Nanometer-scale growth of cone cells tracked in living human eye

December 20, 2011

Humans see color thanks to cone cells, specialized light-sensing neurons located in the retina along the inner surface of the eyeball. The actual light-sensing section of these cells is called the outer segment, which is made up of a series of stacked discs, each about 30 nanometers (billionths of a meter) thick. This appendage goes through daily changes in length. Scientists believe that a better understanding of how and why the outer segment grows and shrinks will help medical researchers identify potential retinal problems. But the methods usually used to image the living human eye are not sensitive enough to measure these miniscule changes. Now, vision scientists at Indiana University in Bloomington have come up with a novel way to make the measurements in a living human retina by using information hidden within a commonly used technique called optical coherence tomography (OCT). They discuss their results in the Optical Society's (OSA) open-access journal Biomedical Optics Express.

To make an OCT scan of the retina, a beam of light is split into two. One beam scatters off the retina while the other is preserved as a reference. The light waves begin in synch, or in phase, with each other; when the beams are reunited, they are out of phase, due to the scattering beam's interactions with . 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.

Instead of measuring the phase of a single , the researchers measured phase differences between patterns originating from two reference points within the retinal cells: the top and bottom of the outer segment. The team used this hidden phase information to measure microscopic changes in hundreds of cones, over a matter of hours, in two test subjects with normal vision. Researchers found 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 work shows that the 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.

Explore further: Rapid, high-resolution 3-D images of the retina

More information: Paper: "Phase-sensitive imaging of the outer retina using optical coherence tomography and adaptive optics," Biomedical Optics Review, Vol. 3, Issue 1, pp. 104-124 (2012).

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not rated yet Dec 20, 2011
" Instead of measuring the phase of a single interference pattern, the researchers measured phase differences between patterns "

Funny, that's kind of how human eyes work, by measuring the difference in colors.

5 / 5 (1) Dec 21, 2011
It is already known that opsin discs get recycled on a daily basis, with new ones constantly being generated at the outer end of the inner segment, near the connecting cilium, and old ones chewed off as a single cartridge at the rear end of the outer segment. What I didn't know was the numbers. It turns out that the daily turnover amounts to 120 discs, given the numbers supplied by this article. The "chewing off" is done by retinal pigment epithelial (RPE) cells, which send them to the blood stream in the choroid layer at the back of the retina.
Why does all this happen? The only explanation that makes sense is that the opsin molecules gradually get damaged by exposure to light (what I would call "bad absorption" as opposed to the ''good absorption" that results in light sensing), so that they are irreversibly bleached, and can no longer be reset by the process described below:
5 / 5 (1) Dec 21, 2011
... Not many cells have their own mechanism for disposing of used proteins, but it seems cilial photorecptors do. This turns out to be crucial to the long lives of vertebrates. Invertebrates have to choose between three options: have a very short life, spend most of it in darkness, or (the lobster option) have cheap, throw-away eyes. Ants have an inteersting compromise: the queen spends nearly all her life in the dark, and maintains the colony (eg, honey ant queens can live to nearly 30 years, relying of the eyes of their cheap, throw-away workers to do what has to be done above ground).
5 / 5 (1) Dec 21, 2011
... This mechanism helps explain why the vertebrate retina seems to be so "badly designed". Their is a trade-off between sensitivity (lower than the otherwise "similar" cephalopod eye, which does not renew opsins), and durability.
5 / 5 (1) Dec 21, 2011
Perhaps I should rephrase my most recent post, because there was no direct trade-off between sensitivity and durability. The vertebrate imaging eye could not have evolved from a cephalopod imaging eye. Rather, it evolved from a non-imaging eye, as an add-on function, so that there was no pre-existing image sensitivity to compete with. Vertebrates acqired their strange imaging eyes by being forced from deep, dark water into shallower, less dark water, where they would be vulnerable to predation by sighted invertebrates, unless they evolved imaging eyes. The most likely circumstances for this to occur is the Ordovician-Silurian period, when Gondwana was drifting over the south pole, and acquiring a large ice-cap, causing the oceans to get shallower, and giving some populations of hagfish a big problem!
not rated yet Dec 21, 2011
David, what's your take on " visual snow/stochastic resonance " in visual perception ?

not rated yet Dec 21, 2011
Isaacsname, I haven't concerned myself that much with the details of what determines the lower limit of vision. Clearly, vertebrate eyes compare poorly to the best invertebrate ones in that respect, but our eyes did not evolve in competition with those eyes, but in competition with sensitive, but non-imaging eyes of the hagfish. This explains the original inversion of the retina in vertebrates -it was the sensitivity of non-imaging vision that had to be maintained (through the use of melanopsin-expressing retinal ganglion cells, which therefore ended up overlying the rods and cones, to the consternation of many, who complained how "badly designed" they were, and therefore what a flop natural selection was!).
Is that the issue you had in mind?
not rated yet Dec 21, 2011
... On further thought, I suppose you might have been thinking of what determines the exact spacing of the opsin discs. In that case, I think the discs are as closely spaced as they can be without hampering the other main function of RPE cells, which is the rapid flushing of retinal molecules through the outer segment, because the vertebrate visual pigment comlex, unlike the invertebrate ones, needs to be reset chemically, not optically, and this is done in the RPE cells, so transport is required.
I don't see how optical interference could be important in determining disc spacing - they are too close for that.
not rated yet Dec 21, 2011
I guess what I'm asking is if opsin would get replaced at the same rate in organisms that have stochastic resonance .


Sorry If the questions seem obtuse, I'm not too familiar with the subjects at hand, I just am curious, as I have lived seeing " static " over everything for ~ 40 years, but my color perceptions seem as strong as ever. I just haven't found much research on the top of visual snow/static.

Thanks !
not rated yet Dec 21, 2011
W/r to our first point, Isaacsname, I must admit that I haven't considered this issue. I suppose it is relevant to the very earliest hagfish types that were beginning to evolve a scotopic vision, but not enough information is ever likely to be available about such eyes, as disc spacing doesn't fossilise!
On your second point, I am not a doctor, or a professional ophthalmologist, but I am sorry to hear what you say.
I presume that you have seen an opthalmologist about it, because it can't be treated without knowing what exactly is causing it, and even then, not necessarily.
One question: are you sure that your colour perception is as strong as it was before you started getting visual snow? It seems unlikely to me that the snow isn't associated with reduced signal strength sent to the visual cortex, but the question then is: at what point in the path is the signal weakened? Also, do you think you also suffer from "night blindness" - could your rod vision also be snowier than it was?
not rated yet Dec 21, 2011
More questions, Isaacsname:
Is the snow equal in both eyes?
Is it equal for all colours?
not rated yet Dec 21, 2011
Never talked to a specialist about it.

Had it as long I can remember.

Yes, equal in both eyes.

Equal for all colors, -in the dark I'm as good as blind, very heavy static, I'm very nearsighted as well, but all light sources appear as starbursts, it's like my own personal light show :P

The interesting thing about it, at least from what I think I understand, is that it's similar to " dithering ", I know that SR is being exploited in fingerprint/image analysis.

It does allow me to see very small changes in my visual field in daylight, I'm always trying to point things out to people, for some reason they never see.

It is one of the things behind my anal-retentive focus on detail, I think. I don't feel like I " suffer " from it though, thx for the condolences all the same, David.
not rated yet Dec 21, 2011
So, your whole vision is "noisy", it seems. Is their any evidence that the noise is of genetic origin - does it run in the family?

What sort of "very small changes" are you referring to, because this aspect of it is odd? I would not expect snow to actually improve sensitivity to.

I think dithering is an essential aspect of vision, because it introduces the temporal gradients essential to maintaining a signal. In your case, it also surely should reduce snow.

Are you suggesting that sigma reject (SR) techniques are being applied by your eyes more effectively than by those of others?

Do any of your relatives have a similar kind of snowy vision?

I think you should have a colour resolution test with controlled brightness, if you want to understand how your vision compares with the average.

More soon...
not rated yet Dec 21, 2011
... Perhaps you need a controlled brightness colour vision test, with another person as control, using a range of brightnesses, to establish how different your vision is from the norm.
BTW, as you probably know, our eyes have a highly non-linear response to brightness, so that we can see over a wide range of illumination, and still not suffer from saturation effects. I suspec this was by natural selection for spotting predators/prey in the shadows. It left us with some interesting colour optical illusions, though!
not rated yet Dec 24, 2011
... One minor oversight I made in my first comment was not to allow for a gap between discs. The number of discs in a "cartridge" must therefore be significasantly less than the figure of 120 I calculated ignoring the (unknown) gap.

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