Here's one for you Michael...
Wednesday, April 4, 2007
Power naps enhance memory consolidation
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 the Sleep and Neuroimaging Laboratory at
Harvard Medical School’s Department of Psychiatry, enlisted 26 healthy
participants for the study. One morning, the participants were trained to
perform a simple motor task with the left hand; the task involved learning a
sequence of 5 key presses. The participants were split into two groups; one
group had an afternoon 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 an
electroencephalogram (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.
This 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 afternnon power nap - includes the
corresponding stages (NREM), newly-encoded memories will be consolidated.
Reference:
Nishida, M. & Walker, M. P.
