Research On The Presence Of Inhibitory Control Deficits In Adhd Patients

Inhibitory control is the ability to voluntarily inhibit prepotent attentional or behavioural responses, it is the basis of proper functioning of all the executive functions, as well as it being one of the most used cognitive function (Barkley & Wasserstein, 2000), and 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’ (Mishel & Ebbesen, 1972) - where children must use inhibitory control to resist eating one marshmallow to get two marshmallows – or the Stroop task (Stroop, 1935) - 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 analyse 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” (American Academy of Paediatrics, 2011). Based on research executed throughout westernised cultures, behaviours severe enough to meet the guidelines issued in the Diagnostic and Statistical Manual of Mental Disorders-IV (DSM-IV) to be diagnosed with ADHD can be seen in around 3-5% of school aged children (Berlin, 2003). Additionally, there is a prevalence of these behaviours in children, without them fulfilling the full criteria for an ADHD diagnosis (American Psychiatric Association, 1994).

This makes hyperactive, impulsive and inattentive the most frequent of all behavioural problems seen 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 exhibit at least six symptoms of hyperactivity, as well as six symptoms of inattention (Berlin, 2003). For the other two subtypes, the child needs only to meet the criteria for one or the other.

Although there have been various theories presented to account for the deficits associated with ADHD. The most comprehensive of these models is the hybrid model, as proposed by Barkley. In this model ADHD is considered as a loss of inhibitory control. Per Barkley, inhibition is the most dominant of all executive functions – he believes that the initial action must be to inhibit one response, therefore creating a delay where alternative executive functions can take place (Barkley, 1997).

Stemming from this view, Barkley nominates the ideal that children with ADHD don’t just have problems with inhibitory control but also struggle with other executive functions, these being: non-verbal working memory (the ability to hold an event in mind and use it to control a response (Goldman-Rakic,1995)), internalisation of speech (more commonly known as verbal working memory, it is short term memory based on what we hear (Daneman & Carpenter,1980)), 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 (Barkley, 1998)) and reconstitution (the analysis of and synthesis of a sequence of events into sections, before manipulating these sections and reconstructing them as new events (Barkley, 1997a)). 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 model, there are still competing theories that must be acknowledged. The neuropsychological theory of BIS and BAS was presented by Gray in 1982. It consists of three systems that interact with each other: the behavioural activation system (BAS), the behavioural inhibition system (BIS) and the nonspecific arousal system (NAS).

Each of the three systems works in response to different motivational stimuli. The BAS acts in response of a stimuli to gain a reward and/or be relived of punishment. The BIS similarly works for the relief of punishment and for novelty however, there is no reward motivation present. The NAS is a fight or flight response, reacting to unconditional pain and punishment. Initially, Gray (1982) used this basis as a way of explaining troubles with anxiety, suggesting these could be attributed to an overactive BIS. In 1997 Quay, utilised Gray’s theory of BIS and BAS to explain deficits seen in those with ADHD and proposed that these deficits are associated with an underactive BIS. Quays conclusions meant that both anxiety problems and ADHD were associated with the BIS but in opposing directions.

This would indicate that the two conditions cannot co-occur, yet around 25% of those diagnosed with ADHD also show enough indicators of anxiety disorder to qualify for an official diagnosis (Cohen et al., 1993 as cited in Berlin, 2003). Additionally, the definitions of inhibition as proposed by Gray -motivational inhibition – and Barkley – executive inhibition - are completely different, with Gray’s theory implying that BIS functioning can only occur when executive tasks are performed under motivational conditions (Nigg, 2000). This distinction is important, as when it made there is minimal evidence supporting the presence of an underactive BIS in ADHD (Nigg, 2000).

Another theory linking deficits in inhibitory control to ADHD is Rothbarts’ theory of effortful control (Rothbart & Derryberry, 1997). This theory evaluates the role of the fear/behavioural inhibition system and the effortful control system. The fear/behavioural system is primarily a motivational system that is thought to supress reward orientated behaviours, it also has regulatory effects via relationships with other systems. Effortful control is an active, self-regulated control system that allows a dominant response to be inhibited, for a subdominant response to be performed. In ADHD, a weak fear regulation could potentially result in impulsive behaviour, especially as the individual is unable to stifle their behaviour with use of effortful control.

When looking at research that supports these theories, studies showing deficits of inhibitory control in ADHD are plentiful. Mostly research is based on Barkley’s hybrid model and these studies generally support the idea that there are lower levels of executive inhibition in those with AHDH, accompanied by deficits in the other four executive functions.

In terms of executive inhibition, it has been shown that those with ADHD have lower levels of impulse control as compared to control groups when tested using either stop-signal tasks (Oosterlaan & Sergeant, 1996) or the go/no-go paradigm (Shue & Douglas, 1992). It has also been shown in two separate studies carried out by Dunn et al. (1998;2000) 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 (Douglas, 1983). However, when diversions are inserted into a task, for example the Stroop task (Stroop, 1935) those with ADHD perform significantly worse than the control group. (Barkley, Grodzinsky & DuPaul, 1992).

Respectively analysing the executive functions described in the hybrid model (Barkley, 1999a), it has been shown that there are many deficits 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 (Mariani and Barkley, 1997). 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 (Berk & Landau 1992; Winsler et al 1999).

Further to this, there are various studies indicating a significant difference in measures of self-regulation and verbal fluency between controls and subjects with an ADHD diagnosis (Berlin, 2003). In contrast, when assessing Barkley’s theory, it is also important to consider the research neglecting to find anything noteworthy when investigating group differences in executive functions such as working memory (Kerns, McInerney & Wilde, 2001) and verbal fluency (Fisher et al. 1990), as well as executive inhibition (Kuntsi, Oosteralaan & Stevenson, 2001).

Building upon the theoretical and experimental support for inhibitory control there are 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 (Aron et al. 2004). 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 (Diamond, 2002). 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 (Vaidya, 2011), 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.