<qa>  What is the AXBX- paradigm?2. To compare 15 subjects with PTSD, 15 subjects with a history of combat exposure but no PTSD, and 15 normal controls using this acoustic startle methodology. We hypothesize that PTSD subjects will have normal baseline startle, intact fear potentiation, and impaired inhibition of fear potentiation as measured by the AXBX- paradigm. We predict that combat controls will have results intermediate between PTSD subjects and normals. 