Emotional memory: in search of systems and synapses.

The neural system underlying the conditioning of autonomic and behavioral fear responses to auditory stimuli is now understood in some detail. It involves projections through the auditory system to the medial geniculate body and from there directly to the amygdala. The lateral nucleus is the sensory interface and the central nucleus the motor interface of the amygdala. The lateral nucleus projects to the central nucleus indirectly, by way of the basolateral nucleus. Projections from the amygdala central nucleus to the midbrain central gray region mediate the behavioral responses whereas projections from the amygdala central nucleus to the lateral hypothalamus mediate the autonomic responses. Emotional memories established through such pathways bypass the neocortex and may contribute to the unconscious processing of emotion. Such memories are indelible (highly resistant to extinction). The sensory input pathway exhibits long-term synaptic potentiation and appears to utilize glutamate in synaptic transmission. An excitatory amino acid-mediated form of synaptic plasticity in the lateral amygdala may be responsible in part for emotional learning.