Abstract
We quantified age-related differences in processing task-irrelevant stimulus properties. In each trial, thirteen young (<32 years old) and ten older (>60 years old) participants saw one of two pre-learnt faces colourised with red or green tones at one of eight phase noise levels while scalp ERPs were recorded. In one condition, subjects had to discriminate between the identities of the two faces ignoring colour, whereas they had to ignore face identity and discriminate colour in the other condition. Behavioural accuracy followed a noise dependent sigmoid in the identity task: older participants had a significantly higher 75% correct discrimination threshold and a lower maximum accuracy. Whereas in the colour task, performance was independent of the noise level and both groups showed equally high accuracy (above 98%). For each individual in the two tasks the time-course of phase noise sensitivity was obtained by entering ERPs into a single-trial general linear model [http://www.biomedcentral.com/1471-2202/9/98]. The model identified variations in neural activity statistically associated with changes in phase noise while regressing out the main ERP differences due to identity, colour and task. Group level analysis indicated that young adults showed higher noise sensitivity in the identity task than in the colour task between circa 170 and 270 ms post-stimulus, whereas older adults did not show any task differences. Results from our study, as well as from other recent studies that reported the absence of task effects in old participants in a variety of visual attention paradigms, suggest that the ability to process only relevant visual information may be compromised in all older adults. However, individual subject analysis revealed that most older subjects did show task modulations whose timing and duration varied extensively. We suggest that individual subject single-trial analysis is a useful tool which provides a deeper insight into age-related changes in top-down visual processing.
Wellcome Trust, University of Glasgow Settlement.