This clinical trial investigates how Cognitive Behavioral Therapy (CBT), a psychological treatment that helps people change their thought and behavior patterns, affects brain function in people diagnosed with Obsessive-Compulsive Disorder (OCD). OCD is characterized by repeated, unwanted thoughts (obsessions) and behaviors (compulsions) that significantly interfere with daily life.

Although CBT is known to effectively reduce OCD symptoms, exactly how it changes brain activity remains unclear. This study aims to measure the effects of CBT on brain function using electroencephalography (EEG), a safe, non-invasive way to monitor brain activity.

Participants diagnosed with OCD will be randomly assigned to two groups. One group will receive CBT consisting of 12 individual therapy sessions, held weekly, lasting about 50 minutes each. The other group will not receive therapy during the study but will be offered treatment afterward. Brain activity and psychological well-being will be measured three times: before starting treatment, immediately after completing CBT, and again three months later.

By comparing these two groups, the study hopes to answer whether CBT causes measurable changes in brain activity and how these changes relate to improvements in OCD symptoms. Results from this study could help improve future treatments and understanding of OCD.

Obsessive-Compulsive Disorder (OCD) is a psychiatric condition marked by persistent, distressing obsessions (intrusive thoughts) and compulsions (repetitive behaviors aimed at relieving anxiety). Cognitive Behavioral Therapy (CBT), particularly with Exposure and Response Prevention (ERP), is established as an effective treatment method. However, the exact neurophysiological mechanisms underlying CBT-induced symptom improvement in OCD patients remain unclear.

This randomized controlled clinical trial aims to clarify these mechanisms by examining how CBT impacts brain functioning through neurophysiological assessments using electroencephalography (EEG). EEG is a reliable and non-invasive method for measuring brain electrical activity, particularly valuable for observing real-time brain responses to various cognitive tasks and resting-state conditions.

The trial will recruit 60 adult OCD patients (aged 18-65) diagnosed according to DSM-5 criteria from outpatient psychiatric clinics in Istanbul, Turkey. After baseline assessments, participants will be randomly assigned equally into two groups:

Intervention Group (CBT): Participants will receive 12 weekly sessions of individual CBT, each lasting 50 minutes. The CBT protocol includes standard therapeutic techniques such as ERP, cognitive restructuring, identification of maladaptive beliefs, psychoeducation, and relapse prevention strategies.

Control Group: Participants will be placed on a waiting list and will not receive any intervention during the active study period. CBT will be offered to these participants after study completion.

Participants in both groups will undergo comprehensive EEG assessments at three different time points: at baseline (pre-treatment), immediately following the CBT intervention (12 weeks post-baseline), and at a three-month follow-up (24 weeks post-baseline). EEG sessions will include:

Resting-State EEG (eyes closed):

Duration: 10-minute continuous recording.

Analysis Method: Low-Resolution Brain Electromagnetic Tomography (LORETA) to identify cortical activity patterns, specifically focusing on alpha-2 and beta frequency bands within regions implicated in OCD, such as the anterior cingulate cortex (ACC) and orbitofrontal cortex (OFC).

Emotional Oddball Paradigm:

Stimuli: Anxiety-provoking and neutral images or words presented randomly.

EEG measure: Event-Related Potentials (ERPs), particularly P300 amplitude and latency, to investigate attentional allocation and emotional processing alterations associated with OCD pathology and potential CBT-induced normalization.

Go/No-Go Task:

Purpose: Evaluate inhibitory control mechanisms and cognitive flexibility.

EEG measure: ERPs such as Error-Related Negativity (ERN), N2 (conflict monitoring), and P3 (response inhibition), known to be altered in OCD populations. This task will clarify whether CBT influences cognitive control functions at a neural level.

In addition to EEG, participants will complete several validated clinical measures at each time point to evaluate OCD symptom severity, anxiety, and depression levels. These scales include the Yale-Brown Obsessive-Compulsive Scale (Y-BOCS), Obsessive-Compulsive Inventory-Revised (OCI-R), Beck Depression Inventory-II (BDI-II), and Beck Anxiety Inventory (BAI).

EEG data will undergo rigorous preprocessing steps (bandpass filtering, independent component analysis for artifact removal, and segmentation into epochs) followed by ERP analysis using standardized software. Statistical comparisons will employ repeated measures ANOVA and linear mixed-effects models to identify significant differences between groups and across time points. Correlation analyses will examine the relationships between clinical improvements and neurophysiological changes observed via EEG.

Ultimately, this research aims to provide greater understanding of the biological processes underlying CBT effectiveness, potentially informing future targeted treatments and personalized therapeutic strategies for OCD.

Inclusion Criteria:

• Aged between 18 and 65 years
• Diagnosed with Obsessive-Compulsive Disorder (OCD) according to DSM-5 criteria
• Yale-Brown Obsessive Compulsive Scale (Y-BOCS) total score ≥16
• Not currently receiving psychiatric medication (at least 2-week washout required)
• No psychotherapy (including CBT) in the past 6 months
• Willing and able to participate in EEG procedures
• Capable of providing written informed consent

Exclusion Criteria:

• Use of any psychiatric medication in the past 2 weeks
• History of epilepsy, head trauma, or neurological disorders
• Current diagnosis of schizophrenia, bipolar disorder, or substance use disorder
• Severe physical illness or unstable medical condition
• Pregnancy or breastfeeding
• Prior CBT treatment for OCD within the past 6 months
• Currently participating in another clinical trial

The outcomes assessor conducting EEG assessments and psychological evaluations is blinded to group assignment. Participants and therapists cannot be blinded due to the nature of the psychotherapy intervention.