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Response Inhibition also known as impulse control, is the intentional inhibition of a response elicited by a particular stimulus. Inhibition of a behavioural response originates during cognitive processing and results in suppression of a motor response normally elicited by a stimulus. Stimulus information is filtered by higher order processes in the prefrontal cortex to limit the amount of irrelevant information in working memory. Attention and goal-relevant thoughts can then be focused to increase ability to cancel dominant motor responses. A greater ability to inhibit irrelevant information from one’s mind increases performance efficiency, while a lesser ability to inhibit irrelevant information decreases the ability of selective attention. Much advancement in the understanding of inhibitory mechanisms has been made by studying patients with prefrontal lobe damage.

Development of Inhibition
The development of inhibition within an individual has a linear relationship with age, meaning young children will be less able to inhibit unwanted actions than older children, and older children will be less able than adults. A second relationship between age and time of inhibited responding also exists. A young child may inhibit a response half way through a button press inhibition task, while an older child will not even initiate the response.

Inhibition is correlated with the extensive development of the lateral prefrontal cortex during early childhood (3 to 7 years). Inhibitory processes become more specialized and efficient which increases complex cognitive ability. Response inhibition has been shown to activate the inferior frontal gyrus. Decreased response inhibition correlates with a decrease in inferior frontal gyrus activity.

In older adults, there is controversy over whether a relationship between age and inhibition exists. The inhibition-deficit hypothesis states that as adults age, inhibitory processes decline. However, many studies have indicated no decline in inhibitory response with age.

Tasks
Testing response inhibition can be problematic because it involves other cognitive processes such as working memory and attention. The most commonly used tests include the Stroop task, Go/Nogo task, Stop Signal Task, and Antisaccade task

Go/Nogo

A participant is asked to respond to one type of stimulus(go-stimulus), and to inhibit a response to a second type of stimulus(no-go-stimulus). The more often the go-stimulus occurs than the no-go-stimulus makes it more difficult to inhibit a response when the no-go stimulus is presented.

Stop Signal Task

A participant is required to respond quickly and accurately to a stimulus every time it is presented. The participant is then presented with a second stimulus, such as a tone, and asked to inhibit their response to the first stimulus while the second stimulus is present. The stop trials measure the participant’s inhibition by using the stop signal reaction time. Lower inhibition will result in a longer stop signal reaction time.

Antisaccade task

A saccade is a quick movement of both eyes, therefore in the antisaccade task, the participant is asked to avert their gaze from a stimulus. A visual cue is presented to the left or right on a screen, followed by a target on the opposite side. Participants are asked to inhibit looking at the cue and respond to the target by pressing a button indicating the direction of the target. Participants can be monitored for activity in the frontal cortex and eye movements.

Stroop task

Participants are asked to verbally name the color of a congruent color word as fast as possible (ie. The word BLUE written in the color blue). Subsequent trials involve the participant identifying the color of an incongruent color word (ie. The word BLUE written in the color green). This involves the participant inhibiting their tendency to read the word, rather than simply identifying the color.

Inhibitory Control in Psychological Disorders
Inhibition deficits are currently thought to be an important problem for individuals with Attention Deficit Hyperactive Disorder (ADHD), which decreased impulse control is characteristic. MRI studies on individuals with ADHD have identified structural abnormalities in the response inhibition networks of the prefrontal cortex.

Deficits in inhibition are also currently thought to contribute to Obsessive-Compulsive Disorder (OCD) and schizophrenia. ERP and fMRI studies on individuals with OCD have also found physiological abnormalities in the prefrontal cortex. Similar deficits in response inhibition are also found in drug abusers.