Individual Brain Cells Track Where We Are and How We Move
Leaving the house in the morning may seem simple, but with every move we make, our brains are working feverishly to create maps of the outside world that allow us to navigate and to remember where we are.
Take one step out the front door, and an individual brain cell fires. Pass by your rose bush on the way to the car, another specific neuron fires. And so it goes. Ultimately, the brain constructs its own pinpoint geographical chart that is far more precise than anything you'd find on Google Maps.
But just how neurons make these maps of space has fascinated scientists for decades. It is known that several types of stimuli influence the creation of neuronal maps, including visual cues in the physical environment -- that rose bush, for instance -- the body's innate knowledge of how fast it is moving, and other inputs, like smell. Yet the mechanisms by which groups of neurons combine these various stimuli to make precise maps are unknown.
To solve this puzzle, UCLA neurophysicists built a virtual-reality environment that allowed them to manipulate these cues while measuring the activity of map-making neurons in rats. Surprisingly, they found that when certain cues were removed, the neurons that typically fire each time a rat passes a fixed point or landmark in the real world instead began to compute the rat's relative position, firing, for example, each time the rodent walked five paces forward, then five paces back, regardless of landmarks. And many other mapping cells shut down altogether, suggesting that different sensory cues strongly influence these neurons.
Finally, the researchers found that in this virtual world, the rhythmic firing of neurons that normally speeds up or slows down depending on the rate at which an animal moves, was profoundly altered. The rats' brains maintained a single, steady rhythmic pattern.
The findings, reported in the May 2 online edition of the journalScience, provide further clues to how the brain learns and makes memories.
Place cells in the real world (l) and in virtual reality. Place cells in the real world (left): The picture on the left shows the top-down view of a room in which a rat runs. There are four colorful walls and a check pattern on the floor. In the middle of the room, there is a narrow track (white line) on which the rat runs. Spikes fired by a neuron are shown by blue tick marks above and below the white track. Spikes fired when the rat runs from left-to-right are shown below the track and those spikes fired when the rat runs from right-to-left are shown above the track. These data were recorded in the real world. Notice that the spikes (blue tick marks) occur at the same absolute position hence these are called place cells. Place cells in virtual reality (right): These data were recorded in the virtual world. Note that the virtual world looks similar to the real world (other image). When the rat runs in this virtual world, the neurons fire at the same relative distance on the track in both movement directions, not at the same position as it does in the real world. Scientists hypothesize that the neurons are actually computing relative distances using cues on the walls and self-movement in the virtual world. In the real word, the presence of nonspecific sensory stimuli on the track, such as smells and textures, are hypothesized to make the neurons active at the same position. (Credit: Image courtesy of University of California - Los Angeles)
The mystery of how cells determine place
"Place cells" are individual neurons located in the brain's hippocampus that create maps by registering specific places in the outside environment. These cells are crucial for learning and memory. They are also known to play a role in such conditions as post-traumatic stress disorder and Alzheimer's disease when damaged.
For some 40 years, the thinking had been that the maps made by place cells were based primarily on visual landmarks in the environment, known as distal cues -- a tall tree, a building -- as well on motion, or gait, cues. But, as UCLA neurophysicist and senior study author Mayank Mehta points out, other cues are present in the real world: the smell of the local pizzeria, the sound of a nearby subway tunnel, the tactile feel of one's feet on a surface. These other cues, which Mehta likes to refer to as "stuff," were believed to have only a small influence on place cells.
Could it be that these different sensory modalities led place cells to create individual maps, wondered Mehta, a professor with joint appointments in the departments of neurology, physics and astronomy. And if so, do these individual maps cooperate with each other, or do they compete? No one really knew for sure.
Virtual reality reveals new clues
To investigate, Mehta and his colleagues needed to separate the distal and gait cues from all the other "stuff." They did this by crafting a virtual-reality maze for rats in which odors, sounds and all stimuli, except distal and gait cues, were removed. As video of a physical environment was projected around them, the rats, held by a harness, were placed on a ball that rotated as they moved. When they ran, the video would move along with them, giving the animals the illusion that they were navigating their way through an actual physical environment.
As a comparison, the researchers had the rats -- six altogether -- run a real-world maze that was visually identical to the virtual-reality version but that included the additional "stuff" cues. Using micro-electrodes 10 times thinner than a human hair, the team measured the activity of some 3,000 space-mapping neurons in the rats' brains as they completed both mazes.
What they found intrigued them. The elimination of the "stuff" cues in the virtual-reality maze had a huge effect: Fully half of the neurons being recorded became inactive, despite the fact that the distal and gate cues were similar in the virtual and real worlds. The results, Mehta said, show that these other sensory cues, once thought to play only a minor role in activating the brain, actually have a major influence on place cells.
And while in the real world, place cells responded to fixed, absolute positions, spiking at those same positions each time rats passed them, regardless of the direction they were moving -- a finding consistent with previous experiments -- this was not the case in the virtual-reality maze.
"In the virtual world," Mehta said, "we found that the neurons almost never did that. Instead, the neurons spiked at the same relative distance in the two directions as the rat moved back and forth. In other words, going back to the front door-to-car analogy, in a virtual world, the cell that fires five steps away from the door when leaving your home would not fire five steps away from the door upon your return. Instead, it would fire five steps away from the car when leaving the car. Thus, these cells are keeping track of the relative distance traveled rather than absolute position. This gives us evidence for the individual place cell's ability to represent relative distances."
Mehta thinks this is because neuronal maps are generated by three different categories of stimuli -- distal cues, gait and "stuff" -- and that all are competing for control of neural activity. This competition is what ultimately generates the "full" map of space.
"All the external stuff is fixed at the same absolute position and hence generates a representation of absolute space," he said. "But when all the stuff is removed, the profound contribution of gait is revealed, which enables neurons to compute relative distances traveled."
The researchers also made a new discovery about the brain's theta rhythm. It is known that place cells use the rhythmic firing of neurons to keep track of "brain time," the brain's internal clock. Normally, Mehta said, the theta rhythm becomes faster as subjects run faster, and slower as running speed decreases. This speed-dependent change in brain rhythm was thought to be crucial for generating the 'brain time' for place cells. But the team found that in the virtual world, the theta rhythm was uninfluenced by running speed.
"That was a surprising and fascinating discovery, because the 'brain time' of place cells was as precise in the virtual world as in the real world, even though the speed-dependence of the theta rhythm was abolished," Mehta said. "This gives us a new insight about how the brain keeps track of space-time."
The researchers found that the firing of place cells was very precise, down to one-hundredth of a second, "so fast that we humans cannot perceive it but neurons can," Mehta said. "We have found that this very precise spiking of neurons with respect to 'brain-time' is crucial for learning and making new memories."
Mehta said the results, taken together, provide insight into how distinct sensory cues both cooperate and compete to influence the intricate network of neuronal activity. Understanding how these cells function is key to understanding how the brain makes and retains memories, which are vulnerable to such disorders as Alzheimer's and PTSD.
"Ultimately, understanding how these intricate neuronal networks function is a key to developing therapies to prevent such disorders," he said.
Other authors of the study included Pascal Ravassard, Ashley Kees and Bernard Willers, all lead authors, and David Ho, Daniel A. Aharoni, Jesse Cushman and Zahra M. Aghajan of UCLA. Funding was provided by the W.M. Keck foundation, a National Science Foundation career award grant and a National Institutes of Health grant (5R01MH092925-02).
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The main goals of the 2045 Initiative: the creation and realization of a new strategy for the development of humanity which meets global civilization challenges; the creation of optimale conditions promoting the spiritual enlightenment of humanity; and the realization of a new futuristic reality based on 5 principles: high spirituality, high culture, high ethics, high science and high technologies.
The main science mega-project of the 2045 Initiative aims to create technologies enabling the transfer of a individual’s personality to a more advanced non-biological carrier, and extending life, including to the point of immortality. We devote particular attention to enabling the fullest possible dialogue between the world’s major spiritual traditions, science and society.
A large-scale transformation of humanity, comparable to some of the major spiritual and sci-tech revolutions in history, will require a new strategy. We believe this to be necessary to overcome existing crises, which threaten our planetary habitat and the continued existence of humanity as a species. With the 2045 Initiative, we hope to realize a new strategy for humanity's development, and in so doing, create a more productive, fulfilling, and satisfying future.
The "2045" team is working towards creating an international research center where leading scientists will be engaged in research and development in the fields of anthropomorphic robotics, living systems modeling and brain and consciousness modeling with the goal of transferring one’s individual consciousness to an artificial carrier and achieving cybernetic immortality.
An annual congress "The Global Future 2045" is organized by the Initiative to give platform for discussing mankind's evolutionary strategy based on technologies of cybernetic immortality as well as the possible impact of such technologies on global society, politics and economies of the future.
Future prospects of "2045" Initiative for society
The emergence and widespread use of affordable android "avatars" controlled by a "brain-computer" interface. Coupled with related technologies “avatars’ will give people a number of new features: ability to work in dangerous environments, perform rescue operations, travel in extreme situations etc.
Avatar components will be used in medicine for the rehabilitation of fully or partially disabled patients giving them prosthetic limbs or recover lost senses.
Creation of an autonomous life-support system for the human brain linked to a robot, ‘avatar’, will save people whose body is completely worn out or irreversibly damaged. Any patient with an intact brain will be able to return to a fully functioning bodily life. Such technologies will greatly enlarge the possibility of hybrid bio-electronic devices, thus creating a new IT revolution and will make all kinds of superimpositions of electronic and biological systems possible.
Creation of a computer model of the brain and human consciousness with the subsequent development of means to transfer individual consciousness onto an artificial carrier. This development will profoundly change the world, it will not only give everyone the possibility of cybernetic immortality but will also create a friendly artificial intelligence, expand human capabilities and provide opportunities for ordinary people to restore or modify their own brain multiple times. The final result at this stage can be a real revolution in the understanding of human nature that will completely change the human and technical prospects for humanity.
This is the time when substance-independent minds will receive new bodies with capacities far exceeding those of ordinary humans. A new era for humanity will arrive! Changes will occur in all spheres of human activity – energy generation, transportation, politics, medicine, psychology, sciences, and so on.
Today it is hard to imagine a future when bodies consisting of nanorobots will become affordable and capable of taking any form. It is also hard to imagine body holograms featuring controlled matter. One thing is clear however: humanity, for the first time in its history, will make a fully managed evolutionary transition and eventually become a new species. Moreover, prerequisites for a large-scale expansion into outer space will be created as well.
Key elements of the project in the future
• International social movement
• social network immortal.me
• charitable foundation "Global Future 2045" (Foundation 2045)
• scientific research centre "Immortality"
• business incubator
• University of "Immortality"
• annual award for contribution to the realization of the project of "Immortality”.