Impact of stimulus integrity on age differences in letter matching

Young and older adults were tested in both a letter-identification and a letter-matching task in which the integrity of the letter stimuli was manipulated through contrast reduction and low-pass spatial frequency filtering. The use of the contrast and filtering manipulations was an attempt to increase encoding difficulty in an effort to examine whether stimulus integrity impacts more than just the initial encoding of the letter pairs in a letter-matching task, namely the comparison process as indexed by fast-same and false-different effects. Of interest in terms of aging is whether a decline in information-processing performance often reported in the aging literature is related to the known encoding deficits of older adults. In the letter-identification task, both contrast reduction and filtering slowed letter-identification speed for both groups, with the effect being larger for the older adults. In the letter-matching task, decreased processing efficiency produced by the contrast-reduction and low-pass-filtering manipulations led to an overall increase in reaction time and errors, but it did not interact with the magnitude of the fast-same effect or false-different effects for either subject group. These findings suggest that the stimulus integrity manipulations only impact the encoding of the letter pairs in the matching task and not the comparison process. The results of the present study support a dual-process model of the matching task consisting of separate encoding and comparison processes. The finding of a larger fast-same effect for older adults suggests that the age effect is occurring at the comparison stage, but it is not impacted by the stimulus integrity manipulations. The findings are described within a generalized slowing framework.