This double-blind randomized controlled clinical trial aims to test whether transcranial direct current stimulation (tDCS) can be used to modulate fear extinction learning during exposure therapy for pathological fear, including fear of spiders, snakes, or germs / contamination. Participation takes place over three laboratory visits, including (1) a pre-treatment visit, (2) a treatment and post-treatment visit, and (3) a 1 month follow-up visit. During treatment, participants will receive either 20 minutes of active or sham tDCS, followed by 30 minutes of in vivo exposure therapy.

In a trans-diagnostic sample with marked pathological fear and behavioral avoidance, this study aims to: (1) evaluate whether excitatory tDCS of the mPFC and inhibitory tDCS of right dlPFC enhances exposure therapy relative to sham tDCS; (2) determine whether tDCS effects are moderated by baseline negative prognostic indicators; and (3) determine whether tDCS effects are mediated by pre-post changes in vigilance to threat, in-session fear reduction, and contextual memory for the exposure context. If successful, the project may discover a potentially effective exposure therapy augmentation, and may enhance knowledge of the behavioral, cognitive, affective, and neurobiological factors that moderate and mediate acute treatment response and maintenance of treatment gains. This knowledge may inform treatment development efforts for more debilitating forms of pathological fear.

Inclusion Criteria:

1. Age 18-65.
2. Fluent in English.
3. A score on at least 1 fear domain-specific prescreen measure > 2 SDs above the subject pool prescreen mean. These measures include (a) FSQ, and (b) OCI-R.
4. Peak fear ≥ 50 on BATs 1 and 2.

Exclusion Criteria:

1. Currently receiving treatment for the primary fear domain (based on clinical interview).
2. Unstable dose of psychotropic medications within 6 weeks prior to baseline assessment (based on the DMQ; see measures).
3. Medical condition that would contraindicate participation in treatment or assessment activities (e.g., cardiovascular problems; based on the DMQ; see measures).
4. Pregnancy (based on the DMQ; see measures).
5. Current major depressive disorder (based on MINI; see measures).
6. Current, or history of bipolar disorder (based on MINI; see measures).
7. Current, or history of psychotic symptoms (based on MINI; see measures).
8. Serious suicidal risk, as determined by self-report (C-SSRS, BDI-II) and clinical interview (MINI; see measures).
9. Active neurological conditions, including seizures, stroke, unexplained loss of consciousness or concussion (based on DMQ and tDCS Safety Screening Form; see measures)
10. Contraindications for tDCS: Metal in the head or implanted brain medical devices.

Participants, treatment administrators, and outcome assessors will be blind to tDCS condition.