More than 50 percent of our brain is dedicated to visual processing. Yet there is so much going on. How do our brains process the information that is coming in from our eyes?
One research project at Max Planck Institute of Neurobiology in Munich, University of California Berkeley and Harvard University looked at the retinal ganglion cells of zebrafish to learn which pathways each signal follows. Why zebrafish? They are used as research models because they breed rapidly and the embryos develop externally, which allows their genetic makeup to be manipulated. So, researchers used these fish to create a catalog of specific retinal ganglion cells.
Optical signals produced by photons hit the retina. Next neurons in the retina collect and process the visual information. As this is going on, the retina focuses on certain details, such as color, size or whether the object is moving or stationary. When these are sorted out, the retinal ganglion cells send them to the brain, where they lead to certain behaviors.
The retinal ganglion cells play an important role in the visual system, since they are the only connection between the retina and the brain. It is known that specific types of retinal ganglion cell types send different signals to different parts of the brain. It wasn’t clear how they differ on the molecular level, what their functions are and how they regulate context-dependent behavior.
To learn what molecules do what task, scientists analyzed the genetic diversity of the zebrafish’s retinal ganglion cells. They found patterns of active genes in the retinal ganglion cells and assigned each cell a unique fingerprint to an overall dataset of over 30,000 retinal ganglion cells. A computational analysis identified at least 32 different retinal ganglion cell types based on similarities.
In this catalog of cell types, researchers found genes that are only active in certain retinal ganglion cell types. With the help of both these genes and targeted genome editing, they were able to get access to select retinal ganglion cell types. Since the zebrafish is almost transparent, it was possible to fluorescently label the cell types and record the brain regions where their axonal projections or information flow ends. Also, as a result of showing zebrafish larvae various visual stimuli and studying which of them activate a particular cell type, it was possible to see which visual detail a particular retinal ganglion cell type prefer.
Of course, if this cell type doesn’t function, what happens to the fish? Since fish larvae prefer a bright environment in order to find food and navigate, when scientists shut off the cell type for measuring light conditions, the fish lose their ability to navigate. This project links retinal ganglion cells to a specific structure, function and behavioral response.
Now that it is known which cells do what for vision, what does the brain do with the information? That will be covered in the next blog entry.