Abstract: An important aspect of executive function involves the ability to inhibit prepotent responses (inhibition). It has been argued that this aspect of executive function might contribute to adult age-related effects on fluid cognition. However, this argument has been challenged by various individual differences studies, especially when processing speed was considered. Specifically, age-related effects on fluid abilities through the mediation of executive construct are much smaller than those through speed construct. Most of the relations between executive construct and both age and other cognitive abilities are shared with other variables, especially speed variables. The purpose of this study was to investigate the relative roles contributions of inhibition and processing speed to age-related differences in fluid intelligence, and to find empirical evidence for the executive decline hypothesis.
A sample of 142 normal adults between 18 and 85 years of age performed a set of inhibition, processing speed, and fluid intelligence tasks. Different from previous studies, which usually used processing speed as cognitive primitives, this study adopted three types of speed measures (i.e. inspection time, reaction time, and perceptual/motor speed), which were assumed to involve different level of executive control resources. All assessments (including 13 tasks) were taken in two sessions, administered during a 2-week period. Participants were encouraged to have a break whenever they felt tired.
The results indicated that reliabilities of the referring inhibition, processing speed, and fluid intelligence measures were relatively high. Confirmatory factor analysis showed that three target speed measures were clearly separable. Hierarchical regression analyses indicated that, the proportion of unique age-related variance in executive measures was related to the complexity of speed measures. There was a larger proportion of unique age-related variance in executive measures after controlling for the variance in simpler speed measures. Moreover, structural equation modeling showed that more complex speed measures, rather than inhibition, played a larger role in mediating age-related differences in fluid intelligence. However, this result was reversed when simpler speed measures were considered.
In summary, these results suggest that inhibition and processing speed are an important mediator between age and fluid intelligence, and provide support for executive decline hypothesis of cognitive aging. In addition, the mediation of age-related effects on cognitive abilities through processing speed as indicated by previous studies is at least partially derived from the executive component of speed measures. These findings provide support for executive decline hypothesis of cognitive aging as well as the processing speed theory

Key words: inhibition, executive control, processing speed, fluid intelligence, age