Age-related decline in inhibitory control contributes to the increased Stroop effect observed in older adults ROBERT WEST and CLAUDE ALAIN Rotman Research Institute of Baycrest Centre for Geriatric Care and Department of Psychology, University of Toronto, Canada Abstract Past research has demonstrated an age-related increase in the Stroop effect. Some theorists have suggested that this increase results from a decline in the ability to inhibit word information on incongruent trials, whereas others have suggested that the decline reflects general slowing. These two hypotheses were evaluated using event-related brain potentials ~ ERPs! measured while younger and older adults performed the Stroop task. As expected, the Stroop effect was greater for older than younger adults. The ERP data revealed a selective age-related attenuation of two modulations reflecting the inhibition of word information on incongruent trials. Latency of the P3 wave did not increase to a greater extend for older than younger adults from the congruent to incongruent trials as expected based on the general slowing hypothesis. Taken together, these findings support the inhibitory deficit hypothesis by demonstrating an age-related decline in a conceptual level inhibitory process that supports the suppression of word information in the Stroop task. Descriptors: Aging, Stroop task, Inhibitory deficit, ERPs, Evoked potentials The inhibitory deficit hypothesis ~ Hasher & Zacks, 1988! has been one of the leading models within the field of cognitive aging over the past decade. Within this model, age-related declines across a number of domains of cognition, including attention, retrospective and working memory, and social cognition are proposed to result from an inability to inhibit the influence of task irrelevant infor- mation. In comparison, cognitive processes supporting the facili- tation or enhancement of task relevant information are proposed to be largely immune to the aging process. A number of studies provide evidence supporting this theory, indicating that age-related declines in inhibitory efficiency contribute to the poor perfor- mance of older adults on tasks requiring selective attention ~Stoltz- fus, Hasher, Zacks, Ulivi, & Goldstein, 1993!, controlled memory search ~ Hartman & Hasher, 1991!, and sustained attention ~ Bunce, Warr, & Cochrane, 1993!. In agreement with these behavioral data, evidence from a num- ber of electrophysiological studies supports the inhibitory deficit hypothesis. For instance, in a study examining the skin conduc- tance orienting response, habituation of the orienting response to a tone was faster for younger adults instructed to ignore the tone than for younger adults instructed to count the number of tones presented ~ McDowd & Filion, 1992!. In comparison, the rate of habituation in older adults was similar in the ignore and attend conditions. This difference suggests that increasing age resulted in a reduced ability to inhibit the processing of task irrelevant infor- mation. Complementing this finding, a number of researchers have observed an age-related increase in the amplitude of auditory- evoked potentials including the Pa wave, a middle latency auditory- evoked potential measured between 25 and 35 ms poststimulus onset ~Chambers & Griffiths, 1991; Woods & Clayworth, 1986!, and the N1 wave, a long latency potential measured between 80 and 120 ms poststimulus onset ~Alain & Woods, 1999; Karay- anidis, Andrews, Ward, & Michie, 1995; Laffont et al., 1989!. Chao and Knight ~1997! suggested that this increase in the ampli- tudes of the Pa and N1 waves results from an age-related decline in inhibitory processes supported by the prefrontal cortex. This proposal is supported by the finding that a similar enhancement of the Pa wave is observed in patients with damage to the prefrontal cortex ~ Knight, Scabini, & Woods, 1989!. One potential limitation of the current electrophysiological data is that it provides evidence only for age-related decline in inhibi- tory processes at sensory and perceptual levels. This limitation makes it difficult to determine whether or not declining inhibitory processes in older adults contribute to age-related differences in cognitive efficiency observed at higher conceptual levels of infor- mation processing. The potential importance of this limitation be- comes apparent when one considers findings from a number of behavioral studies indicating that age-related decline in inhibitory control is not observed universally across tasks reputed to tap inhibitory processing ~ Burke, 1997; McDowd, 1997!. For instance, Connelly and Hasher ~1993! reported that age-related declines in inhibitory efficiency were observed in identity but not location- based negative priming. Similarly, in a multivariate study, age- related declines in inhibitory efficiency were observed in a stop This research was supported by a postdoctoral fellowship sponsored by a grant from the National Institute of Aging ~ RO1 AG13845-01! awarded to R.W. and a grant from the Medical Research Council of Canada awarded to C.A. We thank two anonymous reviews for their helpful comments on an earlier version of the paper. Address reprint requests to: Robert West, 118 Haggar Hall, University of Notre Dame, Notre Dame, IN 46556, USA. Psychophysiology, 37 ~2000!, 179–189. Cambridge University Press. Printed in the USA. Copyright © 2000 Society for Psychophysiological Research 179