Task switching performance reveals heterogeneity amongst patients with mild cognitive impairment.

OBJECTIVE To assess executive function in patients with mild cognitive impairment (MCI) and to determine whether task switching ability is associated with transition to Alzheimer's disease. METHODS Twenty-seven MCI patients and 19 older controls were tested using a cued letter-digit classification switching task. Sixteen patients could perform the task (MCI-able), 6 could not (MCI-unable), and 5 were able only with cognitive support (MCI-cue). Demographic, neuropsychological, event-related potential (ERP), MRI, and genetic data were also collected. RESULTS The four groups did not differ on age, gender, and APO E4 frequency. Compared to the controls, the MCI-unable group had significantly poorer performance on the Trail Making task (η2 = .430), lower education (η2 = .234), and smaller cortical volume (η2 = .245). Most MCI patients exhibited task-switching deficits but to vastly different degrees and with varying outcomes. The combined pattern of neuropsychological and task switching performance indicates that the MCI-able patients displayed memory retrieval difficulties (F(2,39) = 3.6, p = .036, MSE = 1.44), generally preserved task switching abilities, and had a high probability of remaining dementia-free at follow-up. The MCI-cue patients had increased mixing costs, F(2,39) = 11.0, p < .001, MSE = .07; the MCI-unable patients showed episodic memory deficits, and both groups had a high probability of poor outcome (i.e., developing AD or dying within four years). CONCLUSION This study demonstrates that variability in performance on measures of task-switching can highlight important heterogeneity in the MCI population.