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Study Shows How Memories Ripple Through the Brain

November 6, 2017

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NIH-funded study suggests increased communication between key brain areas during sleep

Posterior and inferior cornua of left lateral ventricle exposed from the side. The hippocampus is located in the medial temporal lobe of the brain. In this lateral view of the human brain, the frontal lobe is at left, the occipital lobe at right, and the temporal and parietal lobes have largely been removed to reveal the hippocampus underneath. Graphic credit: Henry Vandyke Carter - Henry Gray (1918) Anatomy of the Human Body (See Book section below) Bartleby.com: Gray's Anatomy, Plate 739; Public Domain, Wikipedia

The hippocampus (named after its resemblance to the seahorse, from the Greek seahorse from hippos, horse and kampos, sea monster) is a major component of the brains of humans and other vertebrates. Humans and other mammals have two hippocampi, one in each side of the 1) ___. The hippocampus belongs to the limbic system and plays important roles in the consolidation of information from short-term memory to long-term memory, and in spatial memory that enables navigation. The hippocampus is located under the cerebral 2) ___ (allocortical) and in primates in the medial temporal lobe. It contains two main interlocking parts: the hippocampus proper (also called Ammon's horn) and the dentate gyrus. The hippocampus is widely thought to turn new information into permanent memories while we sleep

In Alzheimer's disease (and other forms of dementia), the hippocampus is one of the first regions of the brain to suffer damage. Short-term memory loss and disorientation are included among the early symptoms. Damage to the hippocampus can also result from oxygen starvation (hypoxia), encephalitis, or medial temporal lobe epilepsy. People with extensive, bilateral hippocampal damage may experience anterograde amnesia (the inability to form and retain new 3) ___. In rodents as model organisms, the hippocampus has been studied extensively as part of a brain system responsible for spatial memory and navigation. Many neurons in the rat and mouse hippocampus respond as place cells: that is, they fire bursts of action potentials when the animal passes through a specific part of its environment. Hippocampal place 4) ___ interact extensively with head direction cells, whose activity acts as an inertial compass, and conjecturally with grid cells in the neighboring entorhinal cortex. Since different neuronal cell types are neatly organized into layers in the hippocampus, it has frequently been used as a model system for studying neurophysiology. The form of neural plasticity known as long-term potentiation (LTP) was first discovered to occur in the hippocampus and has often been studied in this structure. LTP is widely believed to be one of the main neural mechanisms by which memories are stored in the brain.

Just recently, using an innovative NeuroGrid technology, invented by the study authors, it was showed that sleep boosts communication between two brain regions whose connection is critical for the formation of memories. The NeuroGrid consists of a collection of tiny electrodes linked together like the threads of a blanket, which is then laid across an area of the brain so that each electrode can continuously monitor the activity of a different set of neurons. One of the features of the device, is that it provides for ability to look at multiple areas of the brain at the same time. The study, published in Science, was partially funded by the Brain Research through Advancing Innovative Neurotechnologies (BRAIN) Initiative, a project of the 5) ___ ___ of ___ devoted to accelerating the development of new approaches to probing the workings of the brain.

Previous work revealed high-frequency bursts of neural firing called ripples in the 6) ___ during sleep and suggested they play a role in memory storage. The current study confirmed the presence of ripples in the hippocampus during sleep and found them in certain parts of association neocortex, an area on the brain's surface involved in processing complex sensory information. Using the NeuroGrid system, along with recording electrodes placed deeper into the brain, the researchers examined activity in several parts of rats' brains during NREM or 7) ___-___ ___ ___ sleep, the longest stage of sleep. The team was also surprised to find that the ripples in the association neocortex and hippocampus occurred at the same time, suggesting the two regions were communicating as the rats were 8) ___. Because the association neocortex is thought to be a storage location for memories, the authors theorized that this neural dialogue could help the brain retain information. To test that idea, they examined brain activity during NREM sleep in rats trained to locate rewards in a maze and in rats that explored the 9) ___ in a random fashion. In the latter group of animals, the ripples in the hippocampus and cortex were no more synchronized before exploring the maze than afterwards. In the trained rats, however, the learning task increased the cross-talk between those areas, and a second training session boosted it even more, further suggesting that such communication is important for the creation and storage of memories. 

The group hopes to use the NeuroGrid in people undergoing brain surgery for other reasons to determine if the same ripples occur in the 10) ___ brain. The group also plans to investigate if manipulating that neural firing in animals can boost or suppress memory formation in order to confirm that ripples are important for that process. According to the authors, identifying the specific neural patterns that go along with memory formation may provide a way to better understand memory and potentially even address disorders of memory.

The study was funded by NINDS (NS099705, NS090583) and the National Institute of Mental Health (MH107396). The National Institute of Neurological Disorders and Stroke (NINDS) <http://www.ninds.nih.gov> is the nation's leading funder of research on the brain and nervous system. The mission of NINDS is to seek fundamental knowledge about the brain and nervous system and to use that knowledge to reduce the burden of neurological disease. References: Khodagholy et al. Learning-enhanced coupling between ripple oscillations in association cortices and hippocampus. Science. October 20, 2017. doi: 10.1126/science.aan6203.

ANSWERS: 1) brain; 2) cortex; 3) memories; 4) cells; 5) National Institutes of Health; 6) hippocampus; 7) non-rapid eye movement; 8) sleeping; 9) maze; 10) human

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