Unpaired extinction: implications for treating post-traumatic stress disorder.

Extinction of fear is important for treating stress-related conditions particularly post-traumatic stress disorder (PTSD). Although traditional extinction presents the feared stimulus by itself, there is evidence from both clinical and basic research that repeatedly presenting the feared stimulus by itself does not prevent fear from returning. This renewal or relapse can be "thwarted" by unpaired extinction-presentations of the feared stimulus and the event producing the fear. However, no matter how effective standard unpaired extinction may be in the laboratory, repeated presentation of a traumatic event is untenable. To make an unpaired extinction procedure more clinically relevant, we classically conditioned the rabbit nictitating membrane response using electrical stimulation or air puff as the unconditioned stimulus and then during unpaired extinction reduced both the intensity of the unconditioned stimulus and the days of unpaired stimulus presentations. We found unpaired extinction reduced conditioned and exaggerated unconditioned responding (an animal analog of PTSD called conditioning-specific reflex modification) and could be accomplished with a weak unconditioned stimulus as long as extended presentations were used. Surprisingly, brief presentations of a weak unconditioned stimulus or extended presentations of a strong one made the exaggerated responses stronger. One implication is that brief treatment may not just be ineffectual; it may heighten the symptoms of PTSD. Another implication is that using strong stimuli may also heighten those symptoms.