How many connections are there in the human brain

Called FlyEM, it aims to produce a connectome of the central nervous system of the fruit fly Drosophila melanogaster.

His team expects to release data on roughly one-third of the D. Meanwhile, Lichtman is working on the zebrafish Danio rerio connectome, as well as analysing a small piece of the human brain — a sample of the medial temporal gyrus obtained from a person who was undergoing brain surgery for epilepsy.

That piece is also roughly one cubic millimetre in volume, but to capture the full thickness of the human cortex, the sample is shaped like a slab, rather than a cube. Denk and his colleagues are mapping portions of the connectome in the zebra finch Taeniopygia guttata , a small bird whose process of song learning can yield insights into human speech.

And Kasthuri has a number of projects in progress. To that end, Kasthuri aims to map the visual part of the brain in non-human primates, as well as in an octopus Octopus bimaculoides.

Kasthuri is also working on the full connectomes of young mice and octopuses; comparing these immature connectomes to those of adult animals could offer insights into how the brain learns from experience. Owing to its small size, he hopes to map the young-octopus connectome in about one year.

Now that the researchers at the Allen Institute have finished imaging their cubic millimetre of mouse brain, they have passed on the data to Sebastian Seung, a neuroscientist and computer scientist at Princeton University.

Segmentation has long been the rate-limiting step in connectomics. It can take weeks to trace by hand the path of a single neuron through a stack of electron micrographs. But now, artificial intelligence is getting involved. Computers can perform segmentation faster than the human eye, which cuts down the time it takes to trace neurons to a matter of minutes or hours. People are therefore still needed to check the reconstruction.

Seung is tackling this requirement through crowdsourcing and, specifically, an online game called Eyewire, in which players are challenged to correct mistakes in the rough draft of a connectome. Launched in , Eyewire has , registered users who have collectively put in an effort that is equivalent to 32 people working full time for 7 years, says Amy Robinson Sterling, executive director of Eyewire.

The Developing Human Connectome Project is imaging nerve fibres in the brains of newborns. So far, players have been tracing cells in the mouse retina. Sterling and her team are preparing a new version of the game, called Neo, that will be used with the mouse visual-cortex data set. Many nanoscale connectome-mapping efforts use the program to visualize data. Google has also developed an algorithm for neuron segmentation. A team led by Viren Jain at Google AI, in Mountain View, California, has designed a machine-learning algorithm called a flood-filling network, which builds structures from a point in an image, rather than trying to define the boundaries of all neurons at once.

His team is applying the technique to FlyEM data and has constructed a rough-draft connectome of a whole fly brain that was imaged by another team at Janelia Research Campus. They are also working with data from the labs of Denk and Lichtman. Jain strikes a more cautious note, and points out that as scientists take on ever larger projects, segmentation algorithms have to become more accurate to keep feasible the amount of human checking that is required.

Meanwhile, scientists are honing microscopy techniques to produce sharper, more-detailed images at a much quicker pace, in anticipation of taking on the nanoscale connectomes of large, mammalian brains.

The conventional approach to microscopy in connectomics is a type of electron microscopy known as serial-section electron microscopy. Researchers embed neural tissue in plastic, and cut it into slices that are a fraction of the thickness of a human hair. They then mount the slices on a specialized tape and feed the result — which looks remarkably similar to film on a reel — through the microscope. The advantage of this method is that the sample is preserved and can be re-imaged, if needed.

But no matter how precisely it is done, cutting the sample inevitably results in distortions that make it difficult to align the images. A newer approach, known as focused ion beam scanning electron microscopy FIB-SEM , uses a beam of charged ions to shave away a thin layer of a tissue sample.

The microscope captures an image of the freshly exposed surface, and then the process is repeated. The FlyEM sample represents the first substantial volume to be imaged by this method.

Although it lacks speed, one advantage of FIB-SEM is that the resolution of the images produced is the same in all three dimensions, rather than being coarser along the vertical axis. However, samples can be imaged only once, because they are vaporized in the process.

In addition, the field of view is very small, which makes it difficult to apply to larger samples. Even the fruit-fly brain, which is roughly the size of a poppy seed, has to be chopped into smaller chunks. A method called gas cluster ion beam scanning electron microscopy GCIB-SEM , developed by Kenneth Hayworth, a neuroscientist at Janelia Research Campus, works similarly but has a larger field of view, which makes it more feasible for use in imaging larger brains.

GCIB-SEM might also be more compatible with multibeam electron microscopes, which researchers hope will speed up image acquisition. Numerous electron beams scan a sample at the same time, which enables the microscope to capture hundreds of millions of pixels per second. But doing so might not be worthwhile. Without understanding how information is stored, the data would be gibberish, he says.

A more immediate benefit would be to explore how the cell map differs in people with mental health conditions, says Dulac. Journal reference: bioRxiv , DOI: Sign up to our free Health Check newsletter for a round-up of all the health and fitness news you need to know, every Saturday. Read more: Every single neuron in an animal mapped out for the first time. Trending Latest Video Free. New mineral davemaoite discovered inside a diamond from Earth's mantle Paralysed mice walk again after gel is injected into spinal cord Will a scramble to mine metals undermine the clean energy revolution?

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The remarkable, yet not extraordinary, human brain as a scaled-up primate brain and its associated cost. Kohl J, Jefferis GS. Neuroanatomy: decoding the fly brain. Curr Biol. Williams RW. Mapping genes that modulate mouse brain development: a quantitative genetic approach. Results Probl Cell Differ. Dogs have the most neurons, though not the largest brain: trade-off between body mass and number of neurons in the cerebral cortex of large carnivoran species.

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