Critical flicker fusion threshold: A potentially useful measure for the early detection of Alzheimer's disease

Critical Flicker Fusion Threshold (CFFT) is a psychophysical threshold and, in psychological terms, it is regarded as a measure of information process ing capacity. CFFT has previously been shown to be a valid and reliable mea sure in young healthy volunteers and it also has a long history of use as a psychopharmacological measure in this group. Furthermore, the test satisfi es many of the requirements of an 'ideal' measure for monitoring change, es pecially in a psychopharmacological context. Despite this, CFFT has been ne glected as a research tool in elderly and Alzheimer's disease (AD) populati ons and was therefore investigated further in this regard. CFFT in communit y-based healthy elderly subjects was normally distributed, but CFFT and asc ending and descending thresholds were not significantly correlated with age . The difference between ascending and descending thresholds was, however, significantly correlated with age and this relationship appeared to be due almost entirely to a change in the descending threshold. In addition, desce nding thresholds were found to be significantly greater than ascending thre sholds in healthy elderly subjects. In contrast, patients with AD were foun d to have significantly lower CFFT and descending scores compared with heal thy elderly subjects. Interestingly, descending thresholds were significant ly lower than ascending thresholds in the patient group, a feature that may be a characteristic of AD. Mean CFFT and ascending and descending threshol ds were found to have a high test-retest, split-half and inter-rater reliab ility, in addition to being significantly correlated with a number of psych ometric measures, clinical scales and neuropsychological instruments common ly used to assess patients with AD. CFFT is a quick and simple measure to a dminister and patients had no difficulty completing the test. Because the m easure is a psychophysical threshold, it is free from educational and cultu ral bias and there are no floor or ceiling effects. From the results of thi s work, CFFT appears to be a useful research tool in AD. It may be a suitab le measure for monitoring cognitive change over time, either in community s tudies of AD or a clinical trial context, but further work is required. The technique might also contribute to the early detection of AD. This applica tion would be particularly important because it would enable effective phar macotherapies to be started early during the course of the illness before n euronal damage is too advanced and this would have significant benefits for patients. Copyright (C) 2000 John Wiley & Sons, Ltd.