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    Neurocognitive endophenotypes of impulsivity and compulsivity: towards

    dimensional psychiatry

    Trevor Robbins, Claire Gillan, Dana Smith, Sanne de Wit, and Karen Ersche

    The authors of this article aim to fix the problem of the lack of biological footing in the

    DSM by encouraging a new approach to psychiatry based on neurocognitiveendophenotypes.

    Currently, diagnoses in clinical psychology can be difficult due to complex and

    heterogeneous symptoms in any given disorder combined with the array of

    comorbidities and other disorders a patient may have. DSM criteria are often based

    more upon descriptions of superficial behavioural signs and verbal reports of patients

    and associates rather than being rooted in biology.

    Neurocognitive endophenotypes are well-defined behavioural or cognitive processes

    associated with discrete deficits in defined neural systems. Often these present in

    first-degree relatives of patients due to genetic components. Methods involving theseendophenotypes allow for early detection of disorders, avoid exclusive uses of clinical

    rating scales, and deal with the issue of comorbidities and multiple diagnoses. The

    article focuses on a broad range of disorders: substance dependence, OCD, ADHD,

    eating disorders all of which could be labeled impulsive-compulsive disorders.

    Impulsivityis a trait leading to actions which are poorly conceived, prematurely

    expressed, risky or inappropriate to the situation and that often result in negative

    consequences. Compulsivityis similar to Impulsivity in that it involves a failure of

    response inhibition. However, it leads to actions inappropriate to the situation which

    persist and have no obvious relationship to the overall goal.

    OCD

    OCD patients and their families are predisposed to Tourettes syndrome. They share

    common dysfunctions in frontal-striatal circuits and various other deficits rooted in

    biological causes. These deficits provide a potential biological basis for compulsivity

    as a neurocognitive endophenotype.

    Drug addiction

    Impulsivity and compulsivity are both evident in drug-dependent individuals. High

    impulsivity is predisposing for the development of compulsive drug-taking as seen in

    animal and human studies detailed in the article. Drug users and their siblings both

    rate high in impulsivity, which could indicate a familial vulnerability. This fitsendophenotype criteria and indicates a genetic basis. Drug users and their siblings

    were also found to be almost equivalently impaired on a test of response inhibition

    which involves white matter changes.

    ADHD

    Impulsivity is a key symptom of ADHD. ADHD patients have a propensity for drug

    abuse as the disorder is highly comorbid with drug-dependency.

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    OCD and Drug Dependence

    The compulsive nature of drug abuse is supported by similarities in brain structure

    and functional abnormalities that are found in both substance dependent individuals,

    and OCD patients (such as decreases in OFC gray matter density). Drug-dependent

    individuals and their non-dependent siblings report high levels of obsessive-compulsive behaviour indicating an underlying factor or endophenotype.

    OCD, Substance abuse, and Compulsive Eating Disorders

    There is high comorbidity between drug use and binge eating. The conditions share

    numerous behavioural and physiological similarities, and diagnostic criteria. Both are

    characterized by altered activity of the midbrain dopaminergic system and various

    other physical changes.

    Conclusion

    Due to the common behaviours and biological traits seen among substance users,

    compulsive eaters and patients with OCD or ADHD, the authors argue that there is astrong case for an underlying link between these disorders. Eventually, tests of

    compulsivity and impulsivity will hopefully enhance diagnostic systems such as the

    DSM by providing objective, quantitative assessments of impulsive-compulsive

    syndromes.

    Discussion:

    Do Robbins et al. have sufficient evidence to claim the existence of these

    neurocognitive endophenotypes? Why or why not? Does a fully objective system

    ignore the value of the insights of experienced clinicians?

    Drug addiction, gambling, and eating disorders are not higher in OCD patients.Robbins et al. claim this lack of comorbidity does not necessarily mean they dont all

    stem from a common endophenotypes as they could be expressed in superficially

    contrasting ways. Considering the fact that correlations would be expected if there

    were an underlying factor, is the authors justification adequate?

    Multiple Choice Question:

    Which of the following characteristics is a trait associated with the neurocognitive

    endophenotype of compulsivity and NOT impulsivity:

    A Action performed is inappropriate for situation

    B Action leads to undesirable consequences

    C Major component of drug dependenceD Failure of response inhibition (top-down control)

    E Actions persist without specific goals