Connectomics: a window to the mind
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The human brain has 100 billion neurons, connected to each other in networks that allow us to interpret the world around us, plan for the future, and control our actions and movements. Mapping those networks, creating a wiring diagram of the brain could help scientists learn how we each become our unique selves. Understanding the brain and all its connections is Connectomics – a word soon to become as familiar as ‘genetics’.
In three papers appearing in Nature, scientists report their first step toward this goal: Firstly using a combination of human and artificial intelligence, they have mapped all the wiring among 950 neurons within a tiny patch of the mouse retina. While a second group look at a classic problem of neural computation – the detection of visual motion – in the eye of a fruitfly.
The eye of the mouse
The retina is technically part of the brain, as it is composed of neurons that process visual information. Neurons come in many types, and the retina is estimated to contain 50 to 100 types, but they’ve never been exhaustively characterised. Their connections are even less well known. Neurons in the retina are classified into five classes: photoreceptors, horizontal cells, bipolar cells, amacrine cells and ganglion cells. Within each class are many types, classified by shape and by the connections they make with other neurons.
In this study, the research team focused on a section of the retina known as the inner plexiform layer, which is one of several layers sandwiched between the photoreceptors, which receive visual input, and the ganglion cells, which relay visual information to the brain via the optic nerve. The neurons of the inner plexiform layer help to process visual information as it passes from the surface of the eye to the optic nerve.
By mapping all of the neurons in a 117-micrometre-by-80-micrometre patch of tissue, researchers were able to classify most of the neurons they found, based on their patterns of wiring. They also identified a new type of retinal cell that had not been seen before. To map all of the connections in this small patch of retina, the researchers first took electron micrographs of the targeted section generating high-resolution three-dimensional images of biological samples.
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