Sleep : the architect of memory

The influence of emotions

As mentioned above, Pierre Maquet and his team have recently published the results of a second study in PLOS Biology (October 2007). It is known empirically that "emotional" (i.e. emotionally charged) events tend to be remembered more easily than "neutral" ones. This led the Liège researchers to question whether a particular treatment is reserved for consolidation of this type of memory during sleep.

They first asked volunteers to look at pictures of a neutral and emotional nature, without telling them that their memories would be tested afterwards. This procedure is called incidental encoding : the volunteers were told that the research was investigating how emotion influences perception.

The pictures were selected from the International Affective Pictures System (IAPS), an American data bank containing hundreds of pictures whose emotional charge has been tested on a large sample population. Some of the pictures are neutral, some evoke positive emotions – a smiling child, for instance – while others have a negative connotation – a dead animal, a dirty toilet seat, etc.

The volunteers were presented with images and asked to grade them from -3 (very negative emotion) to +3 (very positive emotion), with 0 representing a neutral emotion. Half of the volunteers were deprived of sleep the following night, while the other half were allowed to sleep. Both groups were permitted uninterrupted sleep the following two nights. All volunteers came back refreshed for the memory test on day 3. Again presented with a series of pictures, they were asked to determine whether they had seen them before or not. If the volunteers claimed to have seen a picture before, they then had to specify whether they remembered having encoded it seventy-two hours earlier as well as the encoding context. This procedure, known as "remember/know", is used to isolate the episodic part of a memory from the more automatic component which is linked to familiarity and relies on a memory system relatively independent of the hippocampus. Cerebral activity during the test was monitored by fMRI.

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