By Adrian Galbreth
Vision scientists have come up with a novel way to measure changes in the human eye, which could lead to better ways of treating disease or making products to enhance vision correction.
Specialists at Indiana University in Bloomington claim they can make the measurements in a living human retina by using information hidden within a commonly used technique called optical coherence tomography (OCT).
In a study published in the Optical Society's open-access journal Biomedical Optics Express, they explained that in order to make an OCT scan of the retina, a beam of light is split into two, where one beam scatters off the retina while the other is preserved as a reference.
The light waves begin in synch with each other, but when the beams are reunited, they are out of phase due to the scattering beam's interactions with retinal cells.
Scientists can use this phase information to procure a precise measurement of a sample's position, but since in this case their samples were attached to live subjects, the researchers had to adapt these typical phase techniques to counteract any movements that the subjects' eyes might insert into the data.
Rather than measuring the phase of a single interference pattern, the researchers analysed phase differences between patterns originating from two reference points within the retinal cells - the top and bottom of the outer segment - and used the hidden phase information to measure microscopic changes in hundreds of cones, over a matter of hours, in two test subjects with normal vision.
They discovered that they could resolve the changes in length down to about 45 nanometers, which is just slightly longer than the thickness of a single one of the stacked discs that make up the outer segment.
The conclusion was that outer segments of the cone cells grow at a rate of about 150 nanometers per hour, which is about 30 times faster than the growth rate of a human hair – a discovery that could have significant implications for eye research in the months and years to come.
by Alexa Kaczka