Posts tagged grid cells
Articles and news from the latest research reports.
The Nobel Assembly at Karolinska Institutet has decided to award the 2014 Nobel Prize in Physiology or Medicine with one half to John O´Keefe and the other half jointly to May-Britt Moser and Edvard I. Moser for their discoveries of cells that constitute a positioning system in the brain.
How do we know where we are? How can we find the way from one place to another? And how can we store this information in such a way that we can immediately find the way the next time we trace the same path? This year´s Nobel Laureates have discovered a positioning system, an “inner GPS” in the brain that makes it possible to orient ourselves in space, demonstrating a cellular basis for higher cognitive function.
In 1971, John O´Keefe discovered the first component of this positioning system. He found that a type of nerve cell in an area of the brain called the hippocampus that was always activated when a rat was at a certain place in a room. Other nerve cells were activated when the rat was at other places. O´Keefe concluded that these “place cells” formed a map of the room.
More than three decades later, in 2005, May-Britt and Edvard Moser discovered another key component of the brain’s positioning system. They identified another type of nerve cell, which they called “grid cells”, that generate a coordinate system and allow for precise positioning and pathfinding. Their subsequent research showed how place and grid cells make it possible to determine position and to navigate.
The discoveries of John O´Keefe, May-Britt Moser and Edvard Moser have solved a problem that has occupied philosophers and scientists for centuries – how does the brain create a map of the space surrounding us and how can we navigate our way through a complex environment?
How do we experience our environment?
The sense of place and the ability to navigate are fundamental to our existence. The sense of place gives a perception of position in the environment. During navigation, it is interlinked with a sense of distance that is based on motion and knowledge of previous positions.
Questions about place and navigation have engaged philosophers and scientists for a long time. More than 200 years ago, the German philosopher Immanuel Kant argued that some mental abilities exist as a priori knowledge, independent of experience. He considered the concept of space as an inbuilt principle of the mind, one through which the world is and must be perceived. With the advent of behavioural psychology in the mid-20th century, these questions could be addressed experimentally. When Edward Tolman examined rats moving through labyrinths, he found that they could learn how to navigate, and proposed that a “cognitive map” formed in the brain allowed them to find their way. But questions still lingered - how would such a map be represented in the brain?
John O´Keefe and the place in space
John O´Keefe was fascinated by the problem of how the brain controls behaviour and decided, in the late 1960s, to attack this question with neurophysiological methods. When recording signals from individual nerve cells in a part of the brain called the hippocampus, in rats moving freely in a room, O’Keefe discovered that certain nerve cells were activated when the animal assumed a particular place in the environment (Figure 1). He could demonstrate that these “place cells” were not merely registering visual input, but were building up an inner map of the environment. O’Keefe concluded that the hippocampus generates numerous maps, represented by the collective activity of place cells that are activated in different environments. Therefore, the memory of an environment can be stored as a specific combination of place cell activities in the hippocampus.
May-Britt and Edvard Moser find the coordinates
May-Britt and Edvard Moser were mapping the connections to the hippocampus in rats moving in a room when they discovered an astonishing pattern of activity in a nearby part of the brain called the entorhinal cortex. Here, certain cells were activated when the rat passed multiple locations arranged in a hexagonal grid (Figure 2). Each of these cells was activated in a unique spatial pattern and collectively these “grid cells” constitute a coordinate system that allows for spatial navigation. Together with other cells of the entorhinal cortex that recognize the direction of the head and the border of the room, they form circuits with the place cells in the hippocampus. This circuitry constitutes a comprehensive positioning system, an inner GPS, in the brain (Figure 3).
A place for maps in the human brain
Recent investigations with brain imaging techniques, as well as studies of patients undergoing neurosurgery, have provided evidence that place and grid cells exist also in humans. In patients with Alzheimer´s disease, the hippocampus and entorhinal cortex are frequently affected at an early stage, and these individuals often lose their way and cannot recognize the environment. Knowledge about the brain´s positioning system may, therefore, help us understand the mechanism underpinning the devastating spatial memory loss that affects people with this disease.
The discovery of the brain’s positioning system represents a paradigm shift in our understanding of how ensembles of specialized cells work together to execute higher cognitive functions. It has opened new avenues for understanding other cognitive processes, such as memory, thinking and planning.
(Source: nobelprize.org)
Researchers discover workings of brain’s ‘GPS system’
Just as a global positioning system (GPS) helps find your location, the brain has an internal system for helping determine the body’s location as it moves through its surroundings.
A new study from researchers at Princeton University provides evidence for how the brain performs this feat. The study, published in the journal Nature, indicates that certain position-tracking neurons — called grid cells — ramp their activity up and down by working together in a collective way to determine location, rather than each cell acting on its own as was proposed by a competing theory.
Grid cells are neurons that become electrically active, or “fire,” as animals travel in an environment. First discovered in the mid-2000s, each cell fires when the body moves to specific locations, for example in a room. Amazingly, these locations are arranged in a hexagonal pattern like spaces on a Chinese checker board.
“Together, the grid cells form a representation of space,” said David Tank, Princeton’s Henry L. Hillman Professor in Molecular Biology and leader of the study. “Our research focused on the mechanisms at work in the neural system that forms these hexagonal patterns,” he said. The first author on the paper was graduate student Cristina Domnisoru, who conducted the experiments together with postdoctoral researcher Amina Kinkhabwala.
Domnisoru measured the electrical signals inside individual grid cells in mouse brains while the animals traversed a computer-generated virtual environment, developed previously in the Tank lab. The animals moved on a mouse-sized treadmill while watching a video screen in a set-up that is similar to video-game virtual reality systems used by humans.
She found that the cell’s electrical activity, measured as the difference in voltage between the inside and outside of the cell, started low and then ramped up, growing larger as the mouse reached each point on the hexagonal grid and then falling off as the mouse moved away from that point.
This ramping pattern corresponded with a proposed mechanism of neural computation called an attractor network. The brain is made up of vast numbers of neurons connected together into networks, and the attractor network is a theoretical model of how patterns of connected neurons can give rise to brain activity by collectively working together. The attractor network theory was first proposed 30 years ago by John Hopfield, Princeton’s Howard A. Prior Professor in the Life Sciences, Emeritus.
The team found that their measurements of grid cell activity corresponded with the attractor network model but not a competing theory, the oscillatory interference model. This competing theory proposed that grid cells use rhythmic activity patterns, or oscillations, which can be thought of as many fast clocks ticking in synchrony, to calculate where animals are located. Although the Princeton researchers detected rhythmic activity inside most neurons, the activity patterns did not appear to participate in position calculations.
Grid cell firing patterns signal environmental novelty by expansion
The hippocampal formation plays key roles in representing an animal’s location and in detecting environmental novelty to create or update those representations. However, the mechanisms behind this latter function are unclear. Here, we show that environmental novelty causes the spatial firing patterns of grid cells to expand in scale and reduce in regularity, reverting to their familiar scale as the environment becomes familiar. Simultaneously recorded place cell firing fields remapped and showed a smaller, temporary expansion. Grid expansion provides a potential mechanism for novelty signaling and may enhance the formation of new hippocampal representations, whereas the subsequent slow reduction in scale provides a potential familiarity signal.
Primates’ brains make visual maps using triangular grids
Primates’ brains see the world through triangular grids, according to a new study published online Sunday in the journal Nature.
Scientists at Yerkes National Primate Research Center, Emory University, have identified grid cells, neurons that fire in repeating triangular patterns as the eyes explore visual scenes, in the brains of rhesus monkeys.
The finding has implications for understanding how humans form and remember mental maps of the world, as well as how neurodegenerative diseases such as Alzheimer’s erode those abilities. This is the first time grid cells have been detected directly in primates. Grid cells were identified in rats in 2005, and their existence in humans has been indirectly inferred through magnetic resonance imaging.
Grid cells’ electrical activities were recorded by introducing electrodes into monkeys’ entorhinal cortex, a region of the brain in the medial temporal lobe. At the same time, the monkeys viewed a variety of images on a computer screen and explored those images with their eyes. Infrared eye-tracking allowed the scientists to follow which part of the image the monkey’s eyes were focusing on. A single grid cell fires when the eyes focus on multiple discrete locations forming a grid pattern.
"The entorhinal cortex is one of the first brain regions to degenerate in Alzheimer’s disease, so our results may help to explain why disorientation is one of the first behavioral signs of Alzheimer’s," says senior author Elizabeth Buffalo, PhD, associate professor of neurology at Emory University School of Medicine and Yerkes National Primate Research Center. "We think these neurons help provide a context or structure for visual experiences to be stored in memory."
"Our discovery of grid cells in primates is a big step toward understanding how our brains form memories of visual information," says first author Nathan Killian, a graduate student in the Wallace H. Coulter Department of Biomedical Engineering at Georgia Tech and Emory University. "This is an exciting way of thinking about memory that may lead to novel treatments for neurodegenerative diseases."
(Image credit: Mark Snelson)