were associated with: sub-maximal physical performance stance scores (Odds Ratio, OR, 3.2 to 3.8), lower self-reported balance confidence (OR 2.6), higher activity avoidance (OR 2.1), mobility disability (OR 4.0), and cautious walking out-door (OR 3.0). However, these other measures of physical function failed to provide any guidance to fall risk in this population of seemingly able older persons. In conclusion – Fall risk assessments may guide clinicians in two directions, either in ruling in or in ruling out a high fall risk. A single cut-off score, however, does not necessarily give guidance in both directions. Staff experienced knowledge is superior to a single assessment of mobility performance for ruling in a high fall risk. Clinicians need to consider the day-to-day variability in mobility when interpreting the TUG score of a physically dependent individual. DTC of gait can, depending on the type of secondary task, indicate a functional limitation related to an increased fall risk or a flexible capacity related to a decreased fall risk. DTC in mean step width seems to be a valid measure of balance control in physically independent older people and may be a valuable part of the physical examination of balance and gait when screening for fall risk as other measures of balance control may fail to provide any guidance of fall risk in this population.