Experiments in rats show some bad memories can be forgotten

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It is believed that exaggerated response to bad memories is similar for all negative memories. Now, a team of Indian scientists have shown that exaggerated response and difficulty to get rid of bad memories could depend on whether the bad memory was formed before or after a stressful event.

The finding is based on experiments done in rats using a technique called fear conditioning. When a rat is presented with a sound tone along with an aversive cue, it forms a memory that the tone is bad. The rat freezes in fear whenever the tone is played. But when the tone is repeated without the aversive cue, the animal learns to forget aversive memory and realises that the tone is not bad.

When rats underwent stressful experience before fear conditioning, they showed increased fear response and inability to forget aversive memory. In contrast, when they underwent the stressful experience afterwards, they did not show any enhanced response fear or inability to extinguish the fear memory.

Researchers also recorded brain activity of the rats as they underwent fear conditioning and stressful experience. It was found that although amygdala (emotional hub of the brain) remained hyperactive in stressed animals, it did not affect expression of fear memory. The prefrontal cortex which remained relatively unaffected in stressed animals seemed to control the normal fear response.

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