The role of the amygdala in memory

The amygdala plays a key role in the modulation of memory consolidation. Following any learning event, the long-term memory for the event is not instantaneously formed. Rather, information regarding the event is slowly put into long-term storage over time, a process referred to as "memory consolidation", until it reaches a relatively permanent state.

During the consolidation period, the memory can be modulated. In particular, it appears that emotional arousal following the learning event influences the strength of the subsequent memory for that event. Greater emotional arousal following a learning event enhances a person's retention of that event. Experiments have shown that administration of stress hormones to a person immediately after the person learns something enhances that person's retention when they are tested two weeks later.

The amygdala, especially the basolateral amygdala, plays a key role in mediating the effects of emotional arousal on the strength of the memory for the event, as shown by many laboratories including that of James McGaugh. These laboratories have trained animals on a variety of learning tasks and found that drugs injected into the amygdala after training affect the animals' subsequent retention of the task. These tasks include basic Pavlovian tasks such as inhibitory avoidance (where a rat learns to associate a mild footshock with a particular compartment of an apparatus) and more complex tasks such as spatial or cued water maze (where a rat learns to swim to a platform to escape the water). If a drug that activates the amygdala is injected into the amygdala, the animal has better memory for the training in the task. If a drug that inactivates the amygdala is injected into it, the animal has impaired memory for the task.

Despite the importance of the amygdala in modulating memory consolidation, however, learning can occur without it, though such learning appears to be impaired.

Evidence from work with humans indicates that the amygdala plays a similar role. Amygdala activity at the time of encoding information correlates with retention for that information. However, this correlation depends on the relative "emotionalness" of the information. More emotionally arousing information increases amygdala activity and that activity correlates with retention.

Papers

 * Da Cunha, C., Wolfman, C., Huang, C., Walz, R., Koya, R., Bianchin, M., Medina, J. H. and Izquierdo, I. (1991). Effect of posttraining injections of flumazenil into the amygdala, hippocampus and septum on retention of habituation and of inhibitory avoidance in rats, Brazilian Journal of Medical and Biological Research 24 301-6.
 * Izquierdo, I., Quillfeldt, J. A., Zanatta, M. S., Quevedo, J, Schaeffer, E., Schmitz, P. K. and Medina J. H. (1997). Sequential role of hippocampus and amygdala, entorhinal cortex and parietal cortex in formation and retrieval of memory for inhibitory avoidance in rats, European Journal of Neuroscience 9, 786-93
 * McGaugh, J. L. (2002). Memory consolidation and the amygdala: A systems perspective, Trends in Neurosciences 25, 456-61.
 * McGaugh, J. L., Ferry, B., Vazdarjanova, A. and Roozendaal, B., (2000) Amygdala: Role in modulation of memory storage. In Aggleton, J. P. (ed.), The Amygdala: A Functional Analysis, Oxford University Press, London.
 * Packard, M. G, Cahill, L. and McGaugh, J. L. (1994). Amygdala modulation of hippocampal-dependent and caudate nucleus-dependent memory processes, Proceedings, National Academy of Sciences, USA 91, 8477-81
 * Packard, M. G., and Teather, L. (1998). Amygdala modulation of multiple memory systems: Hippocampus and caudate-putamen, Neurobiology of Learning and Memory 69, 163-203.
 * Tomaz, C., Dickinson-Anson, H. and McGaugh, J. L. (1992). Basolateral amygdala lesions block diazepam-induced anterograde amnesia in an inhibitory avoidance task, Proceedings, National Academy of Sciences, USA 89, 3615-19.
 * Walker, D L., Ressler, K. J., Lu, K. T. and Davis, M. (2002). Facilitation of conditioned fear extinction by systematic administration or intraamygdala infusions of d-cycloserine as assessed with fear-potentiated startle in rats, Journal of Neuroscience, 22, 2343-51.