Always on guard: Test of high vs. low control conditions in obsessive-compulsive disorder patients
Introduction
Obsessive-compulsive disorder (OCD) is a highly debilitating disorder with a lifetime prevalence of approximately 2.3% (Huppert et al., 2009, Ruscio et al., 2010). OCD is characterized by recurrent intrusive thoughts or impulses (obsessions), and repetitive, irresistible behaviors (compulsions) aimed at feared consequences and to reduce anxiety and/or distress (American Psychiatric Association, 2013). Compulsive behaviors cause immediate effects of relief from distress, though in the long run these compulsions inflict paradoxical effects of increasing rather than decreasing the distress caused by obsessions, effectively perpetuating compulsions (Salkovskis, 1999, van den Hout and Kindt, 2003, van den Hout et al., 2008). Understanding factors affecting individual proneness to developing OCD is paramount to improving OCD treatment, particularly since knowledge of etiological factors underlying OCD is lacking (for reviews see Gava et al., 2007, Grabill et al., 2008).
Executive control is a key human function that mediates the ability to guide behavior in accordance with internal goals (Shallice and Norman, 1986, Miyake et al., 2000, Miller and Cohen, 2001). It has been suggested that executive control is composed of three processes, of which the first two are working memory and set switching (e.g., Banich, 2009). The third process, and perhaps a hallmark of executive function, is the suppression of irrelevant information (Verbruggen and Logan, 2008). Current approaches to OCD suggest that neurobiological abnormalities play a crucial role in the etiology and course of the disorder (Stein, 2002). Neuroimaging studies have provided evidence for abnormal metabolic rates in the orbitofrontal cortex, caudate nuclei, and thalamus (Saxena and Rauch, 2000, Nakao et al., 2005), and dysfunctions in the cortico-striato-pallido-thalamic (Swerdlow et al., 1993) areas that have been linked to executive control – affecting gating, or inhibition of irrelevant information (Swerdlow and Koob, 1987). Corresponding with this neurological finding, many studies found a deficit in executive control in OCD patients (e.g., Lucey et al., 1997, Penades et al., 2005, Abramovitch et al., 2011, Meiran et al., 2011; see also Greisberg and McKay, 2003, Kuelz et al., 2004, Shin et al., 2014, for reviews and meta analysis). Some researchers suggested executive control impairments are a core symptom of OCD (e.g., Muller and Roberts, 2005, Anholt et al., 2012, de Wit et al., 2012, Linkovski et al., 2013). The most robust and stable differences between OCD patients and healthy controls were found on tasks that required response inhibition (Bannon et al., 2002, Penades et al., 2007) and some researchers proposed this to be an endophenotype of OCD (Chamberlain et al., 2005, Menzies et al., 2007, de Wit et al., 2012). On the other hand, other researchers have suggested that executive control deficit is an epiphenomenon caused by wearing out of the “continuously active” control mechanism (Abramovitch et al., 2012). Though the current study does not aim to determine which of the endophenotype and the epiphenomenon theories is more accurate, we will refer to this issue in Section 4.
One very widespread laboratory task to study executive control is the Stroop task (Stroop, 1935; see Fig. 1). In this task participants are presenter with color words (or a meaningless letter string) in different font colors and are required to identify the color in which a color-word is printed, while ignoring the word meaning. Since word reading is automatic, this presents participants with a challenge when presented with incongruent stimuli (e.g., GREEN written in red). In such cases, one has to ignore the irrelevant word (GREEN) and respond instead to the color (red). It has been shown that reaction time (RT) for incongruent trials is longer than RT for congruent trials. The Stroop effect (i.e., longer RT for incongruent than for congruent stimuli; e.g., RED written in red), the interference effects (i.e., slower RT for incongruent than for neutral stimuli; e.g., XXXX written in red), and the facilitation effect (i.e., faster RT for congruent than for neutral stimuli) demonstrate that participants have difficulty in ignoring an irrelevant word altogether (MacLeod, 1991). The low error rate in normal participants and the increased error rate in participants with executive and frontal deficits (Cohen and Servan-Schreiber, 1992) demonstrate the role of executive control and of the prefrontal cortex in guiding task-relevant behavior and suppressing automatic responses (Cohen et al., 1990). Friedman and Miyake (2004) suggested that stopping in the stop-signal task (as a measurement for response inhibition) and Stroop interference belong to the same latent variable. In recent years, a number of studies distinguished between two types of conflict in the Stroop task: the information (or response) conflict, which is between the contradictory information that arises from the word meaning and the information that arises from the word color (e.g., in the case of GREEN written in red – should I respond red or should I respond green); and the task conflict, which is a conflict between the relevant task of color identification and the irrelevant but automatic reading task (e.g., in the case of GREEN written in red – should I read or should I name the color). The information conflict involves the content of the stimulus and the response needed, and differs for congruent and incongruent Stroop stimuli, whereas the task conflict involves the task associated with the stimulus and differs for congruent and neutral nonword Stroop stimuli (Goldfarb and Henik, 2007, Kalanthroff and Henik, 2014). In two recent studies we demonstrated that the control mechanism responsible for managing the task conflict in the Stroop task is contingent upon inhibition control (Kalanthroff et al., 2013b, Kalanthroff and Henik, 2013).
A number of investigations have suggested that executive control constitutes an adaptive and strategic way to respond to conflict (Botvinick et al., 2001, De Pisapia and Braver, 2006, Braver, 2012). This means that participants do not allocate executive control equally in all Stroop trials, but rather, that control is reduced when a conflict is not likely to occur. This suggestion is supported by Tzelgov et al.׳s (1992) study in which the magnitude of the interference effect increased (indication for information conflict) and the magnitude of the facilitation effect decreased (indication for task conflict) in blocks having 75% neutral trials compared to blocks having a majority of color-word Stroop stimuli (see also Goldfarb and Henik, 2007, Kalanthroff et al., 2013c). As suggested by Tzelgov et al. (1992), and consistent with a role of conflict in the allocation of executive control in the Stroop task, an increase in the proportion of neutral trials may result in “letting the executive control guard down”‘, resulting in enhanced Stroop interference.
The performance of OCD patients on the Stroop task has been in investigated before, though researchers mainly focused on the interference effect. Most studies find an increased interference effect, suggesting deficient executive control (e.g., Hartston and Swerdlow, 1999, Penades et al., 2005, Abramovitch et al., 2011). Cohen et al. (2003) found that the increased interference effect in OCD participants was more extreme when an anxiety producing statement was presented. Some researchers even suggested that the performance difficulty in the Stroop task is due to functional difficulties in the left prefrontal cortex (Martinot et al., 1990). On the other hand, some studies did not find any differences between patients with OCD and healthy controls on the Stroop task (see Shin et al., 2014, for review). For example, Moritz et al. (2002) found that non-depressed OCD patients performed similar to normal participants on the Stroop task. Rao et al. (2008) found only a trend towards significance in the differences between OCD patients and controls on Stroop interference scores.
OCD patients experience a constant need to inhibit internal (intrusive) information in order to attend to relevant information and the relevant task in their environment (e.g., Gillan et al., 2011). This requires constant activation of the executive control system. Their inadequate executive system makes this even more challenging. In light of this, it is reasonable to assume that these patients will be much more careful in letting their “executive guard down” or even reducing it. In other words, OCD patients may suffer from a non-adaptive control system. Soref et al. (2008) presented a flanker arrow task to low vs. high obsessive-compulsive (OC) students and instructed them to classify a briefly presented target letter (S or H) that appeared in the center of compatible (e.g., SSSSS) or incompatible (e.g., HHSHH) noise letters. The researchers found that a high level of obsessive compulsions was related to reliance on a focused and serial, rather than a parallel, information processing style. The parallel processing index was defined as incompatible trial RT minus compatible trial RT, with a large difference indicating a more parallel processing style. Importantly, these researchers ran an additional block of trials with a high proportion of congruent trials. Under this high congruent condition, those with a low level of obsessive compulsions tended to shift to a parallel processing style, and those with a high level continued to rely on a focused and serial processing style. Soref and colleagues concluded that a high level of obsessive compulsions was related to a non-adjusting processing style.
The aim of the present study was to investigate whether people suffering from OCD could adjust their executive control and specifically, whether they could reduce it when a conflict is unlikely to appear. In order to manipulate executive control levels, we used two conditions of the Stroop task: high control – with equal distribution of congruent, neutral and incongruent trials; and low control – with most trials being neutral. We predicted that OCD participants would have inadequate executive control and that unlike healthy controls, OCD participants would not adjust their executive control to the changing conditions.
Section snippets
Participants
Fifty-two participants took part in the experiment in return for a small payment (50 Israeli Shekels (NIS) equivalent to approximately 15 USD). All participants had normal or corrected-to-normal vision, were native Hebrew speakers, and all were naive as to the purpose of the experiment. None of the participants had a history of attention deficit/hyperactivity disorder (ADHD) or dyslexia (in both groups, participants stated that they were never diagnosed with ADHD or learning disabilities, both
Results
As expected, participants in the OCD group had significantly higher scores for the OCI-R (mean=27.17, S.D.=7.70, ranging between 20 and 44) than healthy controls did (mean=10.85, S.D.=5.85, ranging between 0 and 19), t(49)=8.58, p<0.01. Participants in the OCD group had significantly higher depression scores as measured by the BDI (mean=12.96, S.D.=5.58, ranging between 2 and 25) than healthy controls did (mean=5.7, S.D.=3.61, ranging between 0 and 12), t(49)=5.57, p<0.01. As could be expected,
Discussion
In order to investigate executive control and executive control adjustment in OCD patients, we used a version of the color-word Stroop task with congruent, neutral and incongruent trials. All participants completed two blocks of the task, each varying in the proportion of neutral trials: 33% vs. 75% – a manipulation that had proved to be useful for reducing control levels (e.g., Tzelgov et al., 1992, Goldfarb and Henik, 2007, Kalanthroff et al., 2013c).
In the equal proportions Stroop block
Acknowledgment
We thank Ms. Chen Aslan for her assistance in collecting the data, Mr. Omer Linkovski for his helpful insights, and Ms. Desiree Meloul for her useful input on this article.
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