Prospective and retrospective timing in mild cognitive impairment and Alzheimer's disease patients: A systematic review and meta-analysis.

Performance on timing tasks changes with age. Whether these changes reflect a real "clock" problem due to aging or a secondary effect of the reduced cognitive resources of older adults is still an unsettled question. Research on processing of time in aged populations marked by severe mnemonic and/or attentional deficits, such as patients with Alzheimer's disease (AD) and Mild Cognitive Impairment (MCI), may help elucidate the role of cognitive resources in age-related temporal distortions. To this end, we conducted a systematic review and meta-analysis of timing studies in AD and MCI patients; both prospective and retrospective timing tasks were considered and analysed separately. As concerns prospective timing, a first random-effect model showed a medium overall effect of neurodegeneration on timing performance. When considering the role of moderator variables(i.e., neurodegenerative condition, type of measure, participants' age and years of education, interval length, and type of timing task), mean score appeared to be a less sensitive measure than accuracy and variability, and the observed temporal impairment was smaller in older samples. In addition, AD patients only exhibited medium-to-high impairment on prospective timing tasks, whereas MCI patients did not significantly differ from controls. However, assuming a mean age of 70 years old and absolute error as dependent variable, a second fitted meta-regression model predicted a significant outcome also for MCI patients. Concerning retrospective timing, a significant but small effect of neurodegeneration was observed for retrospective judgments. None of the moderators, however, explained between-studies variability. Collectively, our findings highlight a clear deficit in prospective timing for AD patients and underscore several issues that future work should carefully consider to better investigate the effect of MCI on prospective temporal judgements. Results from retrospective timing also point to a possible impairment of retrospective judgments in neurodegenerative conditions, albeit more studies are needed to substantiate this finding.