It would make the perfect question for the popular television show "Are You Smarter than a 5th Grader:" What parts of the eye allow us to see?
The conventional wisdom: rods and cones. The human retina contains about 120 million rods, which detect light and darkness, shape and movement, and about 7 million cones, which in addition detect color. Without them, or so we are taught, our eyesight simply would not exist.
But that might not be true, according to a study -- published July 15 in the journal Neuron -- that provides new hope to people who have severe vision impairments or who are blind.
A team led by biologist Samer Hattar of The Johns Hopkins University's Krieger School of Arts and Sciences found that mice that didn't have any rods and cones function could still see -- and not just light, but also patterns and images -- courtesy of special photosensitive cells in the rodents' retinas. Until now, it was presumed that those cells, called intrinsically photosensitive Retinal Ganglion Cells, (or ipRGCs), didn't play a role in image formation, but instead served other functions, such as dictating when the animals went to sleep or woke up. (All mammals, including humans, have ipRGCs, as well as rods and cones.)
"Up until now, it was assumed that rods and cones were the only cells capable of detecting light to allow us to form images," said Hattar, who as an assistant professor in the Department of Biology, studies mammals' sleep-wake cycles, also called "circadian rhythms." "But our study shows that even mice which were blind could form low-acuity yet measurable images, using ipRGCs. The exciting thing is that, in theory at least, this means that a blind person could be trained to use his or her ipRGCs to perform simple tasks that require low visual acuity."
"Visual acuity" refers to the sharpness or clarity of a person's (or animal's) vision. Someone with so-called "20/20 vision" can see clearly at a
|Contact: Lisa DeNike|
Johns Hopkins University