Unconscious Exposure to a Feared Stimulus Can Reduce Fear, Study Shows

For many years, exposure therapy has been a first-line behavioral treatment for people diagnosed with anxiety disorders such as social phobia and PTSD that involve a paralyzing or disabling fear. War veterans, for example, whose trauma may be linked in memory with the sound of an explosion can sometimes be helped by being exposed under controlled clinical conditions to loud sounds that have the power to trigger their fear response even when no threat is really present.

Exposure therapy is well known to be effective for many PTSD and anxiety disorder patients. But it has a significant drawback: some fearful or anxious people find it hard to endure even controlled exposures to feared stimuli. This can lead them to drop out of exposure therapy, or to avoid treatment entirely.

Paul Siegel, Ph.D., of Purchase College, the State University of New York, devoted his 2014 BBRF Young Investigator project to exploring a phenomenon called the “Very Brief Exposure Effect.” In a paper newly published in Lancet Psychiatry, Dr. Siegel and associates report early success in a clinical experiment that sought to make use of this effect, hoping it might one day help people with fear and anxiety disorders who are unable to endure exposure therapy. The team’s senior member was Bradley Peterson, M.D., of the University of Southern California, a 2010 BBRF Distinguished Investigator, 2002 Independent Investigator and 1996 Young Investigator.

Drs. Siegel, Peterson and colleagues recruited a group of 82 women—half of them with a specific phobia, half with no phobias or other disorders—to test if exposure might still be effective even when the person receiving it is not conscious of undergoing exposure to the stimulus that triggers their fear. Their hypothesis was that the brain’s fear mechanisms, including those governing the extinction of fear, rely on automatic processes that occur when one is exposed to a phobic stimulus, be it conscious or not.

This is where the “very brief exposure effect” comes into play. The idea is to present repeated sequences of very brief exposures to the feared stimulus, followed immediately by what psychologists call a “masking” stimulus, which conceals the feared stimulus. Through such repeated exposure to the feared stimulus without the person’s conscious perception, “the fear response is desensitized at an unconscious level of processing,” Dr. Siegel explains.

The women recruited for the study had a carefully assessed, and intense, fear of spiders. This is not usually a disabling fear, the researchers noted—and this was by design. The study would test whether people with a very pronounced fear of something specific could be unconsciously exposed to that stimulus as a way of learning to overcome their fear.

An intense fear of spiders—or a relative absence of that fear—was documented via a test which gauged participants’ willingness to move progressively closer to a live tarantula in a terrarium. Phobic participants met diagnostic criteria for specific phobia, while controls did not meet criteria for any psychiatric disorder. The women were randomly assigned to subgroups: those who would receive very brief exposure (spider images) followed by masking, and those who would receive a placebo stimulus (pictures of flowers) followed by masking. Ten minutes after these exposures were conducted, the women received a functional MRI brain scan, so the researchers could observe, in real time, the changes in brain activity caused by the masked exposures.

Very brief exposure (VBE) via masking had two crucial effects, the team said. First, VBE activated brain circuits known to support the regulation of fear and its associated behavioral responses. Second, after the exposures, participants with phobias were able to move closer to the tarantula; their fear of spiders was reduced.

These results were not inconsistent with the theory behind exposure therapy, the researchers stressed. Just as in standard exposure therapy, the VBE method involves fear-extinction learning. It differs in that the learning in VBE follows from exposures that are not conscious.

For this reason, the researchers propose that if their results are replicated in clinical trials, and in individuals who suffer from more serious and disabling conditions such as PTSD and acute anxiety, VBE might be used as an adjunct to conventional exposure therapy—as a pretreatment, perhaps, that might promote the ability of patients receiving conventional exposure therapy to tolerate the fear that the treatment engenders under controlled conditions.

The fMRI scans of study participants who were helped by VBE suggested that its therapeutic effects may have been mediated by specific brain regions important in automatic (i.e., non-conscious) fear extinction and emotional salience processing. “Our findings challenge the clinical belief that direct confrontation of feared situations—and thus conscious arousal and emotional distress—are necessary to reduce fear,” they wrote.

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