Cognitive sequences in obsessive-compulsive disorder are supported by frontal cortex ramping activity and mediated by symptom severity
Completing sequences is a routine part of daily life. Many are abstract, defined by a rule governing the order rather than the identity of individual steps (e.g., getting dressed). In obsessive-compulsive disorder (OCD), excessive ritualistic behaviors suggest a disruption in abstract sequence completion. Executing abstract sequences requires at least two levels in a hierarchy of cognitive control: abstract sequential control (tracking steps) and task switching (shifting between tasks). While task switching has been studied in OCD, little is known in a sequential context. Understanding both hierarchical control types is key to uncovering how abstract sequences with nested task switches are processed in OCD. Previous studies showed that the rostrolateral prefrontal cortex (RLPFC) supports abstract sequence monitoring in healthy individuals with an increase in activity across each sequence, a dynamic known as "ramping". Ramping outside the RLPFC is potentially indicative of other sequence-related processes such as progress towards a goal and increasing working memory load. Therefore, we hypothesized that abstract sequential control deficits would correspond to altered ramping dynamics in RLPFC and other cortical regions. Second, we predicted task switching deficits in OCD, coupled with altered activity in cortical regions canonically implicated in task level control. We found partial support for both hypotheses. Abstract sequential control did not show behavioral differences in OCD but did show increased overall ramping in the anterior cingulate cortex (ACC) and superior frontal sulcus (SFS) and ramping differences in additional, novel cortical regions according to abstract sequence complexity. In contrast, behavioral differences were observed for task switching in OCD without neural differences between the groups. Together, these results suggest a group of areas support sequential control differentially in OCD than in healthy controls, despite behavioral similarity, and that this observation is likely not the result of neural deficits in task switching. These findings thus provide insight into OCD during complex behaviors more similar to daily life where sequence and task level control are intertwined and may inform future potential treatment.