Scientists have demonstrated for the first time that brain circuitry is modified during sleep. The finding bolsters the hypothesis that memories are made as we snooze, through a process that involves rewiring networks of nerve cells.
A raft of evidence suggests that the tracks of memory are laid down during sleep, as the somnolent brain replays, analyzes, and stores the lessons of the day. The standard view is that this memory consolidation entails remodeling of brain circuits, says Marcos Frank, a neuroscientist at the University of California, San Francisco. This requires "synaptic plasticity," a situation in which some neural pathways presumably send stronger signals and sprout new connections between cells, while other pathways see weaker signals and loss of connections. However, no one had demonstrated that these changes actually happen in neural circuits during sleep, Frank says.
Frank and his colleagues studied kittens that were a month old--a stage when their brains are at their most flexible. They looked for synaptic plasticity by blocking vision in one eye of a young cat; this is known to induce swift changes in the visual cortex and cause neurons to stop responding to impulses from the blocked eye. All the cats in the study had one eye sewed shut and were kept awake for 6 hours, by gently moving their cages or playing tapes of meowing, to allow these modifications to begin. Then some cats were allowed to sleep, while others were kept awake for 6 more hours, some in the dark and others in the light. Frank and his colleagues used microelectrodes to measure the activity of neurons within the visual cortex and gauge the degree of neural remodeling as the brain adapted to the loss of stimulation.
Cats that were allowed to sleep after the initial 6-hour period showed more than twice the degree of neural change compared to those kept awake in the dark. The results suggest that sleep enhances brain remodeling, the researchers report in the April issue of Neuron, and may explain why young mammals with growing, developing brains sleep so much. "The study gets us very close to understanding the function of sleep," Frank says.
Sleep experts are impressed. "I think it is an extremely important paper," says neuroscientist Bruce McNaughton of the University of Arizona in Tucson. He adds that the next step is to show that sleep is also important in "higher level" synaptic plasticity, such as the formation of memories.