Extinction and Reconsolidation of Memory

Extraction (reactivation) of memory on reminding can lead to the process of reconsolidation, in which memory retention occurs, or to extinction, leading to weakening of the existing memory trace or the formation of a competing memory. This review analyzes behavior and also the responses of identified neurons forming the neural network for feeding and aversive behavior in the common snail Helix; an attempt is made to describe the conditions in which reminding leads to extinction or reconsolidation. Studies using a neurotoxin specific for serotoninergic neurons – 5,7-dihydroxytryptamine - demonstrated that the serotoninergic system of the snail is required for formation of memories of dangerous stimuli, though it has no role in maintaining or reproducing these memories. This hypothesis raises the question of the activity of serotoninergic neurons as a key condition for selecting between extinction and reconsolidation triggered by reactivation. If a cell is "silent" as a result of adaptation, acclimation, changes in the surroundings, etc., extinction is observed; if the same neuron responds to the conditioned stimulus, reconsolidation occurs. Memory reconsolidation and extinction are evolutionarily conserved phenomena seen in most vertebrates and many invertebrates, suggesting that they reflect the major characteristics of memory formation and storage. The great variation in brain structure in different vertebrate and invertebrate species prevents the phenomena of reconsolidation and extinction from being regarded as systems properties of the brain; rather, they are the basic neural mechanisms which can be seen in any animal with a sufficiently developed nervous system, regardless of the actual architecture of the brain.

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