XKCD reveals your visual perception quirks

Today's XKCD, "Visual Field," is a terrific mind-bender: a series of optical experiments to try with your computer's screen and a rolled-up piece of paper that demonstrate the quirks of your visual field: your blind-spots, your ability to perceive detail, night vision, the ability to perceive polarization, sprites and floaters, color perception and so on.

Visual Field


    1. close one eye, with both open the blind spot of one is covered by the other eye’s field of view.

      1. Actually according to most tests I don’t.  In the technical sense of the term blind spot, obviously I have one.  But due to having something similar to nystagmus since birth my eyes continuously move.  When tested I show up as having no blind spot since my eyes are basically moving the blind spot around.

        One of the other lovely side effects is the ability to see flickering up to around 120hz or so.  LED Christmas light and taillights drive me up a wall.  And flickering fluorescent tubes have a tendency to make me dizzy (where as strobe lights don’t…)

  1. I have recently noticed floating things when I drive in bright sunshine.  I am glad to know they are blue sky sprites and not indicative of me going blind!  Yay XKCD!

    1. From time to time I notice colored lights floating above the road while I drive.  My passengers tell me they are turn signals…  :-)

  2. You can demonstrate the “brain filling in the holes” effect for stuff in  your peripheral by staring at a surface that has scattered spots on it. A grid of floor tiles works too.. basically you want one dominant tone with a few small interruptions out towards the edge of your vision.

    Keep your eyes super still on one spot on this surface, but pay attention to the grid/spots at the edge of your vision. After about 5-10 seconds they should blend into the background color and disappear. As soon as you shift your eyes, they’ll return.

    I guess the brain only keeps a limited buffer for storing color information. :D

    1.  I got to experience this in an interesting way:  I have amblyopia, so most of my left is blind spot, except for some peripheral vision that I have to concentrate on to use.  When I was young, I was given an eye test that tested each eye individually by showing three columns of black letters on a white background. They did my left eye first and asked me to read the middle column.  I didn’t understand why they were messing with me, because I saw only two columns separated by a pristine, unbroken white expansethat perfectly matched the rest of teh background.  To my eye, there was no middle column.  Mild hilarity of the “who’s on first” variety followed.

  3. Cool idea but my eyes are so bad I can’t see anything when I’m that close to the screen.

  4. I wasn’t aware of the color one.

    I’ve wondered if the reason I see so many floaters is because I’m strongly nearsighted. More of them are in my range of vision? Or I’m just a freak. :-)

     And it’s nice to know that “blue sky sprites” are a thing, because I’ve been seeing those for years!

    (Evidence that good sense is not as easy to follow: I’m so nearsighted that I’m at risk for retinal detachment, and have been told that if I notice anything weird with my sight, I should see an ophthalmologist immediately. I probably should have asked about the sprites and the floaters years ago.)

  5. We worked on an science museum exhibit recently that used eye-tracking hardware. Pretty amazing to look at an image and then have revealed the various paths and fixations. Working on the project, I wondered if you could trick a person into thinking that they were looking at a high-res image by moving a small circular high-res ‘window’ fast enough to track with that central foveal region. To a bystander, it would be obvious that you were looking at a little circle moving over a blurred image. To you, the participant, I wonder if it would just look like an entire high-res canvas? (Actually, I recall that you can’t perceive image changes during eye movements – so you might only need to show the high-res circle at dwell locations.) I have no doubt this has been done many many times – has anyone seen it demo’d?

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