Monday, August 09, 2010

We generalize negative more than positive stimuli

Schechtman et al. find that our learned response to a sound frequency associated with negative reinforcement generalizes over a broader range of adjacent frequencies (has a wider generalization curve) than the response to a positive reinforcement, both being greater than the response to a neutral stimulus. This 'better be safe than sorry' strategy makes sense in terms of survival, as noted in the author's introduction. They discuss the role of the amygdala, and suggest that individual differences in the effect that emotional valence has on generalization could underlie susceptibility to long-term effects of emotional events, possibly explaining why some people seem more susceptible to post-traumatic stress disorder (PTSD).:
If you are a dog, mistaking another dog bark for a lion roar is a shame—you might have missed a friend; but mistaking a roar for a bark can be fatal...Because a miss in this example is much more "costly" than a false-alarm, a more liberal bias for the loss-related stimulus can be expected.... in real-life, it is rarely the exact specific roar or specific bark you once heard, and stimuli rarely repeat with the exact physical properties with which they were first encountered. This means that the same logic used for immediate response should generalize to similar, yet not identical, sounds. We therefore hypothesized that valence associated with a stimulus during learning would influence the scope of generalization, and specifically, that stimuli that were previously associated with loss would generalize more than stimuli that were previously associated with gain.
Here is their abstract, followed by a graphic:
Learning includes the ability to generalize to new situations and respond to similar, yet not identical stimuli. We use stimulus generalization in humans to show that tones that were negatively reinforced induce wider generalization curves than tones that were positively reinforced, and these in turn induce wider curves than neutral memory. Importantly, these wider generalization curves persist even if outcomes for all tones are made identical, indicating that the learning induced a perceptual change, and not merely a decision bias. Moreover, it persists after taking into account loss-aversion, suggesting it is a result of valence per se, and not intensity that reflects overweighting of the aversive stimuli. This effect of emotional valence on learning suggests different locations of plasticity and network mechanisms in the brain. Particularly, it suggests that brain areas that mediate reinforcement and emotions are involved during the learning process to induce a neural representation that can support this broader behavioral generalization. In addition, these findings highlight a model for anxiety and trauma disorders in which aversive experiences affect more than they should, sometimes even in seemingly irrational situations.

Figure - The amount of generalization for negative (aversive), positive (rewarding), and no (neutral)—reinforcement... Average response rate in the generalization stage for tones in different distances from the tones that were reinforced during the acquisition stage.

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