Inhibition of thoughts and actions in obsessive-compulsive disorder: extending the endophenotype?

Abstract

Background: Obsessive-compulsive disorder (OCD) has been associated with impairments in stop-signal inhibition, a measure of motor response suppression. The study used a novel paradigm to examine both thought suppression and response inhibition in OCD, where the modulatory effects of stimuli relevant to OCD could also be assessed. Additionally, the study compared inhibitory impairments in OCD patients with and without co-morbid depression, as depression is the major co-morbidity of OCD.

Method: Volitional response suppression and unintentional thought suppression to emotive and neutral stimuli were examined using a novel thought stop-signal task. The thought stop-signal task was administered to non-depressed OCD patients, depressed OCD patients and healthy controls (n=20 per group).

Results: Motor inhibition impairments were evident in OCD patients, while motor response performance did not differ between patients and controls. Switching to a new response but not motor inhibition was affected by stimulus relevance in OCD patients. Additionally, unintentional thought suppression as measured by repetition priming was intact. OCD patients with and without depression did not differ on any task performance measures, though there were significant differences in all self-reported measures.

Conclusions: Results support motor inhibition deficits in OCD that remain stable regardless of stimulus meaning or co-morbid depression. Only switching to a new response was influenced by stimulus meaning. When response inhibition was successful in OCD patients, so was the unintentional suppression of the accompanying thought.

Fig. 1

Illustration of a typical series of trials in go no-repeat, go repeat and stop repeat conditions in the stop phase (a) and repetition phase (b) of the thought stop-signal task. For each word the participant makes a speeded response via a key press deciding whether the word is a noun or not. Go and stop repeat stimuli appear both in the stop and repetition phases while no-repeat stimuli are replaced with novel stimuli in the repetition phase. Stop signal delays in stop trials were determined online as a function of individual go reaction times (RTs). Stop repeat trials in the repetition phase were analysed based on stop outcome in the stop phase, with words that were previously inhibited successfully compared with responses to words that had previously failed to be successfully inhibited.

Fig. 2

Percentage successful inhibition to the stop signal following obsessive-compulsive disorder (OCD)-relevant () and neutral (□) stimuli in OCD patients with co-morbid depression (OCD+dep), OCD patients without co-morbid depression (OCD−dep) and a control group. Values are means, with standard errors represented by vertical bars. The figure demonstrates impaired inhibition in the OCD groups regardless of stimulus type.

Fig. 3

Reaction time (RT) performance to pressing the space bar following a stop signal following obsessive-compulsive disorder (OCD)-relevant () and neutral (□) stimuli in OCD patients with co-morbid depression (OCD+dep), OCD patients without co-morbid depression (OCD−dep) and a control group. Values are means, with standard errors represented by vertical bars. The figure demonstrates slowed switching in the OCD groups which is specific to OCD-relevant stimuli.