Amygdala role in conditioned associative learning.

Amygdala role in emotion was reviewed in reference to recent amygdala lesion studies and neuronal responses in the rat amygdala to conditioned stimuli. Extensive lesion studies suggest that the amygdala is crucial in various kinds of motivated and emotional behavior, and related autonomic responses. These amygdala functions critically depend on learning and memory. Amygdala lesions, both before and after training of conditioned associative learning, impaired emotional expression without simple sensory-motor deficits. Pharmacological experiments indicated neurotransmission in the amygdala is mediated through NMDA and AMPA receptors. These results strongly suggest the amygdala involvement in acquiring and storing associative memory (i.e. stimulus-affect association), by which animals recognize and evaluate the biological significance of a stimulus. This information is then transferred to the brainstem executing system. In the neurophysiological experiments, there were topographic distributions of sensory-responsive neurons within the amygdala, which were well correlated to anatomical data. The responses of rat amygdala neurons changed plastically during learning. Furthermore, more sensory-responsive neurons were encountered in the amygdala of rats trained to associate the sensory stimuli with a reinforcement than in the amygdala of rats that were not trained. In trained rats, multimodal neurons that responded to conditioned and unconditioned stimuli were frequently found in the basolateral and central nuclei of the amygdala. The results suggest that basolateral and central nuclei are foci where various sensory modalities converge, and which might perform critical functions in acquiring and storing long-term associative memory to link between sensory information and affective significance.