Mapping connections of single neurons using a holographic light beam

November 18, 2017

Controlling single neurons using optogenetics (credit: the researchers)

Researchers at MIT and Paris Descartes University have developed a technique for precisely mapping connections of individual neurons for the first time by triggering them with holographic laser light.

The technique is based on optogenetics (using light to stimulate or silence light-sensitive genetically modified protein molecules called “opsins” that are embedded in specific neurons). Current optogenetics techniques can’t isolate individual neurons (and their connections) because the light strikes a relatively large area — stimulating axons and dendrites of other neurons simultaneously (and these neurons may have different functions, even when nearby).

The new technique stimulates only the soma (body) of the neuron, not its connections. To achieve that, the researchers combined two new advances: an optimized holographic light-shaping microscope* and a localized, more powerful opsin protein called CoChR.

Two-photon computer-generated holography (CGH) was used to create three-dimensional sculptures of light that envelop only a target cell, using a conventional pulsed laser coupled with a widefield epifluorescence imaging system. (credit: Or A. Shemesh et al./Nature Nanoscience)

The researchers used an opsin protein called CoChR, which generates a very strong electric current in response to light, and fused it to a small protein that directs the opsin into the cell bodies of neurons and away from axons and dendrites, which extend from the neuron body, forming “somatic channelrhodopsin” (soCoChR). This new opsin enabled photostimulation of individual cells (regions of stimulation are highlighted by magenta circles) in mouse cortical brain slices with single-cell resolution and with less than 1 millisecond temporal (time) precision — achieving connectivity mapping on intact cortical circuits without crosstalk between neurons. (credit: Or A. Shemesh et al./Nature Nanoscience)

In the new study, by combining this approach with new ““somatic channelrhodopsin” opsins that cluster in the cell body, the researchers showed they could stimulate individual neurons with not only precise spatial control but also great control over the timing of the stimulation. When they target a specific neuron, it responds consistently every time, with variability that is less than one millisecond, even when the cell is stimulated many times in a row.

“For the first time ever, we can bring the precision of single-cell control toward the natural timescales of neural computation,” says Ed Boyden, an associate professor of brain and cognitive sciences and biological engineering at MIT, and a member of MIT’s Media Lab and McGovern Institute for Brain Research. Boyden is co-senior author with Valentina Emiliani, a research director at France’s National Center for Scientific Research (CNRS) and director of the Neurophotonics Laboratory at Paris Descartes University, of a study that appears in the Nov. 13 issue of Nature Neuroscience.

Mapping neural connections in real time

Using this technique, the researchers were able to stimulate single neurons in brain slices and then measure the responses from cells that are connected to that cell. This may pave the way for more precise diagramming of the connections of the brain, and analyzing how those connections change in real time as the brain performs a task or learns a new skill.

Optogenetics was co-developed in 2005 by Ed Boyden (credit: MIT)

One possible experiment, Boyden says, would be to stimulate neurons connected to each other to try to figure out if one is controlling the others or if they are all receiving input from a far-off controller.

“It’s an open question,” he says. “Is a given function being driven from afar, or is there a local circuit that governs the dynamics and spells out the exact chain of command within a circuit? If you can catch that chain of command in action and then use this technology to prove that that’s actually a causal link of events, that could help you explain how a sensation, or movement, or decision occurs.”

As a step toward that type of study, the researchers now plan to extend this approach into living animals. They are also working on improving their targeting molecules and developing high-current opsins that can silence neuron activity.

The research was funded by the National Institutes of Health, France’s National Research Agency, the Simons Foundation for the Social Brain, the Human Frontiers Science Program, John Doerr, the Open Philanthropy Project, the Howard Hughes Medical Institute, and the Defense Advanced Research Projects Agency.

* Traditional holography is based on reproducing, with light, the shape of a specific object, in the absence of that original object. This is achieved by creating an “interferogram” that contains the information needed to reconstruct an object that was previously illuminated by a reference beam. In computer-generated holography, the interferogram is calculated by a computer without the need of any original object. Combined with two-photon excitation, CGH can be used to refocus laser light to precisely illuminate a cell or a defined group of cells in the brain.

Abstract of Temporally precise single-cell-resolution optogenetics

Optogenetic control of individual neurons with high temporal precision within intact mammalian brain circuitry would enable powerful explorations of how neural circuits operate. Two-photon computer-generated holography enables precise sculpting of light and could in principle enable simultaneous illumination of many neurons in a network, with the requisite temporal precision to simulate accurate neural codes. We designed a high-efficacy soma-targeted opsin, finding that fusing the N-terminal 150 residues of kainate receptor subunit 2 (KA2) to the recently discovered high-photocurrent channelrhodopsin CoChR restricted expression of this opsin primarily to the cell body of mammalian cortical neurons. In combination with two-photon holographic stimulation, we found that this somatic CoChR (soCoChR) enabled photostimulation of individual cells in mouse cortical brain slices with single-cell resolution and <1-ms temporal precision. We used soCoChR to perform connectivity mapping on intact cortical circuits.


In ‘Self/less,’ a man cheats death by transferring his mind to another body — here’s how close we are to making that reality

July 16, 2015

sergio canavero

In the new movie “Self/less,” which comes out Friday, July 10, a wealthy man dying of cancer (played by Ben Kingsley) cheats death by transferring his consciousness to the body of a younger man (Ryan Reynolds).

Thanks to the help of a secretive doctor and a lot of money, all the procedure requires is going for a short spin in a device that looks like an MRI machine.

Of course, the mind that originally belonged to the younger body still exists — which creates some problems for the mind that has now taken up residence there — but the new body’s original mind can be “suppressed” by taking special pills.

At first glance, this sounds like a pretty sweet deal. But just how far off is this kind of technology? Do we know enough about the brain to even begin to undertake such a procedure?

In recent years, scientists have been making impressive strides toward understanding and manipulating the brain: We have rudimentary technologies for listening in on, and even altering, the mess of complex activity in the three-pound hunk of flesh in our skulls. Scientists have even developed methods for probing the brain using light, a technique that has been used to implant or erase memories in mice.

But as far as transferring the brain’s consciousness, a concept scientists still have yet to completely understand or define, we’ve got a pretty long way to go.

Problem #1: Everybody has a different brain

While the movie makes it seem like we could simply swap memories between two people, it’s not that simple: We’d have to also transfer the process our brains use to generate thoughts, Wolfgang Fink, a neuroscientist and roboticist at Caltech and the University of Arizona, told Business Insider.

“The reason why [the “Self/less”-style body-swapping procedure] isn’t really possible is that everybody has a different brain,” says Fink. “You would have to transfer not just the memories, but the same thought-generating process.”

In other words, it comes down to not just what we think, but how we think. Each of us has unique mental hardware, which is why it’s likely not possible to simply download your consciousness onto another person, or to a computer, for that matter.

Scientists have developed computer systems modeled on this hardware called artificial neural networks, some of which can flip back and forth between two different “mental” states. “That is sort of the closest to what we saw in the movie,” Fink said — which is why the main character had to take pills to suppress the other personality coming through. You have these two competing personalities, and we can mimic this competition in software. But a computer may not be able to reproduce you.

Which brings us to our next problem.

Problem #2: We can’t just implant memories

For much of its history, neuroscience has been confined to passively studying the brain. But in recent years, a technique has been developed that allows scientists to actively manipulate its activity using light.

Known as optogenetics, the technique involves injecting a harmless virus (containing DNA found in glowing algae) into neurons in the brain, which causes them to produce a protein that makes the cells active in response to light.

By shining a laser onto these cells, scientists can essentially turn them on or off.

In 2013, scientists at MIT used this method to implant a false memory in the brains of mice. In the study, the researchers placed the animals in a chamber where they received mild foot shocks, creating a fearful memory stored in a brain region called the hippocampus. Then, by shining light on the neurons that encoded the shock memory when the mice were in a different environment, scientists made the mice “remember” getting shocked in the new place even though it hadn’t happened.

lab mouse mice ratFlickr/Global Pandora

The same researchers took things a step further in a study this past June, when they activated happy memories in mice that were behaving as if they were depressed. They lost their usual appetite for sugar water, and didn’t put up a struggle when picked up by their tails, for example. But when the experimenters shone light on the mice’s neurons that activated a memory of a happier time (which they’d found by peering into their brains while allowing them to enjoy some time with female mice), it “cured” the animals’ depression, the researchers said.

Of course, the studies were in mice, not humans. And implanting or altering a simple memory is a long way from transferring the entire set of thoughts and memories from one brain to another.

That brings us to our next problem.

Problem #3: There’s an alternative, but it’s even trickier

Rather than transferring someone’s mind to a different brain, it’s more likely we would transplant your entire head to a new body, said Fink.

An Italian neurosurgeon hopes to do just that. Sergio Canavero of Italy’s Turin Advanced Neuromodulation Group announced his plans earlier this year to perform the world’s first head transplant as early as 2017. He has already secured a volunteer for the procedure, a Russian man with spinal muscular atrophy, a disease that causes the muscles to waste away and is ultimately fatal.

TheWhyA clip from the Konami video game “Metal Gear Solid 5: The Phantom Pain,” shows a man who shares Canavero’s exact likeness.

Canavero’s proposed procedure, which he calls the HEad Anatomosis VENture, or “Heaven,” involves finding a brain-dead donor whose body is intact, severing the heads of both the donor and the patient, and attaching the patient’s head and spine to the donor’s body.

While Fink thinks the head transplant may actually happen on schedule, critics say there are a lot of challenges to overcome.

For one thing, the new body’s immune system could reject the head, just as your body can reject a transplanted organ.

In addition, once the patient’s spinal cord is severed and attached to the donor body, the body may end up paralyzed. Canavero has claimed he can get around this problem by using a very sharp knife to make a clean cut that would allow remaining nerve fibers to repair the incision, but this has yet to be demonstrated. (In the past, scientists have performed head transplants with monkeys, but the animals only lived for a few days.)

The takeaway

Even if the procedure were technically possible, it brings up a host of ethical and philosophical issues. Should you be able to inhabit another person’s body? Would you still be you? Furthermore, any life-extending technology would undoubtedly be very expensive, so would it be fair that only the rich could have access to it?