At least four distinct explanations can be proposed to account for patterns of spared and impaired performance in which the age-related effects on some variables are weaker than those on other variables. These are: (a) some variables lack sufficient reliability to exhibit relations with other variables; (b) the variables differ in their dependence on what many of the variables have in common; (c) variables with little or no age differences reflect a qualitatively different form of cognition than variables with moderate to large age differences; and (d) variables with little or no age differences have independent positive age-related influences that offset the negative age-related effects shared with other cognitive variables. These interpretations were examined with three different variables hypothesized to reflect implicit learning obtained from a sample of 183 adults ranging from 18 to 87 years of age. Only an implicit learning measure derived from a sequential reaction time task had acceptable reliability at the level of individual participants, and it was negatively related to age and positively related to variables reflecting fluid cognition. These results therefore suggest that typical measures of implicit learning, when they can be reliably assessed, do not reflect a qualitatively distinct type of cognitive processing nor do they seem to exhibit additional compensatory age-related influences.
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Salthouse et al. (1999) studied this question.