Goodnight sleep

When a rat's eyes are shut in deep sleep, its brain is raging. There are two kinds of waves conflicting inside its head, on the issue of whether information is getting consolidated or forgotten.

It's all about how these conflicts help to resolve how some memories are retained by the brain, while many others get erased, explain researchers in a Cell.

These duelling brain waves determine ideas that appear to contravene each other. For instance, some memories might get reinforced, or they might become undermined even in the same stage of sleep. This gives an insight into learning.

Rats have been taught to monitor a mechanical water spout with just their neural activity, by researchers led by neuroscientist and neurologist Karunesh Ganguly of the University of California, San Francisco. How successful the rats were with the brain-computer interfaces seemed to be dependent on the amount of sleep the rats took after they were trained.

Hence, after the rats finished practising moving a water spout and went to sleep, the team monitored their brains, especially the brain waves washing over the motor cortex, or that part of the brain that controlled the external water spout in the stage of non-REM sleep. Such a stage of sleep constitutes more than half an adult's nocturnal cycle.

There were two kinds of brain waves. One was the slow oscillations that seemed to strengthen memories. Scientists experimented with laser lights and genetic tricks in order to halt the oscillations only a few milliseconds after they started, while the rats were sleeping. After they woke up, the rats took longer to move the water spout, using their brains after they woke up. As these slow oscillations across the sleeping rat's motor cortex were halted, the data or training information did not get absorbed.

The second type of brain waves are called delta waves. These are three to four times in number compared to the slow oscillations. However, what was their role has not been understood, even though the scientists feel that they are very important, as they are so prevalent.

Interestingly, halting delta waves was completely opposite to halting slow oscillations, found the researchers. After they halted the delta waves, the rats could perform their duties better, suggesting that the delta waves speeded up forgetting. The research paper therefore makes it clear that slow oscillations and delta waves have totally the opposite functions.

The key to retaining some memories over others might just be a reward, such as a gift, or a feeling of happiness after a conversation.