Monday, September 12, 2016

Hermann grid

Consider two regions of your retina. One region views an intersection of a white horizontal and vertical band and the other region views a white band between two intersections (the region going away from the intersection). Although, the two regions themselves receive the same amount of light, the situation in their neighboring regions is different. At the intersection, there is light coming in from all four sides, but in the white band that lies between the two intersections it is surrounded by two dark sides.

This leads to a physiological mechanism called lateral inhibition, which has the effect of causing a bright surround to an area appear darker and, conversely, a dark surround will make an area appear lighter.

 The Physiological Explanation

     Your retina is partially composed of many small nerves, which function as receptors of light. These receptors are arranged in rows on the inside of your retina. A number of scientists have shown that it is possible to illuminate and record from a single receptor (A) without illuminating its neighboring receptors. It was discovered that if you illuminate a single receptor (A) you will get a large response; however, when you add illumination to A's neighbors, the response in A decreases. In other words, illumination of receptors "inhibits" of firing of neighboring receptors. This effect is called lateral inhibition because it is transmitted laterally, across the retina, in a structure called the lateral plexus.

In the case of the Hermann grid, there is light coming from the four sides of the intersection, but from only two sides of a band going away from the intersection. The region viewing the intersection is more inhibited than the region of the band going away. Thus the intersection appears darker than the other section. You see dark spots at the intersections of the white bands, but not at the points away from the intersections.

The effect is greater in your peripheral vision, where lateral inhibition acts over greater distances.

Lateral inhibition also explains the illusion of simultaneous contrast seen above. The two center squares reflect the same amount of light into your eyes but, because of the simultaneous contrast effect, they look different. You can prove this to yourself by covering up the areas surrounding the two center squares.

Many magicians use the principle of simultaneous contrast to conceal parts of their magical apparatus. For example, perhaps they want to conceal the supports of a floating body. The magician would make the surrounding parts brightly illuminated, shiny metallic objects, white cloth, etc. The parts the magician wishes to conceal are black, in front of a black background, usually drapery, and appear even darker to the viewer, dazzled by the rest of the display. The eye cannot make out the details in the darker parts.

Inspired by:

1 comment:

  1. An optical illusion image has gone viral as people try to work out why they can't see all 12 dots at the same time.

    Japanese psychology professor Akiyoshi Kitaoka posted the image on Facebook on Sunday morning and within 24 hours it had been shared 7,000 times.

    Game developer Will Kerslake tweeted the image on Sunday night with the caption: "There are twelve black dots at the intersections in this image. Your brain won't let you see them all at once."

    By Monday morning he had received more than 27,000 reactions to the tweet.

    Called Ninio's Extinction Illusion after the French scientist Jacques Ninio, the image shows a grid of horizontal, vertical and diagonal lines with 12 black dots at various intersections.

    Most people are unable to see all 12 at the same time.

    So why does this happen?

    It is when the eye sees for instance a particularly arranged pattern that the brain may not be able to process accurately.

    Ninio explains: "When the white disks in a scintillating grid are reduced in size, and outlined in black, they tend to disappear. One sees only a few of them at a time, in clusters which move erratically on the page.

    "Where they are not seen, the grey alleys seem to be continuous, generating grey crossings that are not actually present.

    "Some black sparkling can be seen at those crossings where no disk is seen. The illusion also works in reverse contrast."

    Twitter user Dan commented: "So its actually your retina that won't let you see them all at once."

    The optical illusion has been confusing the internet over the weekend.

    "I double-checked on different formats. Couldn't catch 'em all," tweeted one person and another said: "Gives you a headache after a while."