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What everyone should know about the human eye (gazehawk.com)
274 points by bkrausz on July 22, 2011 | hide | past | web | favorite | 47 comments

The information about faces and direction of gaze leads to an important tip on presentations. Always choose images that are facing your text. Flip the image if necessary to make it happen.

See http://perl.plover.com/yak/presentation/samples/slide023.htm... for an example of this advice being given. (And read the rest of that presentation if you have to give presentations - it is quite good.)

And if you read the rest of the presentation, don't forget to read along with his notes http://perl.plover.com/yak/presentation/samples/notes.html#s...

Thanks guys I have learned alot from that.

What I was surprised to learn about the eye is that the area of foveal vision, the central part of the image that can see fine details, is about the size of a quarter held at arms length from your face. You can only ever clearly see a tiny dot, but your eyes dart around and paint a picture for your brain to show you.

Plus as they dart around they don't achieve anywhere near 100% coverage and probably rely on something akin to compressed sensing to fill in the gaps.


(beware of the wired dumbing down of it though)

Another interesting fact: every time your eyes move, you're blind for a couple milliseconds. But you never notice this because your brain fills in these gaps in time.


You can see the reason for this if you move your eye slowly (by following a moving object)— say, place your cursor under a line of text and move it across the screen, following it with your eyes. If you do this right, you can notice motion blur in the surrounding text.

Now look back at where you started, a movement that takes tens of milliseconds. Imagine how much blur that creates— that image must be completely useless. You're doing yourself a favor to ignore it.

Edit: Since I mention it, I've wondered before if there'd be any value to training yourself to move your eyes smoothly without following an object. This test suggests probably not; the image just isn't good for anything unless it's correcting for a moving object.

That's why our fonts can be so small! At night, though (or in low light) the cones in fovea are useless and peripheral vision becomes more important.

There's a lot of cool chemistry involved in the eye adjusting to darkness and light. My favorite fun fact is that as you adjust to darkness, the spectrum of light your eye perceives becomes slightly blueshifted: http://en.wikipedia.org/wiki/Purkinje_effect

Knowing this fact came in handy one evening while stargazing with some friends.

This reminds me of a book I read a few years ago - Mind Hacks [1]. It discussed all kinds of interesting mechanisms that our brains use to fool us into thinking that we are always aware of our environment.

In the eye chapter, there was an interesting side-effect of the saccade movement - the "broken watch". This is when your brain fools you into thinking that the picture you see after a saccade has been the same during the movement itself. When you look at your watch and your timing is just right, you will be left with the impression that the seconds arm stays fixed for longer than it should.

I highly recommend the book - it really demonstrates our inner-machinery.

[1] http://mindhacks.com/

The fact that humans like to look at faces and are pretty good at identifying differences between them has been exploited in http://en.wikipedia.org/wiki/Chernoff_face.

Similar in concept, the Robot Hash generator: http://news.ycombinator.com/item?id=2742367

The best one was left out the blind spot everyone has where the optic nerve connects.

Wikipedia has a simple test: http://en.wikipedia.org/wiki/Blind_spot_(vision)

anyone interested in this topic should check out Visual Intelligence, by Don Hoffman:


The book deduces a set of rules that your brain must follow in order to construct a (mostly) correct interpretation of the limited data that it gets. It does this with a series of experiments that the reader can test. While possibly no longer up to date on the latest research, it's simply a delight to read.

With my conscious perception being on the 'processed' side of my neocortex, I used to think that the human eye was basically a hi-res camera .. until I read Jeff Hawkins 'On Intelligence'

In the book he explains how our brain gets a crappy, distorted image from our eyes, and manages to assemble it using a hierarchy of cells [regions] in the neocortex. It's really interesting, I would recommend getting this book, it's only a couple of hundred pages long, and really opened my eyes (ha!) to how the human brain learns.

the eye gives a crappy image? Odd, i dont know many cameras that can detect single photons.

The human eye can't detect single photons. However, it can detect a few simultaneous photons at the retina, which corresponds to a hundred or so entering the cornea.

Rod cells can detect single photons


Sort of. This is from memory, but here's how I remember that experiment working:

In that experiment, they showed that, for some value of n, rod cells consistently activated in response n photons, and did not activate in response to (n-1) photons. Thus, they argue, rods are sensitive to single photons.

In some sense, they're right, in a straw that breaks the camel's back sort of way. However, the common mis-interpretation of these results is that n=1; that is, in utter darkness a rod cell would fire in response to a single photon. But this is not what the experiment showed.

(Note that just because a rod cell activates doesn't mean that the organism would perceive light. In order for a signal to reach the optic nerve, a retinal ganglion cell needs to activate. Ganglion cells only activate in response to the activation of a significant number of photoreceptors. Only in some part of the fovea, where there are no rods, is there a 1:1 mapping of photoreceptor to ganglion cell. In most parts of the retina the ratio is closer to 1 ganglion cell per 100 photoreceptors. Furthermore, even if a ganglion cell activates and a signal reaches the optic nerve, that still doesn't mean that the organism will perceive light. It's likely that further levels of processing may filter out transient activations.)

This was good. If you're interested in some further tech/visual neuroscience crossover, the intros to these two posts I made have some relevant information:



See... I have always felt the opposite. The various parts of your eye are much more like the various parts of a camera than people admit.

Now, your perception of the visual world is a lot more dynamic than a simple still shot, which seems to be what this is really saying. No argument there.

Optically, they're kind of like cameras. I wouldn't be surprised if early camera designers looked to mammalian eyes for inspiration.

But once you get to the retina all bets are off. That's the point of this article.

Right. I just think it would be better said as "people are not cameras." Seems many people think your eyes are somehow immune to the optical qualities that cameras have to deal with.

Good point. But I would argue that, fundamentally, the interaction between the eyes and the brain is much different than the interaction we have with cameras. The optic system is deeply, deeply meshed with the brain's perception of reality.

Somewhat related:

http://www.visionsofjoy.org/pdfs/BatesPerfectSightWG.pdf http://www.i-see.org/perfect_sight/

William Horatio Bates (1860-1931) first published his treatise, The Cure of Imperfect Sight by Treatment Without Glasses (title page), also known as Perfect Sight Without Glasses (cover), in 1920.

This guy has some kind of crazy ideas; but the general thing I learned from him is that the shape of the lens of the eye is a function of three sets of muscles that can be trained/relaxed to help vision come back to 'normal'.

A great read. The HN worthy title would be "hacking your crappy vision"


“Despite continued anecdotal reports of successful results, Bates’ techniques have not been shown to objectively improve eyesight, and his main physiological proposition – that the eyeball changes shape to maintain focus – has consistently been contradicted by observation. In 1952, optometry professor Elwin Marg wrote of Bates, ‘Most of his claims and almost all of his theories have been considered false by practically all visual scientists.’ Marg concluded that the Bates method owed its popularity largely to ‘flashes of clear vision’ experienced by many who followed it. Such occurrences have since been determined to most likely be a contact lens-like effect of moisture on the eye.”

Does it actually work?

The cornea, not the lens, supplies the majority of the eye's refraction, so I would be surprised if this worked for people with any substantial amount of refractive error.

from what I understand (which is admittedly not much) it's actually both the cornea and and lens which are manipulated together by these 3 sets of muscles.

My understanding is that the cornea is essentially fixed and non-adjustable, with all of the tuning done by the lens. I am not an ophthalmologist, however.

Some googling will show you evidence that it does indeed work. For my part, I corrected a slight astigmatism;

This guy claims to have been clinically blind and recovered using similar techniques


>shape of the lens of the eye is a function of three sets of muscles that can be trained/relaxed to help vision come back to 'normal'.

Didn't read your link but could this explain why programmers get poor eyesite as they frequently focus on close objects (monitors) and thus the three sets of muscles become accustom to focusing on close objects?

My eye doctor certainly thinks that's a contributing factor. His recommendation (for me) was that after every hour (or 55 minutes) of PC work, I should spend five minutes looking at things at distance (buildings, trees, etc). It's probably frowned upon by my employers (as I don't have a window office), but it's helped reduce my eye strain quite a bit.

> It's probably frowned upon by my employers

Too bad for them. Your eyes are more important.

I agree. Which is why I'm not overly concerned with what they think about the subject. ;) My supervisor seemed content with "doctor's recommendation" once when I mentioned it to him.

I was recently diagnosed with a condition called Keratoconus... from wikipedia:

Keratoconus (from Greek: kerato- horn, cornea; and konos cone), is a degenerative disorder of the eye in which structural changes within the cornea cause it to thin and change to a more conical shape than its normal gradual curve.

Although my doctor said it was not caused by 10+hrs/day in front of a computer, i suspect otherwise. Its a relatively rare condition, and at my current office there are three of us (out of 25) with the condition.

I would like to know, dear HN readers, do you have this condition?

Definitely. Intact the most important paper on eyesight (IMHO) concludes that the way to guard against myopia is to spend more time outdoors where most objects in your vision will be far away. I'll find the link when I'm not on this stupid iPad!

i think the biggest problem with screens is that the eyes get dehydrated because we blink less often. certainly, the lens muscles may get overused as well though.

the extraocular muscles you refer to control the position of the eye, not the lens. maybe he attempts to change the shape of the cornea (something like squinting)

There is also another kind of movements, smooth pursuit movements where the eyes move smoothly following a target. While we use saccades when reading text, an smooth page animation would trigger them. That's why i find jerky animations annoying and disorienting

Right. I thought about including a description of pursuit movement, but omitted it because I didn't think it came up frequently when viewing web pages. Humans actually have a whole bunch of special-case eye movement patterns, like the "catch-something heuristic" (http://en.wikipedia.org/wiki/Gaze_heuristic), which we share with dogs! (http://www.mansfield.osu.edu/faculty/dshaffer/Shaffer%20Publ...)

That is awesome; thanks for the link! I always expected that we used a more complex algorithm, but this extremely simple one probably works just as well.

I do that all the time when people look at me. Look away like you are staring at something important; they very quickly try to see what you are interested in.

"[P]eople love to look at faces, but we often use them as clues as to where else to look. Following a person’s gaze is almost a reflex. James Breeze demonstrated this really well in a blog post called “You look where they look.” His experiment was simple: about 100 people were shown a picture of an advertisement with a baby and some text. Half the time, the baby was facing the reader, while the other time, the baby was looking at the text. Breeze found that not only did the people shown the baby looking at the text pay more attention to the text, but they actually stopped looking at the baby faster in order to follow its gaze."

Instead of downvoting, I'm going to ask why you're quoting this. So, why are you quoting this?

lol; It was the most useful part of a good article and I thought it would entice more people to read the whole article.

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