4 research outputs found
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Magnetic Resonance Spectroscopy (MRS) and Positron Emission Tomography (PET) Imaging in Obsessive-Compulsive Disorder
Obsessive-compulsive disorder (OCD) is characterised by structural and functional deficits in the cortico-striato-thalamic-cortical (CSTC) circuitry and abnormal neurochemical changes are thought to modulate these deficits. The hypoth- esis that an imbalanced concentration of the brain neurotransmitters, in particular glutamate (Glu) and gamma-amino-butyric acid (GABA), could impair the normal functioning of the CSTC, thus leading to OCD symptoms, has been tested in humans using magnetic resonance spectroscopy (MRS) and positron emission tomography (PET). This chapter summarises these neurochemical findings and represents an attempt to condense such scattered literature. We also discuss potential challenges in the field that may explain the inconsistent findings and suggest ways to overcome them. There is some convergent research from MRS pointing towards abnormalities in the brain concentration of neurometabolite markers of neuronal integrity, such as N-acetylaspartate (NAA) and choline (Cho). Lower NAA levels have been found in dorsal and rostral ACC of OCD patients (as compared to healthy volunteers), which increase after CBT and SSRI treatment, and higher Cho concentration has been reported in the thalamus of the OCD brain. However, findings for other neurometabolites are very inconsistent. Studies have reported abnormalities in the concentrations of creatine (Cr), GABA, glutamate (Glu), glutamine (Gln), Ins (myo-inositol), and serotonin (5-HT), but most of the results were not replicated. The question remains whether the NAA and Cho findings are genuinely the only neurochemical abnormalities in OCD or whether the lack of consistent findings for the other neurometabolites is caused by the lower magnetic field (1–3 Tesla (T)) used by the studies conducted so far, their small sample sizes or a lack of proper control for medication effects. To answer these questions and to further inform the biolog- ical underpinning of the symptoms and the cognitive problems at the basis of OCD we need better controlled studies using clear medicated vs unmedicated groups, larger sample sizes, stronger magnetic fields (e.g. at 7 T), and more consistency in the definition of the regions of interest
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Action sequence learning, habits, and automaticity in obsessive-compulsive disorder
Peer reviewed: TrueFunder: Mental Health Research UK; FundRef: http://dx.doi.org/10.13039/100009981Enhanced habit formation, greater automaticity and impaired goal/habit arbitration in obsessive-compulsive disorder (OCD) are key hypotheses from the goal/habit imbalance theory of compulsion which have not been directly investigated. This article tests these hypotheses using a combination of newly developed behavioral tasks. First, we trained patients with OCD and healthy controls, using a novel smartphone app, to perform chunked action sequences, previously shown to engage habit brain circuitry. The motor training was daily over one month period. There was equivalent procedural learning and attainment of habitual performance (measured with an objective criteria of automaticity) in both groups, despite greater subjective habitual tendencies in patients with OCD, self-reported via a recently developed questionnaire. We then used a combination of follow-up behavioral tasks to further assess the arbitration between previous automatic and new goal-directed action sequences. We found no evidence for impairments of goal/habit arbitration in OCD following re-evaluation based on monetary feedback, although there was a greater preference for engaging in the trained habitual sequence under certain conditions which may have derived from its intrinsic value. These findings may lead to a reformulation of the goal/habit imbalance hypothesis in OCD. Finally, OCD patients with higher compulsivity scores and habitual tendencies showed more engagement with the motor habit-training app and reported symptom alleviation, with implications for its potential use as a form of habit reversal therapy
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Excessive Checking in Obsessive-Compulsive Disorder: Neurochemical Correlates Revealed by 7T Magnetic Resonance Spectroscopy
Background
Compulsive checking, a common symptom of obsessive-compulsive disorder (OCD), has been difficult to capture experimentally. Therefore, determination of its neural basis remains challenging, despite some evidence suggesting it is linked to dysfunction of cingulo-striatal systems. This study introduces a novel experimental paradigm to measure excessive checking and its neurochemical correlates.
Methods
Thirty-one OCD and twenty-nine healthy volunteers performed a decision-making task requiring them to decide whether two perceptually similar visual representations were the same or different under a high uncertainty condition without feedback. Both groups underwent magnetic resonance spectroscopy scans at 7-Tesla on the same day. Correlations between out-of-scanner experimental measures of checking and the Glutamate/GABA ratio in the anterior cingulate cortex (ACC), supplementary motor area (SMA) and occipital cortex (OCC) were assessed. Their relationship with subjective ratings of doubt, anxiety and confidence was also investigated.
Results
OCD patients exhibited excessive and dysfunctional checking, which significantly correlated with changes in the Glutamate/GABA ratio within the ACC. No behavioral/neurochemical relationships were evident for either the SMA or OCC. Excessive checking observed in patients negatively correlated with their confidence levels and positively related to doubt, anxiety and compulsivity traits.
Conclusions
We conclude that experimental measures of excessive and dysfunctional checking in OCD, linked to increased doubt, anxiety and lack of confidence, are related to an imbalance between excitatory and inhibitory neural activity within the ACC. This study adds to our understanding of the role of this region in OCD in providing a laboratory model of the possible development of compulsive checking