Discriminative fear conditioning to context and emotional memory systems in the brain of the rat

In the field of neuroscience there is increased interest in defining the neural substrates that mediate different forms of fear-motivated learning especially fear conditioning. Procedurally, stimuli that co-occur with an aversive event (footshock) acquire predictive properties and act as signals for the aversive event. The learning is expressed when the post-training presentation of those cues elicit conditioned fear responses in the organism. Fear is conditioned not only to discrete phasic cues but also to the surrounding permanent cues that define the context. Contextual cues have no defined temporal relationship with the footshock, but are omnipresent and represent a very potent stressor for the animal. Convergent findings implicate the amygdala and the hippocampus in the acquisition and consolidation of aversive memories that have to do with remembering a fearful environment. Theories in the field diverge on the specific roles and degree of participation of the hippocampus and the amygdala in fear conditioning to context. The context discrimination task used in this set of experiments is: (1) a variant of the standard paradigm and (2) highly dependent on the hippocampus. Multiple behavioural and physiological indices of fear were assessed to examine the role of the hippocampus, amygdala, fornix and nucleus accumbens not only in discriminative fear conditioning to context but also in the performance of unconditioned fear responses in reaction to the aversive event. Damage to either the hippocampus or the amygdala, resulted in selective impairments that were site specific, suggesting that there is a specialization within specific memory circuits that are critical for certain learned responses. The hippocampus and amygdala are both required for another set of context-specific conditioned fear responses suggestive of a synergism between these memory circuits. The lack of a lesion-induced deficit in the performance of unconditioned fear responses in either group supported the idea that the observed deficits are due to a mnemonic impairment. Results from the nucleus accumbens and fornix lesion groups show that these two areas contribute in this fear-motivated context discrimination without participating in the performance of unconditioned fear responses.