200lateral

Lateral Inhibition and the Hermann Grid

Look at the Hermann Grid figure below. Stare into one of the "intersections" where the lines meet. Notice that when you stare into one, the other intersections will appear darker.

Why does this happen?

It is important that we are able to perceive "borders" in the world around us. The border between the doorway and the wall. The border between at the edge of the road. And so on.

Our visual system helps our brain detect these borders by exaggerating them. A grey object on a black background will be made to look lighter while a grey object on a white background will be made to look darker.

Check out the following two figures.

The two shades of grey, one surrounded by a white background and one surrounded by a black background, are identical. The one surrounded by white looks darker than the one surrounded by black. You probably already knew this, that colors surrounded by dark look lighter and colors surrounded by light look darker.

But how does our visual system accomplish this? At the cell level, what exactly is taking place with our eyes and brain?

Lateral Inhibition

Any given ganglion cell in the retina of the eye may receive signals from thousands of photoreceptors (e.g., rods and cones). These photoreceptors constitute the ganglion cell's "receptor field."

When a retinal ganglion cell receives stimulation from its photoreceptors, it will inhibit the firing of neighboring retinal ganglion cells. (Remember inhibitory signals from the section on neurons?) This is where "lateral inhibition" gets it's name.

For instance, look at the figure above, with the grey surrounded by white. Some of the cells in your retina are receiving the grey light and some nearby ones are receiving the white light. The cells that are receiving the grey light are being inhibited by cells receiving the white light. The result of this is that the grey looks darker.

Back to the Hermann Grid.

Why do the dark spots appear in the intersections? Cells receiving the light coming from the intersections (X below) are surrounded by neighboring cells receiving light from the four white areas just off the intersections. So, cells receiving the light from the intersections are being inhibited by four other sets of cells. Cells that are receiving light coming from "the middle of the street" are being inhibited by only the two sets of cells receiving white light on either side of that area. So, those cells in the intersections are receiving twice as much inhibition and, therefore, look darker.

Receptive Field For Intersection That One Is NOT Looking Directly At

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The cell(s)s responding to the light coming from the middle are being inhibited (-) by neighboring cells that are receiving the light coming from the areas just off the middle. Likewise, the cell(s) responding to the light coming from the area off the middle are being inhibited (-) by neighboring cell(s)

This does not happen in the stripes that are not intersections because there is about half as much inhibition occurring (see below).

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Then why does this not happen for the intersection that one is staring directly at?

When you are looking directly into one of the intersections, the light from that intersection is falling into the fovea of your retina. In the fovea, the photoreceptors are more numerous and much denser/tightly packed in together than in the surrounding areas. Therefore, most receptors (in the fovea) receiving the light from the intersection are surrounded by cells that are also receiving the same light, and they are surrounded by cells that are receiving the light also. In other words, the center and surround are receiving the same amount of inhibition. The effect of this is that all of the inhibition signals cancel each other out.

Receptive Field For Intersection That One IS Looking Directly At

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