Executive function
Overview of executive function
Executive function can be described as the exertion of free will over more reflexive attentional mechanisms (Posner, 1990). It enables us to make plans and use selective attention. We can also use executive function to manage emotions and modify thought patterns. Many view executive function at it’s core to consist of three modalities: working memory, cognitive flexibility, and inhibitory control. You may already be familiar with the concept of working memory, which is the ability to rehearse information in your mind and manipulate it.
Theories of executive function
Posner’s attentional networks
Mike Posner’s theory of attentional networks is an older but popular theory to explain which cognitive processes are necessary to complete goal-driven behavior. The theory of attentional networks consists of three parts: orienting, target detection, and alerting behavior (Posner, 1990).
Orienting
Orienting is the process of looking at an object; it’s driven by a covert shift in attention. We call the shift in attention covert because it happens cognitively–that is, there is no observable behavior to tell us when it initially happens. However, just after this shift in attention occurs, the eyes move to observe the stimuli to which it has been shifted. Selective attention is supported by orienting. We can attend selectively to space if we know where to look for a target beforehand. If the target appears outside of the area we are selectively attending to, then there is a cost in terms of reaction time. People with Alzheimer’s disease can identify targets just as quickly as healthy individuals as long as the target appears where expected. However, people with Alzheimer’s take much longer to respond than controls if the target appears outside the anticipated field (Perry, 1999).
Target detection
There are two major states of attention: to be alert and attending generally to the environment, or to be oriented to a specific stimuli and focusing intently on it. The process of target detection is the mechanism by which we move from the first state to the second.
Alerting
Being alert means being in a state of readiness or anticipation. It depends highly on the health of the right brain hemisphere (Posner, 1990).
Miller and Cohen’s integrative theory of prefrontal cortex function
Executive function appears to be controlled most strongly by the prefrontal cortex (Miller, 2001). This area of the brain is located just behind your forehead. It has been shown that information about goals is maintained by neural activity in this area. The neural firing continues while each step of a plan is carried out, and that sustained maintenance is remarkable. This suggests that executive function allows us to remember not only our goals, but information about the steps necessary to achieve them. Neurons in the prefrontal cortex are strongly interconnected among motor and limbic areas of the brain. Executive function enables selective attention, which is a way of filtering out unimportant stimuli so we can focus on what’s really important to us. Actions taken to achieve higher level goals and aspirations are caused by “top-down” processes.
How cognitive tests are used to study executive function
The Stroop task measures our ability to attend to and select stimuli which are weaker but goal-relevant in the face of stronger, yet irrelevant characteristics (Miller, 2001).
The Flanker task tests a person’s ability to inhibit responses in accordance with the context of each trial.
The Simon task measures the ability to inhibit responses and switch your selected attention between two sides of the screen. Click here to try a demo version of the Simon task on your home PC.
Popular tests for measuring executive function in clinical dementia research settings
Seven cognitive tests have been identified as the most frequently used for measuring aging-related changes in executive function (Faria, 2015). Researchers and clinicians are generally interested in measuring mental flexibility, verbal fluency, planning, working memory, and inhibitory control. Most of these executive function tests are relatively simple in terms of the materials needed, making them very accessible and easy to use in a variety of settings. It should be noted that the Stroop test is one of the seven most popular tests of this type, however, we will not cover it in the list below since it has already been described in the section above.
Trail Making
Like the children’s pastime of connect-the-dots, the objective of the Trail Making Test is to draw a line between sequential stimuli as quickly as possible. This could mean a series of dots labeled with “a, b, c…” or “1, 2, 3…”. There are some variants of the task, for example, a sequence of “a, 1, b, 2…” instead of alphabetic or numeric characters alone. In order to perform well on this test, one must be able to maintain the rules in memory while switching attention among the stimuli.
Verbal Fluency
A Verbal Fluency Test measures executive function by requiring the participant to name as many words in a category as possible. Categories can be phonemic (words that start with A) or semantic (farm animals). In cognitively normal participants, it is expected that the rate of word production will slow down over the course of the 60-second trial. Patients with dementia perform significantly worse on this task than healthy controls.
Clock-Drawing
The Clock-Drawing Test is for identifying executive function problems. It does this by requiring participants to draw a clock showing a certain time. Clinicians like to use it as a screening tool because it’s quick to administer and well-tolerated by patients. This test is also sensitive to other neurological disorders besides dementia, such as hemispheric neglect. Results of the clock-drawing test are strongly correlated with those of other cognitive function tests, as well as with neurological evidence of executive function impairment.
Digit Span: forward and backward
Digit Span tests are a very old and simple measure of working memory. They consist of a series of numbers or letters presented in sequence. The participant is asked to recall as many items as possible. There are forward and backward variants of the test; in the backward version, participants are asked to recall items in the opposite sequence as presented. Scores on this executive function test decrease overall with age. For adults over 50, the average score is between 4 and 5.
Wisconsin Card Sorting Test
This test uses cards with colored symbols on them to test whether the participant can follow sorting rules. Rules for sorting the cards change as soon as ten cards have been sorted correctly (Faria, 2015).
References
Miller, E. K., Cohen, J. D. (2001) An Integrative Theory of Prefrontal Cortex Function. Annual Review of Neuroscience 24: 167-202. PDF
Posner, M. I, Petersen, S. E. (1990). The Attention System of the Human Brain. Annual Review of Neuroscience 13: 25-42.PDF
Faria, C. d. A., Alves, H. V. D., Charcat-Fichman, H. (2015). The Nost Frequently Used Tests for Assessing Executive Functions in Aging. Dementia & Neuropsychologia 9(2): 149-155. PDF