According to a study published in Neuron, researchers at Northwestern Medicine have discovered a brand-new variety of retinal ganglion cells, which are the retinal neurons that encode the visual environment and send information back to the brain.
The specific features of this cell, which is called a "bursty suppressed-by-contrast" (bSbC) retinal ganglion cell (RGC), overturn a decades-old assumption about the relationship between the cells” inputs from photoreceptors and outputs to the brain, according to Gregory Schwartz, Ph.D., the Derrick T. Vail Professor of Ophthalmology and senior author of the study.
"The classic view was that these ganglion cells simply integrate their excitation and inhibition inputs, and that will tell you how the cell responds to visual stimulus," said Schwartz, who is also a professor of Neuroscience. "Our findings reveal that these cells have their own intrinsic computation, which has interesting implications for things like retinal prosthetics,” Mr. Schwartz added.
There are more than 40 different types of RGCs that communicate details about the intricate and precise elements of a visual picture, such as motion, direction, orientation, and color. For instance, the "off sustained alpha" (OFFsA) RGC type has a baseline firing rate, but as light levels rise or fall, so does the cell's communication to the brain.
Researchers evaluated the responses of OFFsA and bSbC ganglion cell types to various visual stimuli in the study, measuring the resulting signals that would be sent to the brain.
The authors of the study found that the bSbC has an intriguing signaling mechanism: the cell normally signals to the brain at a constant rate, but both increases and decreases in light lead to a one-way communication pattern in which signaling is reduced. "The signaling can only go down," Schwartz said.
The researchers also put the conventional theory of how these ganglion cells integrate inputs to the test. The prevalent idea said that these inputs must combine to achieve some threshold that results in a change in signaling output. These cells receive excitatory and inhibitory signals from photoreceptors in the retina.
Schwartz and his colleagues tested this by switching the inputs to the OFFsA and bSbC ganglion cells. They discovered that the output signaling in this experiment was the same as that of typical activity.
"This means they have their own ion channels that impact output, their own set of electrical conductances that contribute to the output," Schwartz said.
The research has ramifications for retinal prosthesis and other devices that try to stimulate RGCs that are no longer light-responsive. According to Schwartz, one of the reasons why current prosthetics can only provide rudimentary vision is that they send the incorrect signals to the incorrect types of cells.
"You can't turn on the light to check what kind of retinal ganglion cells they are because the receptors are damaged—it's a catch-22," Schwartz said.
Instead, examining the intrinsic computation in each kind of cell might enable researchers to distinguish between different cell types even in the absence of the proper photoreceptor input.
"If you can get detailed information about these onboard computations, you might be able to send each cell type the correct signaling," Schwartz concluded.