Do chameleons see in 3-D?

Factual Questions
1

Chameleons, like us, usually have forward-facing eyes that I presume allow parallax viewing, which is reconstructed in the brain to represent the 3-D nature of the visual world.

But every so often one of the eyes goes for a look-see to the side or even (I think) backwards, while the other looks straight ahead–completely losing parallax. How does it perceive, or manage to perceive, then in a constant manner? It’s not like when you cover one eye, then uncover it.

(I am not posing the “what-do-fish-think-about” kind of question. I hope.)

Note: FWIW, I have only one eye (hence no 3-D vision), so its very likely I’m coming to this question ass-backwards.

2

So, you have one eye. I bet that, for the most part, you do not have too much trouble in telling, purely visually, how far things are away from you. In real life (as opposed to 3-D movies and the like) binocular disparity is only one amongst several sources of visual depth information, and may not even be the most important. Motion parallax is certainly an important source. Every tiny movement of your head will cause nearer objects to appear to move relative to the more distant background, giving the visual system excellent information about their distance. Visual texture gradients are another source of depth information, and are part of the explanation of how we can normally tell how far away things are even in regular (non-3D) movies and pictures. No doubt chameleons can also make use of these sorts sort of information.

But perhaps you are more puzzled as to why chameleons do not get disoriented when they swivel their eyes in different direction, or get confused because they are seeing two quite different vistas at once. If that is your problem, it probably arises because you implicitly assume (as most people do) that the function of our eyes is to get images into our brains, and that it is these images that we visually experience. This idea is deeply ingrained in the way that most people think about vision (perhaps especially people with a basic knowledge of the optics of the eye and the neuroscience of early visual processing), but it is false. The eyes are not for getting images into the brain (inasmuch as they can be said to do so, it is an incidental by-product of their actual functioning), they are for extracting behaviorally relevant information from the light in the environment. In order to do this, human eyes are constantly on the move, seeking out new bits of information. But the world does not seem to constantly swinging around us as our eyes point in new directions, even though the optical images on the retinas change rapidly and radically, often several times every second. That is because we are not seeing the images, but the world itself, which (normally) remains still. There is no reason to think that it is any different for chameleons, just because they can move their eyes even more freely than we can. When they turn their eyes in two different directions they do not see two conflicting images, because vision is not about seeing images images at all. What they gain by being able to move their eyes independently is the ability to obtain information from a larger proportion of the visual environment, to see more of the world around them than they would be able to see otherwise (without actually turning their whole body around).

3

Njtt has it. Our eyes feed visual data to the brain. The brain process the data and builds a ‘picture’ or a functioning cognitive model of the world around it. Same for chameleons. Their eyes can move and rotate independently, or at least to a much greater degree than ours can. This just means that they can gain more information than we can without having to move their head. But it’s the same process… the eyes feed the data, and the brain stitches it all together into a working model of the vicinity and anything significant in it (prey, predators, comfort, protection).

And yes, they see in 3D.

4

Thanks for the correction. That explains everything. And it’s not based on a false dichotomy or anything.

5

njtt’s post is extremely interesting and valuable, and is useful for interpreting not only chameleon vision, but other types of vision, as well – how do Spiders perceive the world, with their multiple eyes, or the Triops “shrimp”, with its three eyes (one on the back!)

But I do take exception to his abso.lute statement that the eye is not for getting images to the brain. I’ve talked to people who have had accidendents, and now have perpetual double vision. If his statement were correct, this double vision would go away – the relevant information is there, and the brain ought to be able to “recover” and seamlessly “knit the image together”, just as it does over the visual Blind Spot. But it doesn’t – decades later, the person still has double vision. His entire visual system – eye, brain, and internal processing – still treats things as if his eyes were two cameras gathering picture information and passing it along to the brain, not as if they were sensing the environment optically and rendering a map of the outside world.

I’m still not clear, after years of reading and cogitating on how the entire visual system works, exactly how it does so, but the answer lies somewhere between the “it’s just a camera obscura” interpretation (which I know isn’t true – I can see how my perception goes from 2-D to 3-D when I close and open one eye – clearly there’s processing going on) and the “brain makes a map/representation of the world from visual information” idea (for reasons given above).

6

:slight_smile: This is a wonderful thread for me and I am following it closely. May it continue!

7

I’m not sure what might be different about double vision, but there have been experiments involving subjects wearing lenses that inverted, or curved, or did something unusual to the image, and people seem to be able to adjust, reporting that they see things ‘normally’ after a time. Normal at least in the form of being able to ride a bicycle and pass other coordination tests with the lenses on. That does not mean that the image turns itself right side up again, but at least that the person was able to adjust.

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8

CalMeacham, that sounds like some kind of mapping problem.

I’m not an expert on eye/brain functionality, and I know drawing analogies to computer systems is inherently fraught with innacuracies, but consider this example.

Suppose I have a spreadsheet program that compares data in column 1 with data in column 2. Maybe I’m looking for a match, or within some tolerance band. The exact process isn’t important, only that I have two columns side by side being compared. This represents the visual pattern in the brain.

Now suppose the data that fills those columns actually comes from two other spreadsheets. Column 1 data comes from a spreadsheet - let’s call it SH Left, and column 2 data comes from a different spreadsheet, SH Right. The data in an output column from SH Left is directly copied to column 1 of our brain spreadsheet, and the same for SH Right to column 2. The row numbers in SH Left match row numbers in column 1, and the same for column 2.

SH Left and SH Right represent the eyes, obtaining inputs and passing info along to the brain spreadsheet.

Now suppose someone comes along in SH Right and inserts a cell at the top of the output column. Maybe they just added a header, maybe they did it by accident, but what ever they did shifted all the data down one cell, but otherwise left it accurate. Okay, so that spreadsheet is still functioning properly, still computing what it is supposed to, and still filing it in the correct order. Internally, everything is working right, but it just moved where it stores things slightly.

Now the brain spreadsheet tries to pull the data for columns 1 and 2. Column 1 is the same and works perfectly, but column 2 has a slight glitch. All the data copies and pastes correctly, but now the row numbers don’t align the way they used to. Data in row 2 of the SH Right should go into Row 1 of Brain Column 2, but Brain Column 2 is still looking in SH Right Row 1. Suddenly, all the data in column 2 is shifted down one cell from where it belongs.

Now brain spreadsheet is trying to do the comparison and cannot, because the correct data is in the wrong place.

What I am trying to demonstrate is that the visual picture established in the brain is subject to the quality of the data supplied by the eye and also something to do with how that data is transmitted.