The aging of the executive functions: a historical review from the cognitive psychology standpoint

Brief Historical Background in the research of Executive Functions #

As can be found in the Glossary of Diamond’s article on executive functions, Executive Functions are “a collection of top-down control processes used when going on automatic or relying on instinct or intuition would be ill-advised, insufficient, or impossible” (2013, p. 22). The executive functions, or frontal lobe functions, as can be found in earlier literature, have been researched as a result of observations of behaviours of patients with frontal lobe damage or lesions, such as the case of Phineas Gage, as explained by Miyake et al. (2000), or observations such as the one from Luria et al. (1966). It was evident that these patients were lacking certain abilities, as put by Luria et al. in the publication titled Disturbances of active visual perception with lesions of the frontal lobes: “It is easy to see that the patient does not analyze the picture, that all his statements are impulsive and his hypotheses arise from isolated impressions.” (1966, p. 208). With today’s background, we could partially recognize the inability of the patient in this excerpt, as lack of inhibitory control, which now lies at the core of executive functions as explained by Diamond (2013), but back then, what exactly this patient was missing was a mystery.

With the evolution of research on the subject, the executive functions were disconnected from the anatomic principle of being recognized as the results of a well-working frontal lobe, as highlighted by Miyake et al. (2000), and became also associated with other parts of the brain, some of them subcortical (Salehinejad et al., 2021). Additionally, they were disconnected from motor control or behaviour changes, to attain a pure mental nature. The unification between executive functions and actions created by them is evident when taking into account earlier pioneering research, such as the theory of Supervisory Attentional System by Norman & Shallice (1986), which uses examples of real-life actions, classifying them as either deliberate or “automatic”, or publications, such as Guilford’s Executive Functions and a Model of Behaviour (1972), which is also one of the first times the term Executive Function was used. Finally, executive functions were split into well-defined abilities, better decomposing the amalgam of attention control (Norman & Shallice, 1986) and working memory (Hitch & Baddeley, 1976), to purer, more distinct and measurable components.

The later theories have found evidence on such components, with Miyake et al. (2000) enumerating them as mental set shifting, information updating and monitoring and inhibition of prepotent responses, stating them as functions that can be measured satisfactorily individually, to be considered different. Finally, Diamond (2013), states that core executive functions are cognitive flexibility, working memory, and inhibitory control, drawing distinct lines to the results of Miyake et al. (2000).

Executive Functions as of today #

Inhibitory Control #

As stated by Diamond (2013), this is the ability to filter-out, not only our responses and behaviour - as stated by Miyake et al. (2000), but also our internal impulses, as well as our sensory inputs (enabling the cocktail-party effect). Inhibitory control is the ability that enables attention control, by reducing the effect of all mental stimuli, but the chosen one, eliminating distractions (Diamond, 2013).

Working Memory #

This core executive function is what enables the mind to hold information, update it, match it with pre-existing patterns, recall stored information and put it side-by-side with existing to draw conclusions, and generally, can be considered a scratch-pad where all information can be kept shortly to be processed together (Diamond, 2013). While it plays a significant role in speech, its use is broader, storing concepts and ideas (Hitch & Baddeley, 1976), and finally, grants the mind a perception of time (Diamond, 2013).

Cognitive Flexibility #

Finally, the last core executive function, as of Diamond (2013), is responsible for task-switching and perspective change, both spatial (mentalizing a view from a different position) and interpersonal (getting in someone else’s shoes) (Diamond, 2013).

Planning #

As stated by Diamond (2013), planning is considered a higher-order executive function. It is mentioned as it is frequently measured along the core ones, such as in the study of Ferguson et al. (2021), analyzed below. Planning is the ability to identify the goal of a task and create distinct steps towards attaining it (Ferguson et al., 2021).

Why research the effects of aging on executive functions? #

As is evident, a great deal of research is ongoing on executive functions. Apart from acquiring theoretical knowledge on the subject, when this body of research is combined with studying of the mind throughout the human lifetime, it can be impactful on sectors like health and provide suggestions for life-improvement of the elderly.

Early as Brennan et al., (1997), there was no research on normal (non-clinical) aging, with all studies being of a clinical nature. Yet, as explained by Clark et al. (2012) and Harada et al. (2013), decline of executive functions can be symptoms of diseases, such as Alzheimer’s disease, hence having a body of research on normal aging of executive functions, can serve as a control for such studies, comparing healthy and clinical aging patterns (e.g: Kirova et al., 2015; Corbo & Casagrande, 2022).

An additional reason to research the aging of executive functions, is the need to sketch the outline of what tasks can be done safely in older ages (such as driving), and provide assistance when possible (Harada et al., 2013).

Finally, as Ferguson et al. (2021) point out, understanding the cognitive changes during aging can provide information about optimum ages for cognitive training interventions.

Studies on Aging of the Executive Functions #

The study of executive function aging started with the hypothesis that can be simply put as “the older one gets, the more they decline”, as can be seen by the early study by Brennan et al., (1997). This very small study (N=24), with groups of average ages of 19, 65 and 75, confirmed that hypothesis, yet it has done so, during a period when theory was not mature enough to distinguish between the different executive functions. Specifically, in this early study, the Tower of Hanoi has been used as the single instrument of testing, yet, it is known by the later study of Miyake et al., (2000), that such task only taps one of the core executive functions.

Harada et al. (2013), which condenses several studies on cognitive aging, does differentiate between “crystallized” and “fluid” intelligence, explaining that only fluid intelligence does decline with age, peaking between the ages of 30-40. While it directly states that “Executive function, processing speed, memory, and psychomotor ability are considered fluid cognitive domains” (Harada et al., 2013, p. 3), putting core executive functions in the “decline bucket”, it creates the room for other cognitive abilities to stay stable or even enhance during aging. Finally, it states some secondary executive functions that indeed stay stable, such as “similarity appreciation” and “describing the meaning of proverbs” (Harada et al., 2013, p. 5).

It is notable that studies on the matter contained distinct subject groups, of young and elderly people. Yet, a study by Ferguson et al. (2021) committed on exploring the full trajectory of executive functions throughout lifetime, with a sample of 350 subjects in ages 10 to 86 years old, testing for inhibitory control, working memory, cognitive flexibility and planning. The results of this study still generally confirm the hypothesis that executive functions decline in older age, but as its sample size could support, it provided surprising results on the exact onset of this decline, which was found to be 35 years old for inhibitory control, 30 years old for working memory, and late adulthood for cognitive flexibility. This study also seems to unveil that planning, while following the expected decline like the rest executive functions, takes a positive turn during old age. This small finding opens up the possibility of executive functions improvements in older ages, something that disagrees with the initial hypothesis made since the earliest studies.

Finally, the study by Veríssimo et al. (2022) came to further examine executive function improvements on older ages. With a sample size of 702 subjects, in the age group between 58 and 98 years old, this study used the Attention Network model (Petersen & Posner, 2012) that only partially covers the executive functions, as described by Miyake et al., (2000), with inhibitory control being part of its “executive network”. The Attention Network Test (ANT) was used, measuring for attention, orientation and executive networks. The results on executive network revealed a pattern different from the older hypothesis, showing increased efficiency with aging after the average age of 78. It is notable that this study uses a totally different model, and as a result it takes a different approach on tools, drawing further from Tower of Hanoi and Stroop tasks, used in all other studies presented. Finally, in this study the “attention network” showed a steady decline with age, but attention is not a part of executive functions, as explained by Diamond (2013).

Limitations of Studies on Executive Functions #

It is evident that the presented literature cannot provide a congruent result on whether aging only brings decline on executive functions. This can be due to several factors that limit such studies. Harada et al. (2013) states some of them, such as an availability bias on the sample selection. Specifically, the publication states that healthy people would not attend studies as they are too busy and ill people do not attend studies as they cannot, and this behaviour is not controlled for.

Additionally, the socio-economic aspect is mentioned by Harada et al. (2013), regarding sampling. People that attend cohort studies tend to be educated and wealthy enough to be able to spare their time, and socio-economic status does play a role in executive functions, as Ferguson et al. (2021) had to control for it in their study.

Another limitation can be task over-training, as also mentioned by Harada et al. (2013), explaining that Tower of Hanoi and Stroop are tasks that one can become good at, meaning that each new measurement can carry over skill and reflect executive function abilities less. A case of task over-fitting could also be made as the studies of Ferguson et al. (2021) and Veríssimo et al. (2022) delivered very different results on inhibitory control on older age, but also using very different tasks (Stroop and ANT).

Conclusion #

Finally, although a lot of studies have been dedicated to revealing what exactly happens to the aging executive functions, it is evident that there is much more to be learned. As aging is a lifelong process, and not just a matter of the elderly, studies such the one by Ferguson et al. (2021), with very wide age-range samples, can provide great value.