This is a pilot study of 55 adult male veterans with PTSD. The study will test a new mobile application "app" that can be run on an iPhone10. The app, which is called the Mobile Acoustic startle Reflex-monitoring System (a.k.a., MARS), will be used to measure participant's acoustic startle in sitting or standing postures.

The proposed study utilizes a randomized, controlled between-subjects design that will compare and contrast MARS data collected in the sitting versus standing position, and will determine the relationship between PTSD severity and MARS outcome measures. Fifty-five subjects will be recruited from Johns Hopkins and VA facilities, International Association of Fire Fighters Center of Excellence, Veteran Centers in Baltimore/District of Columbia/Northern Virginia area, as well as the Florida panhandle and through social media venues. When potential participants respond to advertisements, study coordinators will explain the purpose of the study and study procedures by phone before arranging a face-to-face meeting with the research team. Once a potential participant indicates willingness to participate, a study team member will meet with the participant at the their residence, or at the Johns Hopkins Bayview Medical Center, where the consent document will be reviewed and MARS testing will take place. Recruitment will continue until a total of 55 participants have been consented.

Volunteers shall sign a participation waiver acknowledging the inclusion/exclusion criteria and will be verbally briefed about the experimental proceedings. After consent procedures are complete, volunteers will provide a saliva sample and breathalyzer for "on the spot" drug screens. If drug screens are negative for sedatives and stimulants (both of which can alter the acoustic startle response), research volunteers will be interviewed using the Structured Interview for PTSD (SI-PTSD).

After completing the SI-PTSD (and it is clear that the subject meets criteria for PTSD), the subject will be be instructed to don earphones as well as noise insulating earmuffs. The MARS evaluation will then proceed in either a sitting or standing position, and the sound stimuli will be conducted at two frequencies [500 and 4000 Hertz (Hz)] at three volumes [65, 90, and 105 decibels (dBs)] for a total of 12 acoustic test sets, in random order. The MARS shall record both left and right eye reactions to each of these 12 acoustic/posture sets. The MARS protocol shall be administered for approximately 6 minutes while the volunteer is sitting/standing quietly and looking forward at the mobile device (which will be located approximately 60 centimeters away).

Within 3 weeks of MARS measurements, the study psychiatrist will conduct a private interview with the participant via remote video. The study psychiatrist (Dr. McCann), who will be blind to MARS data, will interview subjects using the Clinician-Administered PTSD Scale for the Diagnostic and Statistical Manual-5 (DSM-5) (CAPS-5).

Investigators will determine the statistical relationship between PTSD severity (as measured on a numerical scale from 1-5) and eye blink parameters. All data points will be entered into machine learning software (a.k.a. deep learning software) both to analyze interconnections between outcome measures and PTSD severity, but also to make predictions for future studies in PTSD patients evaluated using the MARS.

Inclusion Criteria:

• diagnosis of PTSD

Exclusion Criteria:

• positive drug or alcohol screen

Data analyses will be conducted by study investigators in New Zealand. Both sites (Baltimore and New Zealand) will have access to all study data.

Data will be available to investigators in New Zealand at the time of data analyses, approximately one year after the study has begun.

Only Dr. Yusuf Cakmak and data analyst (who will conduct the data analyses) will have access to the data. They will have a copy of the data which will be saved on a secure server.

The study psychiatrist will be blind to MARS results at the time of the psychiatric interview that will confirm PTSD diagnosis and PTSD severity.