Feb. 6, 2001 Champaign, IL — Flee, freeze or fight. A response to a threat is based on experience and memory. Now scientists have discovered that an area of the brain, the amygdala, which was thought to store painful and emotion-related memories, also initiates memory storage in other brain regions.
There has been a growing debate on the function of the amygdala [pronounced uh-MIG duh-luh], an almond-shaped sub-cortical structure in the temporal lobe. It receives electrical signals carrying auditory information through axons traveling one way from the medial geniculate (MG) nucleus in the thalamus.
New research published in the Jan. 1 issue of the Journal of Neuroscience suggests that the amygdala plays a pivotal role in the initial process of storing memory elsewhere in the brain. The amygdala appears to decide which experiences are important enough to store – a decision based on the emotional significance of the events in a decoding process that affects both learning and memory.
“Our data show that a disabled amygdala leads to a breakdown of learning-related changes in other parts of the brain,” said Michael Gabriel, a professor of psychology at the University of Illinois Beckman Institute for Advanced Science and Technology. “Specifically, disabling the amygdala blocks learning-related changes in the sensory pathway, the media geniculate nucleus. These changes are essential for the ability to discriminate between important and unimportant sounds.”
Amy Poremba of the National Institute of Mental Health in Bethesda, Md., is the co-author of the study. The National Institutes of Health funded the project through a grant to Gabriel, whose lab uses technology that allows for the simultaneous tracking of firing neurons in several places of the brain. In the study, involving 26 male rabbits, researchers temporarily disabled the amygdala. Rabbits with unaltered brains were able to learn the consequences of two differently sounding tones: one that resulted in nothing and another followed in five seconds by a mild shock to the feet. Rabbits that correctly learned the consequences could avoid the shock by moving the wheel under their feet. Those with blocked amygdalas failed to learn such a response to the shock-predicting tone.
In a follow-up study to appear later in the same journal, the researchers blocked the auditory cortex, through which return signals travel from the amygdala to the medial geniculate. Again, rabbits failed to differentiate the tones.
Instead of just getting input from the medial geniculate, Gabriel said, the amygdala appears to send signals back to the medial geniculate, allowing neurons to decipher the significance of the sounds. “This work puts the amygdala in the middle of the circuitry, in a very prominent position in terms of relevance toward learning,” Gabriel said. The findings, he added, also are consistent with a theory originally formulated by James L. McGaugh of the University of California at Irvine. The new data, however, show how the theory is implemented in terms of the activity in the brain circuitry.
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