SUPERVISORY ATTENTIONAL SYSTEM AND AGE
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Brixton error scores. This suggests that some age-related
differences in executive tasks are linked to more specific
factors than processing speed (see also Kramer et al., 1999;
Spieler, Balota, & Faust, 1996). More generally, a produc-
tive approach of cognitive aging taking into account both
common (or shared) and specific (or unique) age-related in-
fluences is needed.
A final point of this article concerned the fractionation of
the SAS. Partial correlation analysis in which we controlled
the general effect of age gave rise to a disappearance of cor-
relations between Hayling and Brixton tests. This confirms
that the SAS can be fractionated into different independent
executive functions, as already suggested by Shallice (1988,
taken alone (without considering the result that elderly par-
ticipants make more inhibition errors), it is difficult to dis-
tinguish between a diminution of the capacity to generate
nonstereotypical responses and a difficulty in inhibiting
strong stereotypical responses. Researchers should conduct
studies to break down the complex executive tasks into their
different cognitive components and to understand which
particular cognitive processes (e.g., working memory) could
contribute to the age-related differences observed in execu-
tive functions.
Acknowledgments
This work was supported by the Spanish Foreign Office (C.G.R.I. in
Belgium) and by the Camille Hela Foundation (University of Liege) in the
form of a project grant to the first author. The article is based on part of the
first author’s doctoral dissertation at the University of Liege. Pilar Andrés is
now Research Associate at the School of Psychology of Cardiff University.
1
994) and Shallice and Burgess (1993, 1996), on the basis
of dissociations in frontal patients.
Globally, these age-related differences in executive func-
tions are consistent with the studies showing a frontal de-
cline in elderly persons. For example, in the study of Raz
and colleagues (1997), age-related structural differences in
the human cerebral cortex were examined in 148 healthy
volunteers (aged 18–77 years) using in vivo evidence from
magnetic resonance imaging. The most substantial age-
related decline was observed in the volume of the prefrontal
gray matter (4.9% per decade). Smaller or no age-related
differences were found in the other regions, including hip-
pocampal formation and prefrontal white matter. Addition-
ally, Raz, Gunning-Dixon, Head, Dupuis, and Acker (1998)
observed that age-related shrinkage of the prefrontal cortex
might specifically mediate age-related differences in execu-
tive functions such as flexibility (evaluated by the WCST).
Researchers should conduct studies to relate the age-related
differences on TOL, Hayling, and Brixton tests and frontal
dysfunction.
We thank Professor Tim Shallice and Dr. Paul Burgess, University Col-
lege of London, for offering us the material for the Hayling and Brixton
tests before it was published. We also thank Fabrice Parmentier and two
anonymous reviewers for many invaluable comments.
Address correspondence to Pilar Andrés, School of Psychology, Cardiff
University, P.O. Box 901, Cardiff CF10 3YG, United Kingdom. E-mail:
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