I’ve seen it stated in several places that other mammals vision (cats, dogs etc) are based much more on movement than a humans. I’ve never been entirely sure what thats supposed to mean, and how can they tell in the first place?
Of course you get it taken to extremes like the T-Rexes in Jurassic Park who can’t see someone if they are standing completely still (so how do they avoid trees, boulders etc?).
I do remember an incident a few years ago when I was walking along a dry stream bed and noticed a small cat approaching from the opposite direction. I hunkered down and stayed as still as possible until it was only about ten feet from me but then had to shift position due to muscle cramp, it immediately noticed me and after a Mexican stand-off (which I broke by saying ‘Hello’) it took off with great haste away from me. Personally I always figured it was just that the cat wasn’t paying attention rather than that it literally couldn’t see me unless I moved.
Bonus question: Unsuccessfully looking for an answer to this question on the interwebs I came across a conflicting opinions on how good the eyesight of a Fox is, some sources say a fox has excellent eyesight, others that its very poor. So what is it? (and again how the feck can they tell?)
Sadly, foxes have very weak eyesight which is mostly geared towards detecting movement. If you see a fox and stand very still, there is a fair chance the fox won’t see you."
It doesn’t sound like much of an evolutionary advantage to me…
btw that links back to a rather good webcomic, if you can get over the cutesy animal stuff.
That a really interesting topic. As a human I am near sighted not too bad 20/40. But I have noticed that I cna identify birds and other animals just by the slightest movement they make even though I could not identify that same bird or animal if it had not moved. I suspect many animals are similar to this. Antelopes have extreme good vision.
I’ve seen this claim made about praying mantises. As I recall understanding it at the time, either the creature or the object had to be moving - only if both were still would it fail to recognize the object. Had to do with the vision being based on the change in where the object was relative to the creature’s eyes.
Not sure if this is accurate, but if it is it would account for it not bumping into things.
Eyes have two basic types of receptor cells. Rods are used to see in lower light levels and are used to see movement, but they aren’t so good at seeing details. Cones see details better, but require higher light levels. Cones are also responsible for color vision.
In humans, the rods are concentrated on the outer edge of the retina and are used for peripheral vision. In dogs and cats, the concentration of rods extends a lot further into the center of the retina and they have a higher proportion of rods to cones throughout much of their eye. This means that they can see in low light levels better than we can and also that they can detect movement better than we can (it’s something that we detect more on the periphery of our vision).
Dogs and cats have fewer cone cells, which means that they can’t see colors as well as we can. They also only have two types of cones where humans have three. This means that in addition to not seeing colors as well as us that they are also partially colorblind.
Dogs and cats also have a structure called the tapetum lucidum behind the retina, which is basically a membrane that reflects light back into the retina. It helps them to see even better in dim light.
The presence or absence of these structures allows you to make some educated guesses about exactly how well animals can see, but there’s still some guesswork involved.
For animals smart enough to be trained, it’s simple to test what they can see: Reward them to respond in some way if they see some particular thing, and then see under what conditions they respond. Most mammals can be so trained, if you’re patient enough.
You may have seen it stated, but it is false. Human vision relies on movement too, although, unlike some other species, much of the necessary movement for humans is supplied by ourselves, by the fact that we move our eyes in their sockets almost constantly. With some relatively minor exceptions and caveats, a visual stimulus is only detected by the eye if its image is moving across the retina, so that the stimulation of the receptor cells is dynamically changing as it moves. Unfortunately, although it is now well established scientific fact, it is still the case that virtually all undergraduate textbooks on human vision, even recent ones, and even some quite advanced ones, have little or nothing to say about the central role of eye movements in seeing, so even some people who think they know quite a lot about about visual science, maybe even some people who teach the subject, have (or show) little or no awareness of the issue.
Some animals, such as frogs, cannot move their eyes at all, and often keep very still for long periods. This works well for them, however, because as soon as a moving object, such as fly, moves across their visual field, they are able to detect it straight away, with no other visual distractions, and shoot out their tongue to catch and eat it. Presumably when they are sitting still and nothing is moving in front of them, they see nothing at all, or very little.
T. Rexes can avoid bumping into boulders because when the T. Rex moves - either its whole body, or its head, or its eyes in their sockets (I don’t know whether T. Rexes could move their eyes in their sockets: some animals can and some can’t) - the image of the boulder is moving across its retina, and can therefore be seen. If you are not moving at all, running into a boulder is not going to happen anyway.
How can you tell (that motion is necessary for vision)? In a number of ways. One is the classic experiments of Hubel and Wiesel using anesthetized cats with their eyes pinned open. They also set up electrodes to record activity in the visual cortex of the cat’s brain, and placed different visual stimuli in front of its eyes. In order to have a nice cleanly controlled experiment, they placed static stimuli in front of the cats with the lights out, then turned on the lights so the cat could see it. They were hoping to see different sorts of responses in the visual cortex to different sorts of stimuli, but to their surprise, the visual cortex hardly responded at all (except momentarily when the light went on or off), no matter what they put there. Then, one day, when they were close to giving the experiment up as a failure, and apparently quite by accident, one of the researchers forgot to switch off the light whilst removing a stimulus object. The moving stimulus set off a great flurry of complex activity in the cat’s brain. They continued the experiment using moving stimuli, and got a whole buttload of interesting results that won them the Nobel Prize.
Another piece of evidence that shows the importance of a moving retinal image to specifically human vision comes from experiments in which an image is artificially stabilized on a person’s retina. This is quite difficult to achieve as a person’s eyes are normally in constant motion (though you may not be aware of it). Some early versions of these experiments involved gluing a tiny projector directly to the eyeball, so that it projects an image into the eye, and moves with the eyeball, thus keeping the image in the same place relative to the retina. (These days the lab technology has advanced such that the methods do not need to be quite so gruesome, and the image is stabilized far better than can be achieved with a glued-on little projector.) The result is that all visual consciousness of the projected, stabilized image disappears after a small fraction of a second. The stimulus is seen very briefly, when it is first switched on, but then very quickly disappears altogether, despite the fact that it is still being projected onto the person’s retina. We only see thanks to changing (which mostly means moving) retinal images. Static ones very quickly become invisible.
Indeed, much visual neuroscience, these days, is done with highly trained monkeys. They have electrodes implanted in their brain, to record neural responses, and are trained to sit very still, but to respond in various ways (often just by making some specific voluntary eye movement, or perhaps pressing a button) to certain visual stimuli. Their reward for doing this, which must take an impressive amount of monkey concentration, is usually a little sip of fruit juice that is squirted into their mouth. Monkeys, it seems, really like juice.
Further to what I said before, it is not that either the T Rex or the cat cannot see that you are there, if you keep still. So long as they, or their eyes, are moving, they can see perfectly well that something is there. However, until they see you move by yourself, they are unlikely to realize that the thing they see is another animal, that may be either food or a threat, so they will largely ignore you.
And sometimes you have to be really, really, really patient. Everyone thought that cats were colorblind for a long time. Even after they discovered that cats had cones in their eyes, cats still did not respond to training for color discrimination. Researchers thought that there was something in the cat’s brain that just wasn’t working with respect to colors, but it turned out that the trainers just didn’t try hard enough.
There are big differences in vision in different animals. For example, a cat’s retina is hilly, so to speak. The result of this is that for a number of objects in a wide distance range, all the objects are in focus at some plane in the retina, and at least in theory, the cat could even help judge the distance based on this. (Another way to get wide focal range is to use a small aperture, but cats are designed to see well in low light.)
Another difference might be foveal density. In humans, we have very dense receptors in a relatively small fovea, allowing us to do things like read small text. Our peripheral vision is quite low-resolution. Other mammals may have a wider distribution, and I’d expect to see it widest in prey animals. (One odd thing is that while we have few color receptors outside the fovea, our peripheral vision seems to be in color. This is our brain tricking us by filling in the blanks based on what it knows.)
Some species of pigeon have 5 different color receptors, compared to our three. Imagine! To them, a TV or computer screen image would seem a lot like a 2-color print does to us.
Vision is fascinating. It’s far more complex than the “TV camera” analogy we like to think of and which does make sense, but only very superficially. Significant processing happens right in the retina, before the signals are passed to the brain, breaking down the analogy that early in the process.
A philosophical friend of mine once argued against the philosophers who posited a creative faculty required for vision, saying in effect that the mind has to create a model of whatever it is we look at (to some extent). He disagreed, saying we “just see” stuff, what’s the need for this model stuff inbetween the reality and our consciousness?
I laughed. While machine vision was still quite crude at the time (80’s) I mentioned how, in order to get computers to recognize things, we had to build in a lot of aspects of physics and had to make kinds of models of the thing seen, remarkably similar to what the philosophers described. He didn’t get it at all. He was assuming the TV analogy worked all the way to perception and understanding. Since then everything I’ve learned about vision and visual processing (as an interested layperson, reading books) reinforced the fact that he was just plain wrong.
While it’s an oversimplification to say that animals see motion and we don’t, I wouldn’t be surprised to learn about much more specific statements that could be made about vision differences (both physical and cognitive) among various animals.
BTW, if you have a steady hand, you can easily catch a landed fly, simply by moving your hand very slowly. They don’t notice the movement. Once you get within a few inches, make a quick grab, and bingo.
True, but the analogy breaks down even before that, before the light even reaches the retina, because of the facts about eye movement that I mentioned above, and because of the smallness of the fovea (which you mention). If a TV camera swung about violently to point in a completely different directions three or more times every second, its output would be unwatchable and utterly confusing, yet that is what the human eye does (even setting aside non-saccadic eye motions such as drift and a constant low amplitude tremor. Furthermore, we would be talking about a TV picture that, even if it could be kept still, would only have decent resolution and anything like good color in the very central part of its screen: about 2º of visual angle, about the width of a thumbnail held at arms length.
No, the eye does not, in any meaningful sense, send a picture back to the brain. Although its optics may somewhat resemble those of a camera, it does not function like a camera at all. Rather, it darts about purposefully (the rapid saccadic jumps of the eye are not random - though they are very far from regular - but are closely controlled by the brain) seeking out points of visual information where the brain anticipates they might be found, and which it anticipates may be useful.
IIRC after the first Jurassic Park movie, further research suggested that the idea T.Rex could not see stationary prey was considered BS… Leading to the scene in the second movie where the bad guy stands perfectly still in the nest, expecting to be safe as T. Rex sniffs aorund. The good guys are watching (on TV?) and the kid says “what’s he doing?” One guy says “T. Rex can’t see stationary objects” after which the expert say “of course they can!” while T. Rex proceeds to eat the bad guy.
My guess is that animals like cats have vision tuned to see small movements like mice. (or laser pointer spots) Big animals are much less interesting (hence, less noticeable) until up close. Same idea that our vision is tuned to see eyes and creatures hidden in the dark, and also to make out faces out of random features, like that Jesus on the toast…
Yep. If T. Rex’s eyesight was that poor, and it didn’t have, say, a good sense of smell to compensate, it’s hard to see how it could have been an effective predator.
Most prey animals have freeze / play dead somewhere in their instinct library; it wouldn’t take much selection pressure for this to become the default action.
The big problem with the responses so far is that you are focusing on the sensor and not the image processing. Just think about it for a moment : most animals have a large portion of their behavior hard-coded as “instincts”. This means their internal rules need to be simple enough to be defined by proteins defined by DNA. Think about what the animal needs to know to identify a static object in the background :
"patterns of these color and shades look like living creature <bird>, this shade pattern looks like living creature <bird2>, this pattern looks like <turtle>, and so on and so forth.
To a limited extent, smarter and larger creatures like cats and dogs can do this.
However, contrast this with the following algorithm :
if <pattern of retinal spots> move, in a different way than the animals sense of it’s own movement, then <PATTERN IS A LIVING CREATURE>. If <pattern> is <BIGGER> (calculated by distance, requires the animal to see in 3d) than <me>, <danger!>
This algorithm continues to work as evolution changes the way living creatures look over the millenia.
Conclusion : yes, if you remain perfectly still, animals will have difficulty seeing you. Smarter animals, especially ones with brains closer to your size (such as big cats), probably can see you just fine. Also, animals with extremely sensitive hearing may be able to hear you breathing or making tiny movements, and of course they can smell you.
One more side note : your visual cortex is about the same size or larger than a dog or cats entire brain! There’s a reason your vision is better, and it has nothing to do with how good your eyes are.
I am not sure that the inference you are insinuating is justified, obvious as it may seem. MRI scans have revealed that some people have about two and a half times as much primary and secondary visual cortex (by surface area) as do some other people. AFAIK, there is no evidence that these differences in visual cortex size are reflected in any differences in how well the people can see.
Size matters. We can discuss what the extra surface area means, but the reason a human is smarter than an animal with a smaller brain of the same body size has a lot to do with the larger brain on a human.
Yeah but the inference about vision is another thing entirely. I think I know what you meant to say, but you went too far with “There’s a reason your vision is better, and it has nothing to do with how good your eyes are”.
For example, many birds have far greater visual acuity than you or I, plus can see a much larger gamut of colours. And of course their entire brain is much smaller than the visual cortex of a human.
The mantis shrimp would do well in an Animal Kingdom Vision oscars, but of course its brain is tiny.
The trouble with thinking about sight, is that it leads on to more and more complex questions. What do we **actually **see? Light is reflected by an object on to the receptors in our eyes. They send signals that our brain interprets as objects, and it is quite easy to fool our brains into thinking they are seeing something different to what is actually there, or not seeing something at all. Think camouflage. Think optical illusions.
Think about driving a car. The sheer volume of information we can ‘see’ would be totally overwhelming, if our brains didn’t simply dismiss the vast majority of it as irrelevant. This has to be learned and is one reason why new drivers have problems. Problems happen if our brain dismisses something that turns out to be relevant, and we make a wrong decision.
Example video. It’s not super visually exciting these days, but I am always in awe at their methods.
Err, cats are colorblind. The normal definition is “sees less color information than a normal human, which does not necessarily mean they can only see shades of grey.” Same as most colorblind humans, who can still see colors, just not as many colors or has less discrimination ability than a color normal human (two hues seem identical when they don’t to most people). Most mammals are colorblind (two cones), save apes (including humans), Old World monkeys, and among New World monkeys only Howler monkeys and certain female monkeys. The rest see color, but not as well. Some mammals, like cetaceans, pinnipeds (seals etc.), and certain nocturnal primates, only see black and white, because their environments don’t require color. But cats and dogs aren’t among them, having confusions between reddish and greenish hues, but bluish is completely different from those.
But yeah, the general experiment was “we didn’t realize cats’ abilities because they were being pains in the ass.”
Yeah, maybe, but that means nothing. A small human’s (dwarf or not) brain is smaller, but there is no intelligence difference suggested by absolute brain size. Absolute size is a poor correlate with abilities. No matter how good your VC is, with two cones plus less acuity, less can reach the brain. Many animals have a tapetum lucidium (the creepy glowing eyes when you take a picture). This improves night vision, at the expense of acuity.