It is well established that memories are consolidated during sleep. Numerous studies show that the formation of different kinds of memories – motor learning and declarative memory, for example – is enhanced as we sleep during the night. Enhanced memory consolidation takes place during the non-rapid eye movement (NREM) stage of sleep. Now, a new study published online in the journal PLoS One, shows that afternoon naps have the same effect on memory formation, and provides further insight into the processes of sleep-dependent memory consolidation.
Masaki Nishida and Matthew Walker, of Sleep and Neuroimaging Laboratory at Harvard Medical School‘s Department of Psychiatry, enlisted 26 healthy participants for the study, and split them into two groups. All of them were trained to perform a simple motor task with the left hand, involving learning a sequence of 5 key presses. One group then had a nap lasting between 60-90 minutes after learning the task, while the other remained awake.
The ability of all the participants to perform the task as quickly and as accurately as possible was then tested. In those who had taken a siesta, there was a significant improvement in performance of the task. By contrast, no significant improvement in task performance was observed in the participants who had remained awake.
Motor learning is known to produce organizational changes in the motor cortex. Consistent with this, Masaki and Walker show a precise correlation between the memory consolidation and activity in the motor cortex. They used electroencephalography (EEG) to record the electrical activity of the brains of participants while they slept.
In the figure on the left, the blue discs correspond to the positions of the electrodes; electrode C4 recorded activity from the motor cortex in the right hemisphere. The left hemisphere of the brain controls the movements of, and receives sensory inputs from, the right side of the body, and vice versa (this is known as contralateral control); the cerebellum, however, controls the ipsilateral (same) side of the body. Hence, because the participants used their left hands to perform the task they had learnt, a subtle increase in activity was recorded in the motor cortex in the right, but not the left, hemisphere.
The stages of sleep are defined by characteristic patterns of electrical activity in the brain. The NREM stage of sleep (during which one does not dream, and is easily awoken) is characterized by “spindles” – bursts of electrical activity with frequency of 12-15 Hz that last for 0.5 – 1.5 seconds. The spindles recorded in the corresponded to the increased neuronal activity in the right motor cortex and, therefore, with the memory consolidation.
his study confirms that the consolidation of motor memories is associated with a particluar stage of sleep (NREM), and that this in turn is correlated with electrical activity in an anatomically discrete region of the brain (the motor cortex). One interpretation of the findings is that power naps trigger accelerated memory consolidation. An alternative hypothesis is that a good night’s sleep consists of multiple stages which are devoted to the consolidation of memories encoded during waking hours; thus, a full night’s sleep may not be necessary for this consolidation to take place; as long as a sleep episode – be it a a short night’s sleep or an afternoon power nap – includes the corresponding stages (NREM), newly-encoded memories will be consolidated.
Reference: Nishida, M. & Walker, M. P. (2007). Daytime naps, motor memory consolidation and regionally specific sleep spindles. PLoS One 2: e341. doi:10.1371/journal.pone.0000341 [Full text]