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Adversity, we are told, heightens our senses, imprinting sights and sounds precisely in our memories. But recent Weizmann Institute research, which appeared in Nature Neuroscience, suggests the exact opposite may be the case: Perceptions learned in an aversive context are not as sharp as those learned in other circumstances. The findings, which hint that this tendency is rooted in our species’ evolution, may help to explain how post-traumatic stress syndrome and other anxiety disorders develop in some people.
To investigate learning in unfavorable situations, Dr. Rony Paz of the Institute’s Neurobiology Department, together with his student Jennifer Resnik, had volunteers learn that some tones led to an offensive outcome (e.g., a very bad odor), whereas other tones were followed by pleasant a outcome or by nothing at all. The volunteers were later tested for their perceptual thresholds – that is, how well they were able to distinguish either the “bad” or “good” tones from other, similar tones.
As expected from previous studies, in the neutral or positive conditions, the volunteers became better with practice at discriminating between tones. But, surprisingly, when they found themselves exposed to a negative, possibly disturbing stimulus, their performance worsened.
The differences in learning were in fact very basic differences in perception. After learning that a stimulus is associated with a highly unpleasant experience, the subjects could not distinguish it from other, similar stimuli, even though they could do so beforehand or in normal conditions. In other words, no matter how well they usually learned new things, the subjects receiving the “aversive reinforcement” experienced the two tones as the same.
Evolution and post-traumatic stress syndrome
Paz: “A reduced ability to distinguish between stimuli likely made sense in our evolutionary past: If you’ve previously heard the sound of a lion attacking, your survival might depend on a similar noise sounding the same to you – and pushing the same emotional buttons. Your instincts, then, will tell you to run rather than to consider whether that sound was indeed identical to the growl of the lion the other day.”
Paz believes that this tendency might be stronger in people suffering from post-traumatic stress syndrome. As an example, he points to the 9/11 terror attacks in New York. Many of those who witnessed the strikes on the towers developed post-traumatic stress syndrome, which, in many cases, could be triggered by the sight of tall buildings. Intellectually, they might know that the building before them bore little resemblance to the destroyed towers; but on a more fundamental, instinctive level, they might perceive all tall buildings as being the same and thus associate them with terrifying destruction.
The scientific team is now investigating this idea in continuing research, in which they hope, among other things, to identify the areas in the brain involved in setting the different levels of perception. Paz: “We think this is a trick of the brain that evolved to help us cope with threats but is now dysfunctional in many cases. Besides revealing this very basic aspect of human perception, we hope to shed light on the development of such anxiety disorders as post-traumatic stress syndrome.”
Dr. Rony Paz's research is supported by the Nella and Leon Benoziyo Center for Neurological Diseases; the Carl and Micaela Einhorn-Dominic Brain Research Institute; the Kahn Family Research Center for Systems Biology of the Human Cell; the Ruth and Herman Albert Scholars Program for New Scientists; Pascal and Ilana Mantoux, Israel; Katy and Gary Leff, Calabasas, CA; the Candice Appleton Family Trust; Sam Revusky, Canada; and Dr. and Mrs. Alan Leshner, Potomac, MD. Dr. Paz is the incumbent of the Beracha Foundation Career Development Chair.
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