Reduction of Inhibitory Control in People with ADHD

Inhibitory control is the ability to voluntarily inhibit prepotent attentional or behavioural responses. Per Barkley (1997a), inhibitory control is the basis of proper functioning of all the executive functions, as well as it being one of the most used cognitive function. It is the way in which the brain corrects a behaviour. It makes it possible to stay quiet when you want to say something inappropriate, to avoid negative thoughts and focus on the positive, to not scratch that bite even though it is itching. Inhibitory abilities have been examined in the laboratory in great depth, using experiments such as ‘The Marshmallow Experiment’ – where children must use inhibitory control to resist eating one marshmallow in order to get two marshmallows – or the Stroop task – a task in which you must say the colour of the word rather than the actual word. In both tasks, you must inhibit the normal response of eating the marshmallow or reading the word. It has been suggested by many psychologists that is a deficit in inhibitory control that causes the behavioural symptoms associated with Attention Deficit Hyperactivity Disorder.

This essay aims to define and describe Attention Deficit Hyperactivity Disorder (ADHD) before recognising existing theories accounting for inhibitory control deficits in ADHD and analysing current research that provides evidence for these deficits. ADHD is a behavioural disorder in which the core symptoms are defined as “inattentiveness, impulsivity and hyperactivity”. Current research throughout westernised cultures shows that between 3-5% of school aged children display hyperactive, impulsive and inattentive behaviours severe enough to meet the guidelines issued in the Diagnostic and Statistical Manual of Mental Disorders-IV to be diagnosed with Attention Deficit Disorder (ADHD). In addition to this, many children experience similar kinds of behaviours without meeting the full criteria for an ADHD diagnosis, making these the most common of all behavioural problems in children (Barkley, 1998). The DSM-IV identifies three different subtypes of ADHD: the combined type, the predominantly hyperactive type and the predominantly inattentive type. In the combined type, a child must display at least six of nine symptoms of hyperactivity, as well as six of nine symptoms of inattention. For the other two subtypes, the child needs only to meet the criteria for one or the other.

There have been theories presented to account for the various inhibitory deficits associated with ADHD. The most comprehensive of these models is the hybrid model of ADHD proposed by Barkley (1997a). According to this model, ADHD is seen mainly as a deficit in inhibitory control. In addition to this, Barkley’s perspective is that inhibition is primary to other executive functions – stating that the first action must always be to inhibit one response, causing a delay in which other executive functions can occur. Stemming from this view, Barkley nominates the ideal what children with ADHD have further problems with different executive functions, these being: non-verbal working memory (the ability to hold an event in mind and use it to control a response), internalisation of speech (more commonly known as verbal working memory, it is short term memory based on what we hear), self-regulation of affect, arousal and motivation (an action an individual directs themselves to result in a change of behaviour or change a future outcome) and reconstitution (the analysis and synthesis of sequences of events into parts, and manipulation these parts, to then reconstruct new events). Therefore, he organised his model in a hierarchical way with a reduced inhibitory control at the top, leading to a reduction in other executive functions below it, all leading to an account of the deficits associated with ADHD, as presented in the model as Motor Control/fluency/syntax. Despite Barkley’s model being the most influential, there are still competing theories that must be acknowledged.

In 1982 Gray presented the Theory of BIS and BAS. This is a neuropsychological theory consisting of three systems that interact with one another: the behavioural activation system (BAS), the behavioural inhibition system (BIS) and the nonspecific arousal system (NAS). The BAS responds to conditioned stimuli for a reward and/or a relief of punishment, the BIS responds to conditioned stimuli for novelty, as well as punishment and a no reward. The NAS responds to unconditional pain and punishment, and is a fight or flight system. Gray (1982) initially used this as a basis for explaining anxiety problems which he stated was as a result of an overactive BIS. In 1997, Quay used this theory to explain the deficits used in ADHD, suggesting that they are associated with an underactive BIS. The fact that anxiety problems and ADHD are both related to the BIS, in opposite directions suggests that the two disorders cannot co-occur, yet around a quarter of those diagnosed with ADHD also meet the criteria for anxiety disorders (Cohen et al. 1993). When explaining, this finding, the BIS, as described by Gray is an inhibitory system that is linked to punishment and reward, completely different to inhibition as described by Barkley. In a working taxonomy of inhibition presented by Nigg (2000), BIS is referred to as a type of motivational inhibition whereas Barkley’s definition of inhibition is referred to as executive inhibition. Making this distinction is important, as it implies that a measure of BIS functioning can only be when executive tasks are performed under motivational conditionsand when the distinction in the types of inhibition is made the evidence of an underactive BIS in ADHD is limited.

Another theory as to how a deficit in inhibition links to ADHD is in Rothbarts theory of effortful control. This evaluates the roll of two different systems, the fear/behavioural inhibition system and effortful control. The fear/behavioural system is a motivational system, but also has modulatory effects via connections with other systems. It is thought to suppress reward-orientated behaviours. Effortful control is an active control system, which is self-regulatory. It allows a dominant response to be inhibited for a subdominant response to be performed. In ADHD, a weakened fear regulation could potentially result in impulsive behaviour, especially as the individual is unable to constrain their behaviour through the system of effortful control.

In terms of research that supports the theories, studies showing a reduced inhibition in those with ADHD are plentiful. For the most part, research is based on Barkley’s model of deficits within ADHD showing lower levels of executive inhibition, as well as a deficit of the other four executive functions. When looking at executive inhibition it has been those with ADHD have lower levels of impulse control as compared to control groups when tested using either stop-signal tasks or the go/no-go paradigm. It has also been shown in two separate studies carried out by Dunn et al. (1998) that poor inhibition is related to hyperactivity. When examining interference control - a type of control that helps filter out distracting information or supress habits or responses that were irrelevant. For tasks with external distractions, those with ADHD are unlikely to differentiate in any way from normal controls. However, when the distractions are inserted into the task, such as a strop task those with ADHD perform significantly worse than those who don’t.

When looking specifically at the other types of executive function included in Barkley’s model, there is many deficits observed in children with ADHD. Through tasks of memory for special location it has been shown that there are significant differences in the non-verbal working memory in children with AHDH. In regards to verbal working memory Mariani and Barkley (1997) used repetition of digit spans to demonstrate a deficit among ADHD sufferers. When a child has severe ADHD symptoms it has been shown that they have delayed internalisation of speech. Further support of deficits in the executive functions in Barkley’s model come from studies that show significant difference between ADHD and controls regarding measures of self-regulation of affect and verbal fluency. It is however important to discuss the studies that have failed to find any significant group differences such as working memory and verbal fluency. On top of this, when evaluating Barkley’s model, it is crucial to consider the studies that have failed to find any significant group differences for inhibition.

Building upon the theoretical and experimental support for inhibitory control there is also several biological studies suggesting there are differences in the physiology of the brain between ADHD and norm. The localizationist theory has associated inhibition with various areas of the frontal cortex. It has also been indicated that Inhibitory control begins emerging within the first postnatal year, continuing to develop through toddler and into preschool years; this pattern is simultaneous with changes in the maturation of the frontal lobe. Structural and functional neuroimaging studies on subjects with ADHD show various abnormalities associated with the disorder. In Structural imaging studies, it has been shown that maturation of the brain is delayed during ADHD, and functional imaging studies indicate abnormalities are predominantly in the frontal cortex (Sowell et al 2003).

Using points described through my discussion, I can conclude that theoretical, experimental and neurobiological research generally support the statement that inhibitory control is significantly reduced in those with ADHD. It is however in debate as to how the deficit in inhibitory control occurs in people diagnosed with ADHD making this an area of psychology that would benefit from further research.

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