Hippocampal and amygdala interactions in the formation of emotional memories to context: Fear and loathing in context
Contextually based fear responses can be elicited after Pavlovian fear conditioning. That is, contexts can be encoded in memory as fearful. Four studies were performed to evaluate the role of the amygdala and hippocampal formation in contextual fear conditioning. In the first study, acquisition of a fear response to foreground (a phasic CS) and background (context) cues was disrupted by lesions of the amygdala, while disruption of conditioning to the background context alone was seen after lesions of the hippocampus. In the second study, hippocampal lesions did not disrupt conditioning to contextual stimuli when no CS was presented in association with the US. Anatomical connections between the hippocampal formation (including the ventral subiculum, area CA1, and the lateral and medial entorhinal cortices) and the amygdala were described in the third study. In the fourth study, subcortical connections with the hippocampus were found to be necessary and sufficient for acquisition of a fear response to background context, as lesions of the fornix (the pathway for hippocampal/subcortical structure connections) disrupted conditioning to the background context. Neocortical connections with the hippocampus, however, were not required for conditioning to background or foreground stimuli, as these cortical lesions did not disrupt conditioning. The amygdala plays a critical role in contextual conditioning as an associative structure, receiving contextual information from the hippocampus which serves to integrate multi-modal stimuli into a conditionable stimulus.