Task interference of prospective memory in healthy ageing and Alzheimer’s disease: differential role of speed processing and response regulation

Abstract

INTRODUCTION: Prospective memory (PM) is the memory for intentions in the future, and interference of these intentions on the present ongoing tasks is called PM task interference. Optimal PM would allow the elderly and patients with Alzheimer’s disease (AD) to live independently. We conducted the present study to examine how healthy or pathological (ie AD) ageing would influence PM, and PM task interference. METHODS: We recruited 26 patients with mild AD, 40 age-matched healthy elderly, and 41 young subjects. We adapted the paradigm of Burgess (2003) to test event-based PM. To explore the underlying mechanism of possible PM deficit and increased PM task interference, we measured general processing speed by simple reaction time and response regulation by arrowhead test. The PM paradigm and simple reaction time were computer-based task programmed with E-prime™. The PM test had an ongoing block and a PM block. PM task interference was calculated by subtracting the pure ongoing task reaction time from that of ongoing task in PM block. Arrowhead task was a paper-based adapted test from Lee et al (2006). RESULTS: The PM task accuracy for the young, elderly, and AD patients were 82%, 87% and 69%, and the PM interference were 78 ms, 174 ms and 274 ms respectively. The accuracy of AD group was significantly less accurate than the other two groups (P<0.05), while no significant difference exists between the young and healthy old groups; the PM interference is bigger in the AD group than that in the health elderly group (P<0.5), which is bigger than that in the young group (P<0.05). Path analysis showed that it was the slow processing speed significantly mediated the ageing effect on increased task interference, and the deficiency in response regulation mediated the AD effect on increased task interference. CONCLUSION: We conclude that the elderly is not impaired at event-based PM, while AD patients are. Further we find there is task interference of PM in all the three groups; this is line with the theory of Preparatory and Attentional and Memory processes (PAM model) in explaining PM interference. The double dissociation of the underlying mechanism lends support that the process of ageing and AD are similar in clinical feather but different in neuropsychological mechanism.