Off-line processing of memory traces during human sleep: Contribution of functional neuroimaging

Sleep is thought to participate in the consolidation of recent memory traces. We tested this hypothesis
in humans by using functional neuroimaging (functional magnetic resonance imaging and
positron emission tomography). First, following the training procedural visuo-motor learning task
(pursuit task), total sleep deprivation on the first post-training night significantly deteriorates the
gain in performance, which is usually observed after one complete night of sleep. In parallel, sleep
deprivation hampers the changes in functional segregation and connectivity, which underpin the
gain in performance usually observed in subjects allowed to sleep on the first post-training night.
Second, following the training on an implicit memory task (probabilistic serial reaction time task),
some brain areas are reactivated during rapid eye movement (REM) sleep on the first post-training
night. The reactivation was shown to be related to the processing of high-level material and to be
modulated by the amount of learning achieved during the training session. These changes in activity
do not involve isolated brain areas but entire macroscopic cortico-subcortical networks. Taken
together, the results suggest an off-line processing of recent memory traces during sleep.