Optical illusion: the Troxler effect

Have a look at the video below, which demonstrates “the Troxler effect” or “Troxler fading”, a phenomenon that occurs when you try to stop the involuntary movements of your eye (“scanning”):

As we focus on a certain point in our perception field, that point becomes the main object of our visual system. When a blurry stimulus appears in a region of the visual field further from the point we are fixating, and we keep our eyes still, that stimulus will disappear even though it is still there. The phenomenon is known as Troxler’s fading. It occurs because even if our eyes move a little when we are fixating a point, away from that point, in the perception field, the movements aren’t large enough to observe other elements. The neurons remain focused on the main object and our visual system doesn’t involve new ones for the other elements.

Here’s an example from Wikipedia:

In this example, the spots in the “lilac chaser” illusion fade away after several seconds when the black cross is stared at long enough. This leaves a grey background and the cross. Some viewers may notice that the moving space has faded into a moving blue-green spot, possibly with a short trail following it. Furthermore, moving one’s eyes away from the image after a period of time may result in a brief, strong afterimage of a circle of green spots.

And here’s the most obvious picture to use to demonstrate the effect: a Cheshire cat!

INSTRUCTIONS. Keep very still and keep your gaze focused on the central black cross. Do not strain your eyes, but try not to let your gaze wander from the cross.

You can read more on the Troxler Effect here, including this:

The Troxler Effect is named after Swiss physician and philosopher Ignaz Paul Vital Troxler (1780-1866). In 1804, Troxler made the discovery that rigidly fixating one’s gaze on some element in the visual field can cause surrounding stationary images to seem to slowly disappear or fade. They are replaced with an experience, the nature of which is determined by the background that the object is on. This is known as filling-in.

The Troxler effect illustrates the importance of saccades, the involuntary movements of the eye which occur even while one’s gaze is apparently settled. If we could perfectly fixate on some point in our visual field by suppressing saccadic movement, a static scene would slowly fade from view after a few seconds due to the local neural adaptation of the rods, cones and ganglion cells in the retina. In brief, any constant light stimulus will cause an individual neuron to become desensitized to that stimulus, and hence reduce the strength of its signal to the brain.

When we attempt to fix our gaze on an object, the eye undergoes extremely rapid and relatively large-scale sudden movements called microsaccades, in contrast to saccadic drifts or small oscillations. Microsaccades cause the pattern of activity which forms the retinal image to shift across hundreds of photoreceptors at a time, providing a constant “refreshing” of the image (Martinez-Conde 2010).

 

14 Comments

  1. Mark R.
    Posted April 10, 2018 at 3:09 pm | Permalink

    Cool. It’s almost the opposite effect of staring and blurring the eyes at a fixed point to reveal a 3D print; they sort of slowly come into view just as these slowly fade away. However, I don’t know if 3D prints are an optical illusion or not. Those 3D pictures were popular in the ‘90’s. Now the fad has disappeared.

    • Mark Reaume
      Posted April 10, 2018 at 3:18 pm | Permalink

      “Those 3D pictures were popular in the ‘90’s. Now the fad has disappeared.”

      Maybe that is just an illusion.

      • Mark R.
        Posted April 10, 2018 at 9:32 pm | Permalink

        HA!

  2. Posted April 10, 2018 at 3:30 pm | Permalink

    This effect would seem to support the claim Robert Jourdain, a neuroscientist and musician, makes in “Music, The Brain, and Ecstasy” that the brain is ultimately only interested in change, and that this is why monotonous singing or playing is boring and considered bad technique and that judicious vibrato, messa di voce, and other dynamic manipulations are considered good technique.

  3. Mark Sturtevant
    Posted April 10, 2018 at 4:11 pm | Permalink

    In the purple chaser illusion, is there a soft green spot or is that an illusion?

    • Posted April 10, 2018 at 4:16 pm | Permalink

      An illusion. You’re seeing the complementary color after-image of the purple dot in the neutral background. Try looking at it with only your peripheral vision. You should be able to see that there is nothing there but gray when the dot disappears.

    • nicky
      Posted April 10, 2018 at 9:17 pm | Permalink

      I found the Lilac Chaser, designed by Jeremy Hinton always one of the most impressive illusions of the lot. It is not limited to lilac. In Michael Bach’s version you can change the colours, sizes, no of gaps, etc. You always get a dot in the complementary colour running around. However for one reason or other it works easiest with the ‘lilac’ (magenta).
      I think it is one of the better understood visual illusions, based on the Troxler fading, negative retinal after-image and the phi phenomenon (a series of still images in quick succession appear as moving).

      http://www.michaelbach.de/ot/col-lilacChaser/index.html

  4. Posted April 10, 2018 at 4:14 pm | Permalink

    Also, it is very hard to suppress saccadic movement. I suppose that’s a feature that’s been selected for.

    • infiniteimprobabilit
      Posted April 10, 2018 at 10:15 pm | Permalink

      Agreed. In trying to ‘focus on the dot’ I was quite conscious of my eyes trying to wander.

      I suppose this is linked with the fact that our neurons transmit signals by ‘firing’, not by sending a steady signal, and the fact that we’re sensitive to movement – we can ‘spot the nightjar’ much more easily if it moves. And links with your previous comment about steady musical notes.

      cr

  5. Posted April 10, 2018 at 6:13 pm | Permalink

    Wow! That moving green circle illusion is impressive.

  6. infiniteimprobabilit
    Posted April 10, 2018 at 10:07 pm | Permalink

    The second pic was very strong – I could clearly ‘see’ the green dot orbiting the centre.

    The first one was more gradual, the colours gradually faded a little, I found that ‘zooming’ (pulling my head back from the screen while still watching the dot) restored the colors temporarily.

    cr

  7. Posted April 10, 2018 at 11:11 pm | Permalink

    A lot of these optical illusions have no effect on me. (Nor those ‘Magic Pictures’, either.)

    I happen to have “off-the-chart” peripheral vision (as tested by my optometrist and the Air Force recruiters back hen I was a teenager.) Does anyone who knows stuff know whether that is relevant?

  8. ThyroidPlanet
    Posted May 1, 2018 at 5:33 am | Permalink

    “Some viewers may notice that the moving space has faded into a moving blue-green spot”

    This sounds like it has something to do with color blindness – the green spot – it appears strong to me. I think there’s “red/green” color blindness, so, they’d miss it?

    What about any other readers who haven’t noticed it? Can we hear about color-blindness?

    … plowing through some old email early this AM, you might have guessed….

    • ThyroidPlanet
      Posted May 1, 2018 at 5:33 am | Permalink

      OH and they wrote “blue-green”, but I see only green.


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