The purpose of this study is to test whether transcranial direct current stimulation (tDCS) can enhance the clinical efficacy of trauma-focused therapy for posttraumatic stress disorder.

Posttraumatic stress disorder (PTSD) is a debilitating, often-chronic psychiatric condition emerging following a severe traumatic event. Trauma-focused therapy techniques, and primarily Prolonged Exposure, constitute the primary first-line treatment. While effective to some degree, these methods have several substantial shortcomings, including limited patient compliance (long process) and responsiveness, sustained therapeutic effect, and susceptibility to spontaneous symptom relapse. Thus, there is a considerable need for enhancing the efficacy of PTSD treatment.

Dominant theories in the field of PTSD emphasize a key role for threat-related learning and memory processes in the underlying etiology and maintenance of PTSD symptoms, such as absent or insufficient extinction of learned fear associations. Indeed, trauma-focused therapy protocols typically involve repeated imaginal or in vivo recall of traumatic memories in a systematic, controlled manner, while employing anxiety-reducing techniques, and without experiencing additional external trauma. Thus, these therapies parallel cue-extinction training within a model of learning and unlearning of conditioned responses, with the patient's diminished fear response over successive extinction trials reflecting the weakening of trauma-induced associations between the fear-provoking stimuli and the conditioned fear response. Extinction of fear responses is thus generally assumed to be one the most important underlying mechanisms of exposure therapy. Noting the limited efficacy of trauma-focused treatment (and in particular the spontaneous relapse), there is much room for improving the effectiveness of this cue-extinction process in a manner that is not dangerous to the patient (cf. extinction-enhancing pharmacological agents that are also toxic).

Transcranial direct current stimulation (tDCS) is a safe method to induce weak transcranial currents (up to 1-2 milliampere). Using 2 rubber electrodes positioned on the scalp, tDCS can be used to manipulate localized brain excitability via membrane polarisation: cathodal stimulation hyperpolarises, while anodal stimulation depolarises the resting membrane potential, whereby the induced after-effects depend on polarity, duration and intensity of the stimulation.

The investigators believe that the therapeutic efficacy of PTSD treatment can be enhanced by employing tDCS during the therapeutic process. That is, tDCS's modulatory effects on existing brain activity may enable us to render the therapeutic mechanisms operating during trauma-focused therapy more effective, leading to a more efficient and efficacious therapeutic process in terms of greater symptom reduction, greater long-term sustainability, a shorter treatment course, and broader compliance.

Inclusion Criteria:

• Clinical diagnosis of PTSD
• Adequate physical health, including vision and hearing

Exclusion Criteria:

• Non-trauma-related major psychiatric/neurological disorder
• History of seizures, fainting spells, diagnosis of epilepsy, history of abnormal (epileptiform) EEG or family history of treatment resistant epilepsy
• Any metal in the brain, skull or elsewhere.
• Pregnancy
• Any medical devices (i.e. Cardiac pacemaker, deep brain stimulator, medication infusion pump, cochlear implant, vagal nerve stimulator)
• Intracranial lesions
• Substance abuse or dependence within the past six months
• Other criteria for MRI/tDCS