Friday, December 22, 2006

Age decline in reasoning - speed and executive abilities

[Double click on table to enlarge for easier viewing]

What may be the reasons for decreased fluid intelligence (Gf) with increasing age?

According to recent study (see reference and abstract below),two of the primary causative factors for decreased fluid intelligence (as established by recent research) are (a) decreased speed of cognitive processing (a generalized cognitive slowing mechanism) and (b) decreases in the executive functions of the frontal lobes as evidenced by decreased frontal lobe volume, alterations in frontal lobel cell morphology, and reductions in cerebral blood flow to the frontal and prefrontal brain lobes. The current study investigated the relative contributions of processing speed and frontal lobe function on decreases in Gf. The abstract for the article (together with URL link) is reproduced below. I will summarize some of the major findings plus add my two cents.

I have 2 cents worth of methodological comments. First, measures of reaction time (Gt) operationally defined cognitive processing speed in this study. According to the CHC taxonomy, these measures represent aspects of the broad domain of Gt (broad reaction time), which is NOT to be confused with broad cognitive processing speed (Gs). Thus, the current results are specific to the influence of Gt and may not be generalizable to Gs abilities. Additional research with valid Gs markers is needed.

Second, the authors continue the unfortunate tradition of using the Wechsler Block Design test as a marker for Gf. Contemporary Gf-Gc/CHC joint exploratory and confirmatory factor studies have convincly indicated that Block Design is a strong measure of visual-spatial processing (Gv)...not Gf. Luckily, the authors also use the Wechsler Matrix Reasoning test which is a valid indicator of Gf. Given the problems with Block Design, I recommend that readers of this article only pay attention to the results focused on understanding the decline in the Matrix Reasoning test (Of course, the Block Design findings can be interpreted in the context of Gv if that is what is of interest.)

Given this caveat, below are the major conclusions regarding possible explanations for age-related declines in Gf. I believe readers should only focus on the composite Gt measure as the indicator of general reaction time (Gt) and only the analyses that included the Gt, frontal function measures, and age in the analysis (as these provide the most valid and comprehensive explanations from the current study). These findings have been highlighted in red in the above table picture.
  • Frontal (executive) function and Gt, collectively, account for approximately 27 % of the decrease in Gf with age. Chronological age explains and additional 15-16 % of the decline in Gf, above and beyond frontal function and Gt abilities.
  • As noted by the authors, a generalized slowing of cognitive speed contributes to decreased Gf abilities...but, speed is not the entire picture. Decreased frontal functions, as well as other unaccounted for variables realated to age, also contribute to decreased Gf with age. Decreased frontal functions and Gt both contribute uniquely to age-related declines in Gf abilities.
  • Declines in cognitive abilities, in this case Gf, are multiply determined. No one single mechanism can explain age-related changes in cognitive ability.
  • Bottom line - age-related decreases in the ability to reason inductively/deductively and solve novel problems (Gf - fluid intelligence) appear due, in part, to age-related decreased speed of cognitive thinking and decreases in ability to think and manage (executive function) one's own thinking processes ("thinking about thinking"), plus additional factors not clearly delineated.
Bugg, J., Zook, N., DeLosh, E., Davalos, D. & Hasker, D, (2006). Age differences in fluid intelligence: Contributions of general slowing and frontal decline. Brain and Cognition, 62, 9–16 (click here to view)

  • The current study examined the contributions of general slowing and frontal decline to age differences in fluid intelligence. Participants aged 20–89 years completed Block Design, Matrix Reasoning, simple reaction time, choice reaction time, Wisconsin Card Sorting, and Tower of London tasks. Age-related declines in fluid intelligence, speed of processing, and frontal function were observed. Hierarchical regression analyses showed that the processing speed and frontal function measures accounted for significant variance in fluid intelligence performance, but there was also a residual effect of age after controlling for each variable individually as well as both variables. An additional analysis showed that the variance in fluid intelligence that was attributable to processing speed was not fully shared with the variance attributable to frontal function. These findings suggest that the age-related decline in fluid intelligence is due to general slowing and frontal decline, as well as other unidentified factors.
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