Are cognitive changes with age best explainedin terms of a deficit in a single general property ofthe brain, or in terms of differential declines inseparate modular functions? This question hasobsessed and infuriated those studying cognitiveaging for the past two decades, and the issue ofgeneral versus specific functions has been dealtwith more openly in the literature on aging than inother areas of psychology (see e.g. Salthouse,2001). At the physiological level, there is a debateas to whether the effects of normal adult aging onbrain pathology are localized or global. At thepsychological level, there is debate about whetherall cognitive functions change at the same rate withage, or whether change is specific to particularcognitive mechanisms. In recent cognitiveneuropsychological literature there has been areasonably cosy consensus that the frontal lobes ofthe brain are affected by normal adult aging earlierand more rapidly than other brain areas. Thiscoincides nicely with evidence that older adults areimpaired on the cognitive functions most closelyassociated with the frontal lobes: executive controlprocesses such as inhibition, planning and taskswitching. This seems like evidence for specificlocalised change with age.

However: maybe the idea of generalised agedecline in intelligence is happily compatible withthe idea of accelerated age decline in frontal lobeexecutive functions. Age changes in executivefunctions statistically overlap with variance in fluidintelligence (e.g. Crawford et al., 2000). But is itmore useful to describe aging as a general changein intelligence or as a specific change in executivefunctions? I would like to argue that both of theseconceptualisations can be useful, and which ismore useful depends on the type of researchquestion that you ask.

Measures of fluid intelligence have been aroundfor a long time, and tests have been developed thatare highly reliable, predictive, and show dramaticeffects of aging on performance. Age variance inperformance in many cognitive domains overlapswith variance in performance on fluid intelligencetests. So, if your research question is one that asks“how do we best predict age-related variance in x,y or z?” it would be sensible to take a measure offluid intelligence, for psychometric, if not fortheoretical reasons.

Measure of executive functioning can be

separated into two types. Firstly there are ‘frontallobe tests’ which were designed to be sensitive tolocalised lesions (although actually deficits onthese tests are not particularly sensitive or specificindicators of frontal lobe damage). These tasks tendto be complex combinations of functions, as in forexample the Wisconsin Card Sorting Task. Thesetasks are poorly understood in terms of thecognitive processes involved. There tend to be ageeffects on most of these frontal lobe tests, althoughgiven the poor reliability and complex nature of thetasks it could be argued that this finding does littleto illuminate the effects of age on cognition. Thesecond type of ‘executive tasks’ have stemmedfrom the cognitive psychology literature in anattempt to isolate specific cognitive processes suchas inhibition, switching or memory updating.Although this literature is very much a work inprogress, these tasks have already provided someinteresting insights into the nature of cognitivecontrol processes, and the time course andlocalisation of such processes in the brain (e.g.West and Alain, 2000). For some types of researchquestion experimentally manipulating suchexecutive tasks may be a more appropriate methodthan the brute force of intelligence tests. Forexample, it might be of interest to investigatewhether age effects on the ability to switchattention between two tasks differ when there areendogenous versus exogenous cues to switch; or toask whether tasks which require automaticinhibitory processing are immune to the effects ofaging. Such results might have importantimplications for the design of living environmentsor recommending effective methods to older adultsfor learning and memorising information.

To sum up: if a researcher wishes to statisticallypredict age differences on a range of psychologicalmeasures, using indices of fluid intelligence will onaverage provide a useful way of mopping up areasonable portion of the age-related variance. Butthis does not necessarily provide a usefultheoretical mechanism to understand aspects ofprocess. A cognitive neuropsychologist mightargue: so what if an intelligence test whichmeasures a complex mix of cognitive processespredicts variance in lots of other cognitive tasks?Does this provide any kind of theoretical advance?In contrast a psychometrician may argue: so whatif tweaking an inhibition task influences the effects

Cortex, (2005) 41, 236-237

FORUM ON INTELLIGENCEBOTH SPECIFIC FUNCTIONS AND GENERAL ABILITY CAN BE USEFUL:

BUT IT DEPENDS WHAT TYPE OF RESEARCH QUESTION YOU ASK

Louise Phillips

(Psychology Department, Aberdeen University, Aberdeen, Scotland)

Cortex Forum 237

of aging? Do these executive tasks have predictivevalidity? Both methods may have their place inunderstanding cognitive aging, but to addressdifferent types of research question.

REFERENCES

CRAWFORD JR, BRYAN J, LUSZCZ MA, OBONSAWIN MC andSTEWART L. The executive decline hypothesis of cognitiveaging: Do executive deficits qualify as differential deficits and

do they mediate age-related memory decline? Aging,Neuropsychology and Cognition, 7: 9-31, 2000.

SALTHOUSE TA. General and specific age-related influences onneuropsychological variables. In F Boller and SF Cappa(Eds), Handbook of neuropsychology (2nd Ed). Amsterdam:Elsevier Science, 2001, pp. 39-49.

WEST R and ALAIN C. Age-related decline in inhibitory controlcontributes to the increased Stroop effect observed in olderadults. Psychophysiology, 37: 179–189, 2000.

Louise Phillips, Psychology Department, Aberdeen University, Aberdeen, Scotland,UK., AB24 2UB. e-mail louise.phillips@abdn.ac.uk.