The purpose of this study is to identify how trauma-focused psychotherapy changes the function of brain circuitry in posttraumatic stress disorder (PTSD) and how this mediates improvements in the diminished ability to experience positive emotions following a traumatic or extremely stressful life event. In this instance, the investigators will be using cognitive processing therapy (CPT), a widely-utilized and evidence-based treatment for PTSD.

The goals of the study are as follows:

1. Quantify, under conditions of safety (no threat), how PTSD psychotherapy alters reward circuit function and information encoding.
2. Identify how presence of threat augments PTSD psychotherapy effects on reward circuit function and information encoding.
3. (Exploratory). Identify how, following psychotherapy, changes in reward circuit function and information encoding under conditions of safety and threat are associated with improvements in symptoms of diminished positive affect (DimPA).

To accomplish the goals of the study, the investigators propose a neuroimaging-coupled, randomized clinical trial of immediate vs. delayed individual cognitive processing therapy (CPT) in individuals (N=120) with a primary diagnosis of chronic PTSD. Individuals will undergo, prior to randomization, clinical and neurobiological assessment with functional magnetic resonance imaging (fMRI) during completion of several reward processing paradigms. Two of these involve both a normal "safe" context and a threat context manipulation (threat of mild electrodermal shock that is periodically cycled throughout the task). Another paradigm involves making decisions to either approach reward or forego a reward when this decision conflicts with the likelihood of an aversive outcome. This is known as approach-avoidance conflict (AAC). This battery will provide a comprehensive characterization of reward processing behavior and circuit function and establish its relationship to treatment processes, as well as how such processes may vary as a function of threat.

Inclusion Criteria:

• English as primary language, and comprehension suitable to understand experimenter instructions.
• Current and chronic syndromic PTSD, defined as being exposed to a DSM-5 Criterion A traumatic event, with the presence DSM-5 qualifying PTSD symptoms for at least 3 months, as assessed by the Clinician-Administered PTSD Scale for DSM-5.
• Able and willing to undergo functional magnetic resonance imaging (fMRI).
• Willingness to participate in repeated assessments and as part of a delayed treatment group.

Exclusion Criteria:

• Evidence of current or prior history of psychosis or bipolar disorder as evidenced by self-report or clinical interview.
• Active substance dependence within the past 6 months as evidenced by clinical interview.
• Current regular psychiatric medication use (i.e. antidepressants), except for as-needed benzodiazepine or opiate medication no more than three times per week, on average, or for short-duration stimulant medication for attention deficit hyperactivity disorder that can be skipped within 24 hours of study visits.
• A recent (<6 months) suicide attempt or current active ideation with intent.
• Unremovable ferrous metal in body.
• History of neurological disorder, stroke, seizures/convulsions (except febrile seizures in childhood), epilepsy, brain surgery, electroconvulsive or radiation treatment, brain hemorrhage or tumor, or thyroid disorder.
• Anyone who is pregnant or trying to become pregnant.
• Current or past year (> 3 sessions), psychotherapy with a prominent exposure or cognitive restructuring component.
• Previous or current (es)ketamine treatment and/ or brain stimulation/neuromodulation treatment.
• Other ongoing treatment that is likely to confound experimental effects.
• Previous penetrating head injury/traumatic brain injury. Mild-to-moderate traumatic brain injury without penetrating injury is allowable.

As participants will necessarily be aware of their arm assignment, symptom raters (research assistants) and study therapists will be blinded to participant arm assignment to reduce risk of ascertainment bias.