Emotional tagging of memory formation--in the search for neural mechanisms

Memory-related areas, such as the hippocampus, should be able to sort out the more significant from the less relevant aspects of an experience in order to transform only the earlier into long-term memory. We have recently suggested the Emotional Tagging concept, according to which the activation of the amygdala in emotionally arousing events mark the experience as important and aids in enhancing synaptic plasticity in other brain regions. Here, we review evidence from both human and animal studies that lend support to the Emotional Tagging hypothesis and to the central role the amygdala may play in its formation. We further speculate on potential neural mechanisms that may underlie emotional tagging. Long-term memory formation is considered to involve lasting alterations in synaptic efficacy, known as synaptic plasticity. It has been suggested that two factors are crucial for obtaining a synapse-specific long-term plasticity: (a) the successful activation of a synapse-specific, protein synthesis-independent tag, and (b) the activation of synapse-non-specific protein synthesis. The activation of protein synthesis can then induce lasting plasticity only in those synapses marked by a tag. Interestingly and relevant to the Emotional Tagging hypothesis, it has been recently shown that the activation of the amygdala could transform transient into long-lasting plasticity. These recent findings seem to fit well with the Emotional Tagging hypothesis. It seems reasonable to assume that the activation of the amygdala triggers neuromodulatory systems, which in turn reduce the threshold for the activation of the synaptic tag, and by this facilitate the transformation of early- into late-phase memory.