The dual mechanism of cognitive control theory predicts that elderly people tend to spontaneously prefer reactive control to the detriment of proactive control which is more costly in terms of attentional resources. Is this really the case in fact? Fabienne Collette’s group showed, by means of an experimental task, that elderly people resort less easily to proactive control than young people (2). Nevertheless, this detail hides another fact. When the researchers compared young and elderly individuals whose speed at processing information and whose average score in terms of fluid intelligence (the ability to resolve problems)(3) were similar (4), the difference between young and elderly with regard to implementing proactive or reactive processes disappeared. What can be deduced from this? “Rather than being an executive deficit as such, perhaps there are other variables underlying these results such as weaker general resources for the processing of information. We aim to examine this question more closely. Therefore, we are going to do some FMRI studies on elderly individuals with the objective of determining how the level of their resources for processing information influences their cerebral functioning with regard to a Stroop task involving several or just a few interferent items”, says Fabienne Collette.

Another iron in the fire for the researchers is genetic. We know that dopamine is a regulator of executive functions. In an article that appeared in 2014 in the journal Cortex (5), they demonstrate that in young subjects, a higher level of dopamine in the cortex modulates the activation of areas underlying proactive control, in particular the frontal areas. The participants in the experiment were divided into three groups according to the allelic variant of the COMT gene (catechol-O-methyltransferase) which they were carriers of (VV, MM and VM respectively), each variant being associated with a greater or lesser deterioration of the available dopamine. Therefore, as it is less active than the others, the MM allele of the gene allows for the maintenance of a higher level of dopamine. In this way, it improves the way the subjects use the cerebral regions involved in proactive control.

“As the concentration of dopamine diminishes in the elderly, we are next going to study to what extent certain genes, particularly the COMT gene, could contribute to the erosion of executive skills during the ageing process”, adds Fabienne Collette.

Disconnection syndrome

This brings us to Alzheimer’s disease. In the beginning, this condition was considered to be a pathology that initially affected memory due to damage to the hippocampus, then to the temporo-parietal-occipital junction. Later, we realized that the executive skills of patients were also affected early on. How could this be explained? In Liege, Fabienne Collette’s team compared two groups of patients whose overall level of dementia was similar (6). The brains of the former showed typical posterior damage; the second group also, but with frontal damage as well. Through a series of tasks, the researchers firstly confirmed that executive function disorder was a constituent element of Alzheimer’s disease.  What was astonishing, however, was that the extent of these deficits was the same for both groups.

“This means that executive function disorder in Alzheimer patients is principally the fact that information no longer passes correctly between the anterior and posterior regions of the brain. With the development of recent MRI techniques, it should be possible to determine if the problem stems from a deterioration of the white matter or a desynchronization between the cerebral activities”, comments the neuropsychologist. One thing is certain however: the results obtained by the researchers from Liege marry with the theory of Robin Morris of the Institute of Psychiatry, Kings College London, according to whom Alzheimer’s disease is above all a disconnection syndrome disease.

Information
In the context of its studies on normal and pathological ageing, the Cyclotron Research Center of ULg is recruiting volunteers aged 65 years or over.  Contact: telephone (0494/18.17.62) or by e-mail (Agitude@ulg.ac.be).

(2) Manard M., Carabin D., Jaspar M., Collette F. (2014). Do fluid intelligence and processing speed protect from age-related decline in cognitive control? BMC Neuroscience, 15:7. F.I.:3.0.
DOI: 10.1186/1471-2202-15-7. URL: http://www.biomedcentral.com/1471-2202/15/7
(3) Fluid intelligence is distinct from crystallized intelligence which encompasses all our knowledge of the world.
(4) In practical terms, it involved the most high-performing elderly subjects and low-performing young subjects.
(5) Jaspar M., Genon S., Muto V., Meyer C., Manard M., Dideberg V., Bours V., Salmon E., Maquet P., Collette F., Modulating effect of COMT genotype on the brain regions underlying proactive control process during inhibition, Cortex (50), 148-161.
DOI: http://dx.doi.org/10.1016/j.cortex.2013.06.003.
(6) Collette F., Van der Linden M., Delrue G., Salmon E. (2002). Frontal hypometabolism does not explain inhibitory dysfunction in Alzheimer's disease. Alzheimer’s Disease and Associated Disorders, 16 (4), 228-238.