In the current study, the investigators will characterize obsessive-compulsive disorder (OCD) patients and healthy controls with a comprehensive clinical and neuroimaging assessment, focusing on the basic psychological processes related and on the neurobiological underpinnings of emotion regulation (ER). After a baseline assessment, the investigators will implement an internet-based cognitive behavioral therapy (iCBT) protocol and assess its efficacy on the modulation of ER and OCD severity. Specifically, the investigators will (1) examine to what extent the iCBT intervention modulates ER and the associated neurobiological mechanisms and (2) identify candidate biomarkers of successful treatment response.

Sixty patients and sixty controls will be recruited at Hospital de Braga, Braga, Portugal (sample size estimated based on a previous systematic review). A power analysis indicated that a sample of 24 individuals per group is adequate to detect an effect size of d=.89, with a Type I error of .05 and a statistical power of .85. A final sample of 30 individuals was defined to account for a 20% dropout rate during the intervention.

All participants will be assessed with a comprehensive psychological and clinical characterization. Following the baseline assessment, OCD patients will be randomly allocated to the intervention (iCBT intervention) or the control group (waiting list). The iCBT intervention will consist of a structured program comprising 10 modules with text and video content, as well as 3 videoconferencing psychotherapy sessions. After the intervention/waiting list, patients will be re-assessed with the clinical and psychological assessment.

In order to characterize the patterns of brain activation in OCD patients during an emotion regulation task in comparison to healthy controls and to identify potential neurobiological markers of symptoms' improvement, a multimodal neuroimaging assessment will also be performed. The task encompasses the presentation of pictures (fearful, neutral and OCD-related stimuli) under two conditions: the instruction to view the pictures naturally ('observe'); or to apply cognitive reappraisal techniques to decrease any negative affective state caused by the stimuli ('regulate'). Fear and OCD-related pictures are presented during both conditions; neutral pictures only during 'observe'. After each stimulus, the level of distress will be assessed by a self-report rating scale.

In the baseline assessment, brain activation patterns (with whole-brain and region of interest approaches) will be compared between OCD and healthy controls in both task conditions. After intervention, emotion regulation ability and the underlying neurobiological mechanisms will be compared between the 'intervention' and 'waiting list' groups.

Inclusion Criteria:

• Primary diagnosis of current OCD according to the fifth Diagnostic and Statistical Manual of Mental Disorders
• YBOCS score ≥ 16
• Psychotropic medication, if stable for at least 2 months prior to inclusion and if the dosage is maintained thoughout the study
• Have the resources to access the online intervention (internet access; computer or tablet)

Exclusion Criteria:

• MRI contraindications
• Prior medical history of neurological disorders or traumatic brain injury
• Having performed CBT treatment for OCD in the last 2 years
• Undergoing any psychological treatment
• Suicidal ideation
• Current alcohol or drug abuse

The psychologist performing the assessment of the main psychometric outcomes will be blind to the group allocation of each participant.