New neural pathway in eyes that aids in vision identified

WASHINGTON: A type of retina cell plays a more critical role in vision than previously known, researchers have found. 

A team of researchers led by Johns Hopkins University found that the ipRGCs — an atypical type of photoreceptor in the retina — help detect contrast between light and dark, a crucial element in the formation of visual images. 

The key to the discovery is the fact that the cells express melanopsin, a type of photopigment that undergoes a chemical change when it absorbs light. 

“We are quite excited that melanopsin signalling contributes to vision even in the presence of functional rods and cones,” said Tiffany M Schmidt, lead author of the study in the journal Neuron. 

Rods and cones are the most well-known photoreceptors in the retina, activating in different light environments. Rods, of which there are about 120 million in the human eye, are highly sensitive to light and turn on in dim or low-light environments. 

Meanwhile the 6 million to 7 million cones in the eye are less sensitive to light; they drive vision in brighter light conditions and are essential for colour detection. 

Rods and cones were thought to be the only light-sensing photoreceptors in the retina until about a decade ago when scientists discovered a third type of retinal photoreceptor — the ipRGC, or intrinsically photosensitive retinal ganglion cell — that contains melanopsin. 

Those cells were thought to be needed exclusively for detecting light for non-image-dependent functions, for example, to control synchronisation of our internal biological clocks to daytime and the constriction of our pupils in response to light. 

“Rods and cones were thought to mediate vision and ipRGCs were thought to mediate these simple light-detecting functions that happen outside of conscious perception,” Schmidt said. 

“But our experiments revealed that ipRGCs influence a greater diversity of behaviours than was previously known and actually contribute to an important aspect of image-forming vision, namely contrast detection,” Schmidt added. 

The Johns Hopkins team along with other scientists conducted several experiments with mice and found that when melanopsin was present in the retinal ganglion cells, the mice were better able to see contrast in a Y-shaped maze, known as the visual water task test. 
In the test, mice are trained to associate a pattern with a hidden platform that allows them to escape the water. Mice that had the melanopsin gene intact had higher contrast sensitivity than mice that lacked the gene.


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