[extropy-chat] [minus1_0_1 at yahoo.de: [rael-science] First Picture of Living Human Retina Reveals Surprise]

Thu Dec 1 06:30:00 UTC 2005

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Date: Mon, 28 Nov 2005 16:42:27 +0100
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Subject: [rael-science] First Picture of Living Human Retina Reveals Surprise
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Source: LiveScience
http://www.livescience.com/humanbiology/051128_eye_image.html

First Picture of Living Human Retina Reveals Surprise

28 November 2005    By Sara Goudarzi   Special to LiveScience

The first images ever made of retinas in living people reveal surprising  
variation from one person to the next. Yet somehow our perceptions don't  
vary as might be expected.

Imaging thousands of cells responsible for detecting color in the deepest  
layer of the eye, scientists found that our eyes are wired differently.  
Yet we all -- with the exception of the color blind -- identify colors  
similarly.

The results suggest that the brain plays an even more significant role  
than thought in deciding what we see.

Inside the eye

The eye, responsible for receiving visual images, is wrapped in three  
layers of tissue [graphic]. The innermost layer, the retina, is  
responsible for sensing color and sending information to the brain.

The retina contains light receptors known as cones and rods. These  
receptors receive light, convert it to chemical energy, and activate the  
nerves that send messages to the brain. The rods are in charge of  
perceiving size, brightness and shape of images, whereas color vision and  
fine details are the responsibility of the cones.

On average, there are 7 million cones in the human retina, 64 percent of  
which are red, 32 percent green, and 2 percent blue, with each being  
sensitive to a slightly different region of the color spectrum. At least  
that's what scientists have been saying for years.

But the first complete imaging of the human retina, mapping the  
arrangement of the three types of cone photoreceptors, revealed something  
surprising about these numbers.

Big variation

The study found that people recognized colors in the same way. Yet the  
pictures of their retinas showed there is enormous variability, sometimes  
up to 40 times, in the relative number of green and red cones in the  
retina.

"[This] suggests that there is a compensatory mechanism in our brain that  
negates individual differences in the relative numbers of red and green  
cones that we observed," Joseph Carroll, a researcher at Center for Visual  
Science at University of Rochester and a collaborator of the study, told  
LiveScience.

The researchers used adaptive optics imaging, which uses a camera  
containing a corrective device that cancels the effects of the eye's  
imperfect optics on image quality, producing a high-resolution retinal  
picture.

Borrowing from astronomy

"Adaptive optics is a technique borrowed from astronomy where it is used  
to obtain sharp images of stars from telescopes on the ground," said David  
Williams, Director of Center for Visual Science at the University of  
Rochester.  "All such telescopes suffer from blur due to the effects of  
turbulence in the Earth's atmosphere. In our case, optical defects in the  
cornea and lens of the eye blur images of the retina."

The measured defects were corrected using deformable mirrors, which bend  
and morph according each person's eye, before taking high magnification  
pictures of the eye. This allowed Williams and colleagues to see and map  
single cells such as the cones.

The researchers hope to use the same techniques to better understand  
various forms of color blindness and different kinds of retinal disease.

The findings were detailed in a recent issue of the Journal of  
Neuroscience.

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