URJHS Volume 7

Inhibitory Control in Psychiatric Disorders—A Review of Neuropsychological and Neuroimaging Research

Thomas Hugh Richardson
Trinity College, Dublin, Ireland

Abstract

Inhibitory control is an important component of executive function that allows for the suppression of actions and resistance to interference from irrelevant stimuli. Dysfunctional inhibitory control is increasingly being recognised as an important component to a number of neuropsychiatric disorders. This paper reviews neuropsychological and neuroimaging research which documents impaired inhibitory control in Attention Deficit Hyperactivity Disorder, Autism, Tourette’s syndrome, Obsessive-Compulsive Disorder, and Schizophrenia, with a focus on different patterns of impaired cognitive and motor inhibition and related activity in the fronto-striatal circuitry.

Introduction

Inhibitory control can be defined as a range of mechanisms that allow the suppression of previously activated cognitions and inappropriate actions and resistance to interference from irrelevant stimuli (Bjorklund & Harnishfegar, 1995). Essentially, inhibitory control is the ability to suppress the processing or expression of information that would disrupt the efficient completion of the goal at hand (Dempster, 1992). This key executive function allows for the control of complex cognition and behaviour and is essential for effective interaction with our environment (Burke et al., 1991). Inhibitory control is important in attention, memory, and intelligence and is related to social competence and emotional regulation (Kochanska et al., 1996). Inhibitory control is also related to impairment in a range of different neuropsychiatric disorders (Peterson et al., 1998), producing different patterns of deficits in clinical populations. This essay will examine inhibitory control as evidenced by neuropsychological findings and attempt to examine the neural correlates of such deficits, focusing on five neuropsychiatric disorders in which inhibitory control is particularly important: Attention Deficit Hyperactivity Disorder, Autism, Tourette’s syndrome, Obsessive-Compulsive Disorder, and Schizophrenia.

ADHD

Attention Deficit Hyperactivity Disorder (ADHD) is a mental illness usually beginning in childhood, consisting of symptoms of inattention, impulsiveness, and hyperactivity that are inappropriate for the child’s age (APA, 2000). Inhibitory control is increasingly being recognised as central to ADHD, as a defining characteristic of ADHD is a persistent inability to control behaviour in a number of areas (Durston, 2003). Barkley (1997) proposed that an initial inability to inhibit a inappropriate pre-potent response causes all the other cognitive deficits observed in ADHD, such as problems with working memory. Thus, for Barkley, ADHD is a disorder that is mediated entirely by poor inhibitory control. In everyday settings, ADHD children demonstrate high levels of behaviour signalling poor inhibitory control, such as talking to themselves (Berk & Potts, 1991). They also have problems stopping specific behaviours when asked to do so (Barkley & Ullman, 1975) and have problems resisting temptation (Hinshaw, Simmel & Heller, 1995) and delaying gratification (Barkley & Ullman, 1975). A number of experimental findings also demonstrate poor inhibitory control in ADHD. In tasks requiring inhibition of a prepotent response, ADHD subjects commit more errors of commission (Vaidya et al., 1998) and demonstrate higher levels of interference suppression (Scheres et al., 2003). Performance is impaired on tasks of motor inhibition, such as delayed response tasks (Sonuga-Barke et al. 1992) and stop-signal tasks (Schachar & Logan, 1990). ADHD children also show deficits on cognitive inhibition tasks such as the Wisconsin Card Sorting Test and Stroop Test (Barkley, Grodzinsky, & DuPaul, 1992). Deficient inhibitory control is also apparent in those at risk of ADHD (Crosbie & Schachar, 2001), suggesting that it may be a causal factor. Whilst most models accept that inhibitory control is not the sole causal factor for ADHD symptomatology (Willicutt et al., 2005), it is certainly a key symptom in the illness, which is ameliorated through successful medication-based treatment (Aron et al., 2003).

Neuroimaging research is beginning to shed light on the neurological mechanisms underlying impaired inhibitory control in ADHD. During a Go/NoGo task, striatal activation has been found to be reduced in children and adolescents with ADHD (Durston et al., 2003; Rubia et al., 1999), whilst activity in ventrolateral prefrontal cortical areas is increased (Schulz et al., 2004). Using the same task, prefrontal cortical activity has been found to be reduced in ADHD children (Durston et al., 2003). This suggests fronto-striatal deficits mediate impaired motor inhibitory control in ADHD. Cognitive inhibition has also been studied, with reduced anterior cingulate cortex activity apparent during performance of the Stroop Task in ADHD adolescents (Bush et al., 1999). Research using transcranial magnetic stimulation suggests impaired inhibitory mechanisms in the motor regulation system in ADHD (Moll et al., 2000). Event-Related-Potential studies have shown reduced N2 amplitudes during NoGo trials in ADHD, and that wave amplitude correlates with performance (Overtoom et al., 2002). Finally, research has suggested greater frontal involvement in correct NoGo trials (Vaidya et al., 1998), essentially suggesting a greater effort to correctly inhibit a response.

Autism

Autism is a pervasive developmental disorder characterised by deficits in language and social interactions and repetitive stereotyped behaviour, with an onset usually before age 3 (APA, 2000). In recent years the executive function model of autism (Ozonoff et al., 1991) has received increasing interest. Within this executive function model, inhibitory control has been found to be disrupted in autism. A number of the symptoms of autism such as context-inappropriate behaviour suggest that response inhibition processes are impaired (Kana et al., 2007). Prepotent inhibition as measured by the Go/NoGo task has been found to be impaired in autistic individuals, even after controlling for IQ (Ozonoff et al., 1994). Performance on the NEPSY Knock-Tap task further demonstrates such deficits (Joseph, McGrath & Tager-Flusberg, 2005), as does performance on eye-movement inhibition tasks (Luna et al., 2006). Cognitive inhibition also appears to be impaired, as those with autism perform poorly on the Stroop task (Kleinhans, Akshoomoff & Delis, 2005) and Wisconsin Card Sorting Task (Ozonoff & McEvoy, 1994). Increased preservative behaviour on cognitive inhibition tasks is also apparent (Ozonoff, Pennington & Rogers, 1991; Prior & Hoffman, 1990). Failures of inhibition of a prepotent response appear to be particularly persistent in autism (Ozonoff et al., 1994). In normal children, inhibitory control has been found to be strongly related to the development of theory of mind (Carlson & Moses, 2001; Carlson, Moses & Claxton, 2004). Therefore it is plausible that an early deficit in inhibitory control may represent a causal factor for impaired theory of mind in autism. Inhibitory control impairments have been hypothesised to underlie a number of autistic symptoms, such as repetitive behaviours and obsessionality (Schmitz et al, 2006), cognitive and motor inflexibility (Baron-Cohen, 2004), and language deficits (Joseph & Tager-Flusberg, 2004).

There is also research suggesting a neurological basis for such impairments in inhibitory control in autism. Anagnostou et al. (2006) used an fMRI scanner during performance of a Go/Nogo task in autistic individuals, finding that subjects displayed decreased activation in the caudate nucleus and anterior cingulate cortex. This suggests that problems with frontostriatal circuitry in autism may relate to repetitive behaviours. Kana et al. (2007) replicated this finding of decreased activation in the anterior cingulate cortex and also found that autistic individuals show poor synchronisation between elements of the inhibition network, namely the insula and anterior and middle cingulate gyrus. Sub-activation was also found in inferior frontal and right inferior parietal regions. The authors suggest that this indicates that in autism, inhibitory neural mechanisms are under-activated and poorly synchronised, so that inhibition is successfully achieved by voluntary control rather than automatic mechanisms (Kana et al., 2007). This is further supported by the finding that autistic individuals show increased activation in the left insula, left inferior, and orbital frontal gyrus, suggesting an increased effort to inhibit responses (Schmitz et al., 2006).

Tourette’s Syndrome

Tourette’s syndrome (TS) is a neuropsychiatric disorder characterised by chronic vocal and motor tics (APA, 2000), which usually present at 5-7 years old (Robertson, 2000). A number of symptoms in TS such as ecoholalia, echopraxia, copropraxia, coprolalia, repetitive movement, urges to perform socially inappropriate acts, and other ‘disinhibited’ behaviours suggest a failure of inhibitory control (Cohen & Leckman, 1992; Kurlan et al., 1996). As such it has been suggested that impaired inhibitory control may relate to the core symptoms of TS (Rankins, Bradshaw & Georgiou-Karistianis, 2006). Until recently, such behaviours and tics were assumed to be involuntary, representing a total failure of inhibitory control. However, recent evidence suggests that these behaviours are at least partially voluntary, suggesting compromised but intact inhibitory control mechanisms. TS patients are impaired in interrupting the execution of intended movements and have difficulty in not processing irrelevant distracter stimuli when instructed to do so (Baron-Cohen et al., 1994; Ozonoff et al., 1998). Abnormalities of movement and timing errors on anti-saccade eye movement trials suggest an inability to inhibit planned motor sequences (LeVasseur et al., 2001). TS patients show increased negative priming, with increased severity of symptoms predicting poorer performance (Ozonoff et al., 1998). Work using the Simon effect paradigm suggests that impaired inhibitory control becomes more pronounced with increasing complex demands (Rankins et al., 2006). Children with TS have been shown to be impaired in their ability to inhibit both motor and verbal prepotent responses (Baron-Cohen et al., 1994).

Such inhibitory control impairments in TS have traditionally been thought to reflect impaired neurological mechanisms, and research is increasingly demonstrating this to be the case. Tic suppression has been found to relate to activity in the basal ganglia and thalamus and the prefrontal cortex (Peterson et al., 1998), suggesting these areas are important for inhibitory control over unwanted impulses. Event-related potentials during performance of a STOP-signal paradigm suggest that impaired frontal inhibitory mechanisms are present in TS (Johannes et al., 2001). The inability of TS patients to inhibit the completion of already planned motor programmes has been related to altered cortical-basal ganglia circuitry (LeVasseur et al., 2001). Peterson et al. (1998) found a number of additional active areas during tic suppression, including the right posterior cingulate, ventral putamen, and ventral globus pallidus and increased activity in right anterior cingulated, right midfrontal cortex, and right ventral caudate. Such activity was correlated with symptom severity, especially in subcortical regions. The authors consequently suggested that successful tic inhibition depends on cortical-striato-thalamo-cortical circuitry (Peterson et al., 1998). Structural abnormalities have been found in the orbital frontal cortex (Peterson et al., 2001), a region implicated in inhibitory control, and the size of the orbital frontal cortex correlates with symptom severity (Goldman-Rakic, 1987), suggesting the importance of disrupted orbitofrontal inhibitory control mechanisms in TS.

OCD

Obsessive-Compulsive Disorder (OCD) is a mental illness characterised by obsessions and compulsions that are inappropriate, frequent, and sufficiently intense to cause distress (APA, 2000). OCD has long been hypothesised to be the result of a dysfunction in inhibitory control mechanisms (Chamberlain et al., 2005). Many of the key symptoms and clinical features of OCD such as intrusive and troubling thoughts and compulsive and repetitive behaviours strongly suggest inhibitory impairment (Krikorian, Zimmerman & Fleck, 2004). On a number of different tasks, patients with OCD demonstrate impaired inhibitory control. In oculomotor tasks requiring the suppression of eye movements, OCD patients are impaired (Rosenberg et al., 1997). OCD patients commit more errors on tasks requiring response inhibition (Bannon et al., 2002) and demonstrate decreased response inhibition when instructed to respond to a stimuli that has previously been ignored (Enright & Beech, 1993). Cognitive inhibition appears to be disrupted in OCD, with poor performance on the Stroop test (Penades et al., 2005). Findings using the negative priming paradigm further support this, and such deficits may be particularly strong for those with symptoms involving compulsive checking (Enright, 1996). Motor inhibitory control also appears to be jeopardised, with OCD patients showing poor performance on the Go/NoGo task (Penades et al., 2007; Watkins et al., 2005), and such measures correlating with symptom severity (Bannon et al., 2002). Such motor impulsivity present not only in OCD patients but also in their relatives, suggesting that impaired motor inhibition may represent an endophenotype for OCD (Chamberlain et al., 2007). Such problems with cognitive and motor inhibitory control may underlie not only the primary symptoms of OCD but also the secondary executive function deficits observed in the disorder (Logan & Cowan, 1984).

Such impairments in cognitive and motor inhibition are in line with neuropsychological models of OCD that suggest poor functioning of the frontostriatal circuits of the brain which mediate functions of inhibitory control (Penades et al., 2007). In particular, inhibitory failings on tasks such as the Go/NoGo task suggest a lateral orbitofrontal loop dysfunction in OCD. During NoGo trials, Herrmann et al. (2003) found reduced frontal electrical activity in OCD patients, and such activity was negatively correlated with symptom severity. Malloy et al. (1989) found that during a Go/NoGo task, the P300 event-related potential had a significantly lower amplitude in OCD patients, and Johannes et al. (2001) found altered event-related potentials during a STOP signal paradigm, suggesting a decreased frontal activity may underlie decreased inhibition in the illness. fMRI research has demonstrated that during a Stop task, OCD patients show reduced activity in a number of areas including the basal ganglia and right orbitofrontal cortex and reduced frontal and tempoparietal activity during interference inhibition (Woolley et al., 2008). These findings suggest that dysregulation of the temporparietal areas of the brain underlie impaired cognitive inhibition observed in OCD, whilst attenuated frontostriatothalamic activity underlies impaired motor inhibition. Reduced activation of the anterior cingulate during an inhibitory task has also been demonstrated (Maltby et al., 2005), and symptom severity has been found to be negatively correlated with anterior cingulate gyri activity during such tasks (Roth et al., 2007). Different types of inhibitory control impairment underlie types of symptoms in OCD, and research suggests that such symptom dimensions may have distinct neural correlates (Mataix-Cols et al., 2004).

Schizophrenia

Schizophrenia is a mental illness characterised by a wide range of symptoms including hallucinations and delusions, disordered thinking, poverty of speech, impaired emotional reactivity, and avolition (APA, 2000). A number of neuropsychological deficits have been observed in this disorder, and impaired inhibitory control is increasingly being recognised as an important marker. Schizophrenia is strongly associated with an inability to inhibit prepotent responses once commenced (Ford et al., 2004; Roberts & Pennington, 1996), and such impairment has been described as a core feature of schizophrenia (Brownstein et al., 2003). Patients have severe difficulty in inhibiting automatic eye movement by suppressing prosaccades (Clementz, McDowell & Zisook, 1994; Clementz & Sweeney, 1990) and make more errors on NoGo trials (Kiehl et al., 2000), suggesting impaired motor inhibition. Deficits are also found on measures of cognitive inhibition, in particular the Stroop task (Braver et al, 2001; Everett, Latplante & Thomas, 1989) and negative priming paradigm (Salo et al., 1997). Such impairments are found in those who have a genetic predisposition to schizophrenia (Ross et al., 1998), and as such a failure of inhibitory control has been hypothesised to represent a key endophenotype for schizophrenia (Cadenhead et al., 2002). Poor inhibitory control may also explain high levels of co-morbid substance abuse associated with the disorder (Chambers, Krystal & Self, 2001) and predict violent offending in patients (Enticott et al., 2008). Impaired intentional control of intrusive cognitions has been linked to a predisposition to hallucinations (Paulik, Badcock & Murray, 2008) and frequency of hallucinations in schizophrenic patients (Waters et al., 2006). Poor inhibitory control also predicts poor occupational and social outcome (Reeder et al., 2004) and greater symptom severity (Donohue et al., 2006). Impaired inhibitory control is particularly evident in patients with high levels of negative symptoms (Ettinger et al., 2006; Karoumi, Ventre-Dominey & Dalery, 1998).

Research is increasingly documenting the neural correlates of such poor inhibitory control mechanisms in schizophrenia. Ford et al. (2004) found that healthy participants showed a wider range of activity during NoGo trials than did schizophrenic subjects, suggesting a less complex neural mechanism for inhibiting responses in schizophrenia. Rubia et al. (2001) showed that even when schizophrenic subjects performed equally well as controls on NoGo trials, they showed reduced activation in the anterior cingulated cortex and left rostral dorsolateral prefrontal cortex. Similar effects have been found in the thalamus and putamen (Rubia et al., 2001) and right ventrolateral prefrontal cortex (Kaladjian et al., 2007), suggesting that neural mechanisms involved in inhibiting prepotent motor responses are consistently underactive in schizophrenia. Schizophrenic patients also demonstrate reduced anterior cingulate activation during performance on measures of cognitive inhibition such as the Stroop task (Kerns et al., 2005) and the continuous performance task (Fallgatter et al., 2003). White matter abnormalities in the right inferior frontal gyrus have been found to predict impulsivity in schizophrenia (Hoptman et al., 2002), and it has been suggested that such impulsivity results from dysfunction of fronto-temporo-limbic circuits (Hoptman et al., 2004).

Conclusion

In conclusion, deficient cognitive and motor inhibitory control represents a key component of a number of different neuropsychiatric disorders. Inhibitory control deficits have been proposed to represent a main symptom in ADHD, from which all secondary symptoms and cognitive deficits arise. Poor response inhibition is evident in autism through a variety of both motor and cognitive tasks and may be a significant neuropsychological casual factor. Recent evidence suggests that tics in Tourette’s syndrome are voluntary and are due to an impaired ability to control such impulses. In OCD, inhibitory control appears to be related to an inability to stop inappropriate thoughts intruding, and consequently relate to the compulsions that characterise the illness. In schizophrenia, a wide range of symptoms such as hallucinations may be related to poor inhibitory control, in particular an inability to inhibit prepotent responses. Within all these disorders, fMRI and Event-related potentials have demonstrated functional and occasionally structural deficits in fronto-striatal circuitry. Such deficits are observed in terms of reduced activation, increased activation, and reduced inter-connectivity between regions. Such findings demonstrate the importance of this system in effective inhibitory control and give insight into how impairment of this system result in symptoms of disinhibition commonly observed in psychiatric patients. As such, focusing on the neural correlates of impaired inhibitory control in clinical populations may lead to important new therapeutic and pharmacological treatments.

References

American Psychiatric Association. (2000). Diagnostic and Statistical Manual for Mental Disorders, 4 th edition, text-revision. Washington, D.C.

Anagnostou, E., Sorrya, L., Fan, J., Stamper, K., & Hollander, E. (2006). fMRI for the study of response inhibition, and face and linguistic processing in autism. Neuropeadiatrics, 37.

Aron, A. R., Dowson, J. H., Sahakian, B. J., & Robbins, T. W. (2003). Methylphenidate Improves Response Inhibition in Adults with Attention-Deficit/Hyperactivity Disorder. Biological Psychiatry, 54, 1465–1468.

Bannon, S., Gonaslvez, C. J., Croft, R. J., & Boyce, P. M. (2002). Response inhibition deficits in obsessive–compulsive disorder. Psychiatry Research, 110, 165–174.

Barkley, R. A. (1997). Behavioural inhibition, sustained attention, and executive functions: constructing a unifying theory of ADHD. Psychological Bulletin, 121, 65–94.

Barkley, R. A., Grodzinsky, G., & DuPaul, G. (1992). Frontal lobe functions in attention deficit disorder with and without hyperactivity: A review and research report. Journal of Abnormal Child Psychology, 20, 163-188.

Barkley, R. A., & Ullman, D. G. (1975). A comparison of objective measures of activity level and distractibility in hyperactive and nonhyperactive children. Journal of Abnormal Child Psychology, 3, 213-244.

Baron-Cohen, S. (2004). The cognitive neuroscience of autism. Journal of Neurology, Neurosurgery and Psychiatry, 75, 945–948.

Baron-Cohen, S., Cross, P., Crowson, M., & Robertson, M. (1994). Can children with Gilles de la Tourette syndrome edit their intentions? Psychological Medicine, 24(1), 29–40.

Berk, L. E., & Potts, M. K. (1991). Development and functional significance of private speech among attention-deficit hyperactivity disorder and normal boys. Journal of Abnormal Child Psychology, 19, 357- 377.

Bjorklund, D. F., & Harnishfeger, K. K. (1995). The evolution of inhibition mechanisms and their role in human cognition and behavior. In: Dempster, F, N. & Brainerd, C, J. (Eds.). Interference and inhibition in cognition . San Diego: Academic Press, 142-69.

Braver, T. S., Barch, D. M., Gray, J. R., Molfese, D. L., & Snyder, A. (2001). Anterior cingulate cortex and response conflict: effects of frequency, inhibition and errors. Cerebral Cortex, 11, 825-836.

Brownstein, J., Krastoshevsky, O., McCollum, C., Kundamal, S., Matthysse, S., Holzman, P. S., Mendell, N. R., & Levy, D. L. (2003). Antisaccade performance is abnormal in schizophrenia patients but not in their biological relatives. Schizophrenia Research, 63, 13–25.

Burke, W., Zencius, A., Wesolowski, M., & Doubleday, F. (1991). Improving executive function disorders in brain-injured clients. Brain Injury, 5, 241–252.

Bush, G., Frazier, J. A., Rauch, S. L., Seidman, L. J., Whalen, P. J., Jenike, M. A., Rosen, B. R., & Biederman, J. (1999). Anterior cingulate cortex dysfunction in attention-deficit/hyperactivity disorder revealed by fMRI and the Counting Stroop. Biological Psychiatry, 45, 1542–1552.

Cadenhead, K. S., Light, G. A., Geyer, M. A., McDowell, J. E., & Braff, D. L. (2002). Neurobiological measures of schizotypal personality disorder: defining an inhibitory endophenotype? American Journal of Psychiatry, 159, 869–871.

Carlson, S. M., Moses, L. J., & Claxton, L. J. (2004). Individual differences in executive functioning and theory of mind: An investigation of inhibitory control and planning ability. Journal of Experimental Child Psychology, 87, 299–319.

Carlson, S. M., & Moses, L. J. (2001). Individual differences in inhibitory control and chldren’s theory of mind. Child Development, 72(4), 1032-1053.

Chamberlain, S. R., Fineberg, N. A., Menzies, L. A., Blackwell, A. D., Bullmore, E. T., Robbins, T. W.. & Sahakian, B. J. (2007). Impaired Cognitive Flexibility and Motor Inhibition in Unaffected First-Degree Relatives of Patients with Obsessive-Compulsive Disorder. American Journal of Psychiatry, 164(2), 335–338.

Chamberlain, S. R., Blackwell, A. D., Fineberg, N. A., Robbins, T. W., & Sahakian, B. J. (2005). The neuropsychology of obsessive compulsive disorder: The importance of failures in cognitive and behavioural inhibition as candidate endophenotypic markers. Neuroscience Biobehavioural Reviews, 29, 399–419.

Chambers, A. R., Krystal, J. H. & Self, D. W. (2001). A neurobiological basis for substance abuse comorbidity in schizophrenia. Biological Psychiatry, 50, 71–83.

Clementz, B. A., McDowell, J. E., & Zisook, S. (1994). Saccadic system functioning among schizophrenia patients and their first-degree biological relatives. Journal of Abnormal Psychology, 103, 277–287.

Clementz, B. A., & Sweeney, J. A. 1990). Is eye movement dysfunction a biological marker for schizophrenia: Amethodological review. Psychological Bulletin, 108, 77-92.

Cohen, A,. J., & Leckman, J. F. (1992). Sensory phenomena associated with Gilles de la Tourette syndrome. Journal of Clinical Psychiatry, 53 , 319–23.

Crosbie, J., & Schachar, R. (2001) Deficient inhibition as a marker for familial ADHD. American Journal of Psychiatry, 158, 1884–1890.

Dempster, F. (1992). The rise and fall of the inhibitory mechanism: Toward a unified theory of cognitive development and aging. Developmental Review, 12, 45–75.

Donohoe, G., Corvin, A., & Robertson, I. H. (2006). Evidence that specific executive functions predict symptom variance among schizophrenia patients with a predominantly negative symptom profile. Cognitive Neuropsychiatry, 20, 13–32.

Durston, S. (2003). A review of the biological bases of ADHD: What have we learned from imaging studies? Mental retardation and developmental disabilities research reviews, 9, 184-195.

Durston, S., Tottenham, N. T., Thomas, K. M., Davidson, M. C., Eigsti, I-M., Yang, Y., Ulug, A. M., & Casey, B. J. (2003). Differential patterns of striatal activation in young children with and without ADHD. Biological Psychiatry, 53, 871–878.

Eneight, S. J., & Beech, A. R. (1993). Reduced cognitive inhibition in obsessive–compulsive disorder. British Journal of Clinical Psychology, 32(1), 67–74.

Enright, S. J. (1996). Obsessive-compulsive disorder: Anxiety disorder or schizotype? In Rapee, R, M. (Ed.). Current controversies in the anxiety disorders , (161–190). New York: Guilford Press.

Enticott, P. G., Ogloff, J. R. P., Bradshaw, J. L., & Fitzgerald, P. B. (2008). Cognitive inhibitory control and self-reported impulsivity among violent offenders with schizophrenia. Journal of Clinical and Experimental Neuropsychology, 30(2), 157-162.

Ettinger, U., Picchioni, M., Hall, M. H., Schulze, K., Toulopoulou, T., Landau, S., Crawford, T. J., & Murray, R. M. (2006). Antisaccade performance in monozygotic twins discordant for schizophrenia: the Maudsley twin study. American Journal of Psychiatry, 163, 543–545.

Everett, J., Laplante, L., & Thomas, J. (1989). The selective attention deficit in schizophrenia: Limited resources or cognitive fatigue? Journal of Nervous and Mental Disease, 177, 735–738.

Fallgatter, A. J., Bartsch, A. J., Zielasek, J., & Herrmann, M. J. (2003). Brain electrical dysfunction of the anterior cingulate in schizophrenic patients. Psychiatry Research, 124, 37–48.

Ford, J. M., Gray, N., Whitfield, S. L., Turken, A. U., Glover, G., Faustman, W. O., & Mathalon, D, H. (2004). Acquiring and inhibiting prepotent responses in schizophrenia. Event-related brain potentials and functional magnetic resonance imaging. Archives of General Psychiatry, 61, 119-129.

Goldman-Rakic, P. S. (1987). Motor control function of the prefrontal cortex. Ciba Found Symposium, 132, 187– 200.

Herrmann, M. J., Jacob, C., Unterecker, S., & Fallgatter, A. J. (2003). Reduced response-inhibition in obsessive–compulsive disorder measured with topographic evoked potential mapping. Psychiatry Research, 120, 265–271.

Hinshaw, S. P., Simmel, C., & Heller, T. L. (1995). Multimethod assessment of covert antisocial behavior in children: Laboratory observations, adult ratings, and child self-report. Psychological Assessment, 7, 209-219.

Hoptman, M. J., Ardekani, B. A., Butler, P. D., Nierenberg, J., Javitt, D. C., & Lim, K. O. (2004). DTI and impulsivity in schizophrenia: a first voxelwise correlational analysis. NeuroReport, 15, 2467–2470.

Hoptman, M. J., Volavka, J., Johnson, G., Weiss, E., Bilder, R. M., Lim, K. O. (2002). Frontal white matter microstructure, aggression, and impulsivity in men with schizophrenia: A preliminary study. Biological Psychiatry, 52, 9–14.

Johannes, S., Wieringa, B. M., Mantey, M., Nager, W., Rada, D., Muller-Vahl, K. R., Emrich, H. M., Dengler, R., Munte, T. F., & Dietrich, D. (2001). Altered inhibition of motor responses in Tourette Syndrome and Obsessive–Compulsive Disorder. Acta Neurol Scand, 104, 36–43.

Joseph, R. M., McGrath, L. M., & Tager-Flusberg, H. (2005). Executive Dysfunction and Its Relation to Language Ability in Verbal School-Age Children With Autism. Developmental Neuropsychology, 27(3), 361–378.

Joseph, R. M., & Tager-Flusberg, H. (2004). The relationship of theory of mind and executive functions to symptom type and severity in children with autism. Development and Psychopathology, 16(1), 137–155.

Kaladjian, A., Jeanningros, R., Azorin, J-M., Grimault, S., Anton, J-L., & Mazzola-Pomietto, P. (2007). Blunted activation in right ventrolateral prefrontal cortex during motor response inhibition in schizophrenia. Schizophrenia Research, 97, 184–193.

Kana, R. K., Keller, T. A., Minshew, N. J., & Just, M. A. (2007). Inhibitory control in high-functioning autism: Decreased activation and underconnectivity in inhibition networks. Biological Psychiatry, 62 , 198–206.

Karoumi, B., Ventre-Dominey, J., & Dalery, J. (1998). Predictive saccade behaviour is enhanced in schizophrenia. Cognition, 68 , 81–91.

Kerns, J. G., Cohen, J. D., MacDonald III, A. W., Johnson, M. K., Stenger, V. A., Aizenstein, H., & Carter, C. S. (2005). Decreased conflict- and error-related activity in the anterior cingulate cortex in subjects with schizophrenia. American Journal of Psychiatry, 162, 1833–1839.

Kiehl, K. A., Smith, A. M., Hare, R. D., & Liddle, P. F. (2000). An event-related potential investigation of response inhibition in schizophrenia and psychopathy. Biological Psychiatry, 48, 210-221.

Kleinhans, N., Akshoomoff ‌ , N., & Delis, D. C. ‌ (2005). Executive functions in autism and asperger's disorder: flexibility, fluency, and inhibition. Developmental Neuropsychology , 27 (3), 379-401.

Kochanska, G., Murray, K., Jacques, T. Y., Koenig, A. L., & Vandegeest, K. A. (1996). Inhibitory control in young children and its role in emerging internalization. Child Development, 67, 490-507.

Krikorian,R., Zimmerman, M. E., & Fleck, D. E. (2004). Inhibitory control in Obsessive-Compulsive Disorder. Brain and Cognition, 54, 257–259.

Kurlan, R., Daragjati, C., Como, P. G., McDermott, M. P., Trinidad, K. S., Roddy, S., Brower, C. A., & Robertson, M. M. (1996). Non-obscene complex socially inappropriate behaviour in Tourette’s syndrome. Journal of Neuropsychiatry and Clinical Neurosciences, 8, 311–317.

LeVasseur, A. L., Flanagan, J. R., Riopelle, R. J., & Munoz, D. P. (2001). Control of volitional and reflexive saccades in Tourette’s syndrome. Brain, 124, 2045-2058.

Logan, G. D., & Cowan, W. B. (1984). On the ability to inhibit thought and action: a theory of an act of control. Psychological Review, 91, 295–327.

Luna, B., Doll, B. S., Hegedus, S. J., Minshew, N. J., & Sweeney, J. A. (2006). Maturation of

executive function in autism. Biological Psychiatry, 61(4), 474-481.

Malloy, P., Rasmussen, S., Braden, W., & Haier, R. J. (1989). Topographic evoked potential mapping in obsessive–compulsive disorder: evidence of frontal lobe dysfunction. Psychiatry Research, 28, 63–71.

Maltby, N., Tolin, D. F., Worhunsky, P., O’Keefe, T. M., & Kiehl, K. A. (2005). Dysfunctional action monitoring hyperactivates frontal–striatal circuitry in obsessive-compulsive disorder: An event-related fMRI study. NeuroImage, 24, 495– 503.

Mataix-Cols, D., Wooderson, S., Lawrence, N., Brammer, M. J., Speckens, A., & Phillips, M. L. (2004). Distinct neural correlates of washing, checking, and hoarding symptom dimensions in obsessive–compulsive disorder. Archives of General Psychiatry, 61(6), 564–576.

Moll, G. H., Heinrich, H., Trott, G.,Wirth, S., & Rothenberger, A. (2000). Deficient intracortical inhibition in drug-naive children with attention-deficit hyperactivity disorder is enhanced by methylphenidate. Neuroscience Letters, 284, 121–125.

Overtoom, C. C., Kenemans, J. L., Verbaten, M. N., Kemner, C., Van Der Molen, M. W., Can Engeland, H., Buitelaar, J. K., & Koelega, H. S. (2002). Inhibition in children with attention deficit hyperactivity disorder: A psychophysiological study of the stop task. Biological Psychiatry, 51, 668–676.

Ozonoff, S., Strayer, D. L., McMahon, W. M., & Filloux, F. (1998). Inhibitory deficits in Tourette syndrome: A function of comorbidity and symptom severity. Journal of Child Psychology and Psychiatry, 39(8), 1109–18.

Ozonoff, S. Strayer, D. L., McMahon, W. M., & Filloux, F. (1994). Executive function abilities in autism and Tourette syndrome: an information-processing approach. Journal of Child Psychology and Psychiatry, 35, 1015–1032.

Ozonoff, S., & McEvoy, R. E. (1994). A longitudinal study of executive function and theory of mind development in autism. Development and Psychopathology, 6, 415-431.

Ozonoff. S., Pennington, B. F., & Rogers, S. J. (1991). Executive function deficits in high functioning autistic individuals: Relationship to theory of mind. Journal of Child Psychology and Psychiatry, 32, 1081–1105.

Paulik, G., Badcock, J. C., & Maybery, M. T. (2008). Dissociating the components of inhibitory control involved in predisposition to hallucinations. Cognitive Neuropsychiatry, 13(1), 33-46.

Penades, R., Catalan, R., Rubia, K., Andres, S., Salamero, M., & Gasto, C. (2007). Impaired response inhibition in obsessive compulsive disorder. European Psychiatry, 22 , 404-410.

Penades, R., Catalan, R., Andres, S., Salamero, M., & Gasto, C. (2005). Executive function and nonverbal memory in obsessive-compulsive disorder. Psychiatry Research, 133, 81– 90.

Peterson, B. S., Staib, L., Scahill, L., Zhang, H., Anderson, C., Leckman J. F., Cohen, D. J., Gore, J. C., Albert, J., & Webster, R. (2001). Regional brain and ventricular volumes in Tourette syndrome. Archives of General Psychiatry, 58, 427–40.

Peterson, B. S., Skudlarski, P., Anderson, A. W., Zhang, H. J. Gatenby, C., Lacadie, C. M., Leckman, J. F., & Gore, J. C. (1998). A Functional Magnetic Resonance Imaging Study of Tic Suppression in Tourette Syndrome. Archives of General Psychiatry, 54, 326-333.

Prior, M. R., & Hoffmann, W. (1990). Neuropsychological testing in autistic children through an exploration with frontal lobes tests. Journal of Autism and Developmental Disorders, 20, 581-290.

Rankins, D., Bradshaw, J. L., & Georgiou-Karistianis, N. (2006). The semantic Simon effect in Tourette’s syndrome and obsessive-compulsive disorder. Brain and Cognition, 61, 225–234.

Reeder, C., Newton, E., Frangou, S., & Wykes, T. (2004). Which executive skills should we target to affect social functioning and symptom change? A study of a cognitive remediation therapy program. Schizophrenia Bulletin, 30, 87–100.

Roberts, R, J., & Pennington, B. F. (1996). An interactive framework for examining prefrontal cognitive processes. Developmental Neuropsychology, 12, 105-126.

Robertson, M. M. (2000). Tourette syndrome: associated conditions and the complexities of treatment. Brain, 123, 425–462.

Rosenberg, D. R., Averbach, D. H., O’Hearn, K. M., Seymour, A. B., Birmaher, B., & Sweeney, J. A. (1997). Oculomotor response inhibition abnormalities in pediatric obsessive–compulsive disorder. Archives of General Psychiatry, 54(9), 831–838.

Ross, R., Harris, J. G., Olincy, A., Radant, A. D., Adler, L. E., & Freedman, R. (1998). Familial transmission of two independent saccadic abnormalities in schizophrenia. Schizophrenia Research, 30, 59-70.

Roth, R. M., Saykin, A. J., Flashman, L. A., Pixley, H. S., West, J. D., & Mamourian, A. C. (2007). Event-related functional magnetic resonance imaging of response inhibition in obsessive-compulsive disorder. Biological Psychiatry, 62, 901–909.

Rubia, K., Russell, T., Bullmore, E. T., Soni, W., Brammer, M. J., Simmons, A., Taylor, E., Andrew, C., Giampietro, V., & Sharma, T. (2001). An fMRI study of reduced left prefrontal activation in schizophrenia during normal inhibitory function. Schizophrenia Research, 52, 47-55.

Rubia, K., Overmeyer, S., Taylor, E., Brammer, M., Williams, S. C. R., Simmons, A., & Bullmore, E. T. (1999). Hypofrontality in attention deficit hyperactivity disorder during higher-order motor control: a study with functional MRI. American Journal of Psychiatry, 156, 891–896.

Salo, R., Robertson, L. C., Nordahl, T. E., & Kraft, L.W. (1997). The effects of antipsychotic medication on sequential inhibitory processes. Journal of Abnormal Psychology, 106, 639–643.

Schachar, R., & Logan, G. D. (1990). Impulsivity and inhibitory control in normal development and childhood psychopathology. Developmental Psychology, 26, 710–720.

Scheres, A., Oosterlaan, J., Swanson J., Morein-Zamir, S., Meiran, N., Schut, H., Vlasveld, L., & Sergeant, J. A. (2003). The effect of methylphenidate on three forms of response inhibition in boys with AD/HD. Journal of Abnormal Child Psychology, 31, 105–120.

Schmitz, N., Rubia, K., Daly, E., Smith, A., Williams, S., & Murphy, D. G. M. (2006). Neural correlates of executive function in Autistic Spectrum Disorders. Biological Psychiatry, 59, 7–16.

Schulz, K. P., Fan, J., Tang, C. Y., Newcorn, J. H., Buchsbaum, M,.S., Cheung, A. M., & Halperin, J. M. (2004). Response inhibition in adolescents diagnosed with Attention Deficit Hyperactivity Disorder during childhood: An Event-related fMRI study. American Journal of Psychiatry, 161, 1650–1657.

Sonuga-Barke, E. J. S., Taylor, E., Sembi, S., & Smith, J. (1992). Hyperactivity and delay aversion-I. The effect of delay on choice. Journal of Child Psychology and Psychiatry, 33, 387-398.

Vaidya, C. J., Austin, G., Kirkorian, G., Ridlehuber, H. W., Desmond, J. E., Glover G. H., & Gabrieli, J. D. (1998). Selective effects of methylphenidate in attention deficit hyperactivity disorder: a functional magnetic resonance study. Proceeds of the National Academy of Science USA, 95, 14494– 14499

Waters, F. A, V., Badcock, J. C., Michie, P. T., & Maybery, M. T. (2006). Auditory hallucinations in schizophrenia: Intrusive thoughts and forgotten memories. Cognitive Neuropsychiatry, 11, 65-83.

Watkins, L., Sahakian, B.J., Robertson, M.M., Veale, D. M., Rogers, R. D., Pickard, K. M., Aitken, M. R. F., & Robbins, T. W. (2005). Executive function in Tourette’s syndrome and obsessive–compulsive disorder. Psychological Medicine, 35, 571–582.

Willcutt, E. G., Doyle, A. E., Nigg, J. T., Faraone, S. V., & Pennington, B. F. (2005). Validity of the executive function theory of attention-deficit-hyperactivity disorder: a meta-analytic review. Biological Psychiatry, 57, 1336–1346

Woolley, J., Heyman, I., Brammer, M., Frampton, I., McGuire, P. K., & Rubia, K. (2008). Brain activation in paediatric obsessive compulsive disorder during tasks of inhibitory control. British Journal of Psychiatry, 192, 25-31.