In the 2004 film Eternal Sunshine of the Spotless Mind, Jim Carrey and Kate Winslet star in the roles of recently separated lovers Joel and Clementine, who both undergo a radical medical procedure to have the painful memories of their relationship erased. The ability to erase memories is no longer restricted to Hollywood script-writers: in an advance online publication in Nature Neuroscience, a group of researchers at New York University’s Center for Neural Science, led by Joseph LeDoux, report that they have succeeded in preventing the transfer of a single fearful memory from short-term to long-term storage, while leaving another memory, which was encoded at the same time, intact.
>LeDoux’s team trained rats to fear musical tones by giving them electric shocks while they heard the tones. This is called classical (or Pavlovian) conditioning: by associating the musical tones with electric shocks, the rats learn to expect an electric shock whenever they hear the tones. Fear behaviour was assessed by the extent to which the rats freeze when they hear the tones associated with the electric shock. Half of the animals were treated with a drug called U0126, which causes limited amnesia. The rats were then played one of the musical tones which they had been conditioned to fear; those treated with U0126 were still under the influence of the drug when they heard the tone.
The rats’ memories of the tones were tested the next day. Whereas the control group remained fearful of both, the animals treated with U0126 no longer displayed any fear behaviour when they heard the tone that had been played to them while under the influence of the drug. Microelectrodes were used to record the electrical activity of neurons in the amygdala, a small, almond-shaped structure on the medial surface of the temporal lobe). It was found that, in the rats treated with the amnesia drug, there was reduced activity in the lateral nucleus of the amygdala.
The transfer of memories from short-term to long-term storage requires reconsolidation. This involves the strengthening of existing synaptic connections, or the formation of new synapses, both of which require the expression of certain genes and the synthesis of their protein products. It is these processes that were disrupted in this study – if the effects of the drug remained during reconsolidation, the memory of the association between the musical tone and the electric shock was erased. The study provides further evidence of the role of the amygdala in the encoding of fearful memories, and could eventually lead to the development of drugs for the treatment of conditions such as post-traumatic stress disorder (PTSD).
>Doyère, V., et al. (2007). Synapse-specific reconsolidation of distinct fear memories in the lateral amygdala. Nature Neurosci. doi:10.1038/nn1871 [Abstract