The representation of a neutral stimulus in the primate amygdala changes after the animal learns to associate the stimulus with another, fear inducing stimulus, reports a study published online in Nature Neuroscience. This change in the neural representation of a previously neutral stimulus could underlie fear generalization, in which an animal’s associations of fear with one stimulus extend to other, similar stimuli. Overgeneralization of aversive memories is thought to contribute to anxiety disorders in humans.
Rony Paz and colleagues studied non-human primates trained to associate an aversive odour (the unconditioned stimulus) with a tone (the conditioned stimulus). Following training, the primates showed fear responses, not only to the conditioned stimulus, but to tones similar in pitch to the conditioned stimulus as well.
Recordings from neurons in the amygdala, a part of the brain involved in fear learning and implicated in anxiety disorders, showed that individual amygdala neurons responding to the tone changed their preference (“tuning”) for tones of different pitch following fear learning. Following fear learning, some neurons shifted their preference towards the conditioned stimulus and narrowed their tuning, whereas others shifted their preference away from the conditioned stimulus and broadened their tuning. These changes in neural tuning for tones could account for the behavioral generalization that occurred following fear learning.