Rhythmic “slow wave” signals that pulse through the brain during deep sleep have been proved to originate in the cerebral cortex. Using light to stimulate and observe these waves in unprecedented detail could provide scientists with a better understanding of learning and memory formation, a new international study suggests. The rhythmic signal pulses, which sweep through the sleeping brain at the rate of about one cycle per second, are assumed to play a role in processes such as consolidation of memory. Previous studies, which relied mainly on electrical measurements, lacked the spatial resolution to precisely map the initiation and propagation of these waves. Now, scientists from the Technical University of Munich (TUM), in collaboration with researchers at Stanford University in California and the Johannes Gutenberg University of Mainz, have ruled out many longstanding hypotheses by showing conclusively that slow waves start in the cerebral cortex, the part of the brain responsible for cognitive functions. A brief pulse of light delivered to a local cluster of neurons through an optical fiber can induce a wave of neuronal activity that spreads across the entire cortex, new research from the US and Germany suggests. Illustrated here using a computer model of the mouse brain, the actual experiment is performed on the intact brain of a live mouse under anesthesia. Courtesy of Albrecht Stroh/©University of Mainz. “The brain is a rhythm machine, producing all kinds of rhythms all the time,” said TUM professor Arthur Konnerth. “These are clocks that help to keep many parts of the brain on the same page.” One such timekeeper produces the so-called slow waves of deep sleep, which are thought to be involved in transmuting fragments of a day's experience and learning into lasting memory. They also can be observed in very early stages of development and may be disrupted in diseases such as Alzheimer's. The researchers also discovered that such a wave can be set in motion by a tiny cluster of neurons. “Out of the billions of cells in the brain, it takes not more than a local cluster of 50 to 100 neurons in a deep layer of the cortex, called layer 5, to make a wave that extends over the entire brain,” Konnerth said. Despite considerable investigation of the brain's slow waves, definitive answers about the underlying circuit mechanism have remained elusive. Where is the pacemaker for this rhythm? Where do the waves start, and where do they stop? This study — based on optical probing of intact brains of live mice under anesthesia — now provides the basis for a detailed, comprehensive view. Optogenetics enables researchers to insert light-sensitive channels into specific types of neurons, shown green in this micrograph. Other neurons are shown in red. Through an optical fiber (right), scientists can use light both to stimulate these cells and record their response. Courtesy of Albrecht Stroh, Arthur Konnerth/©TU Munich. “We implemented an optogenetic approach combined with optical detection of neuronal activity to explore causal features of these slow oscillations, or up-down state transitions, that represent the dominating network rhythm in sleep,” said professor Albrecht Stroh of the Johannes Gutenberg University of Mainz. This allowed for selective and spatially defined stimulation of small numbers of cortical and thalamic neurons. Accessing the brain with optical fibers enabled the researchers to microscopically record and directly stimulate neurons. Flashes of light near the mouse’s eye were used to stimulate neurons in the visual cortex. The researchers made slow waves visible by recording the flux of calcium ions, a chemical signal that can serve as a more spatially precise readout of the electric activity. They also correlated optical recordings with more conventional electrical measurements, making it possible to watch individual wavefronts spread first through the cortex and then through other brain structures. Not only is it possible for a tiny local cluster of neurons to initiate a slow wave that will spread far and wide, recruiting multiple regions of the brain into a single event, but this also appears to be typical. These measurements were made by optically recording the flux of calcium ions in optogenetically transduced neurons in a mouse's brain. In the train of four consecutive brain waves shown here, optogenetically induced waves alternate with waves evoked by visual stimulation (sun symbol). A striking feature is the virtually identical pattern of the visually and optogenetically evoked waves — keeping in mind that less than 100 neurons in the cortex have been optogenetically stimulated. Stimulating these few neurons leads to the generation of a wave recruiting the entire cortex. Courtesy of Albrecht Stroh, Arthur Konnerth/©TU Munich. “In spontaneous conditions,” Konnerth says, “as it happens with you and me and everyone else every night in deep sleep, every part of the cortex can be an initiation site.” Furthermore, a surprisingly simple communication protocol can be seen in the slow wave rhythm. During each one-second cycle, a single neuron cluster sends its signal, while all others are silenced, as if they are taking turns bathing the brain in fragments of experience or learning, building blocks of memory. The investigators are now testing how the slow waves behave during disease. The findings were published in Nature (doi: 10.1016/j.neuron.2013.01.031). For more information, visit; www.tum.de