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Lateral Inhibition

Lateral inhibition refers to the inhibition that neighboring neurons in brain pathways have on each other. For example, in the visual system, neighboring pathways from the receptors to the optic nerve, which carries information to the visual areas of the brain, show lateral inhibition. This means that neighboring visual neurons respond LESS if they are activated at the same time than if one is activated alone. So the fewer neighboring neurons stimulated, the more strongly a neuron responds.

You might expect that such inhibition would decrease the visual system's ability to represent information. In fact, this process greatly increases the visual system's ability to respond to edges of a surface. This happens because neurons responding to the edge of a stimulus respond more strongly than do neurons responding to the middle. The "edge" neurons receive inhibition only from neighbors on one side -- the side away from the edge. Neurons stimulated from the middle of a surface get inhibition from all sides.

The pictures you see taken from space craft are computer enhanced by a process just like lateral inhibition. Computer transformation of the raw image makes neighboring points of the computerized image inhibit each other. This makes the very faint edges in the original image much sharper. This is why lateral inhibition is important: It makes edges stand out

The Hering grid illusion, simultaneous contrast, and Mach bands, described below, are three examples of the visible effects of lateral inhibition (usually you are quite unaware of it). The diagram below shows the Hering Grid.

Look steadily at the center of the left grid. Most people notice faint grey clouds at the intersections of the light bars (except the one you look straight at). The bottom panel of the drawing shows the explanation for this effect. The faint clouds appear because intersections have lateral inhibitions from white areas on 4 sides, whereas the middle of the bars have lateral inhibition from white areas on only 2 sides. So the intersections receive (roughly) twice the lateral inhibition that the bars do. This makes the neural signal triggered by the intersection weaker than the signal triggered by a section between intersections. This difference produces the perception of faint clouds at the intersection.

Simultaneous contrast is illustrated in the two rectangles in the figure below. Most people see the center rectangle on the left as darker than the one on the right, even though physically they are identical.

The center rectangle on the left gets little lateral inhibition from its dark surround. The center rectangle at the right gets considerable lateral inhibition from its light surround. Therefore, the light from the center rectangle on the left sends a stronger neural signal to the brain than does the same light from the center of the right rectangle, so the center rectangle on the right appears brighter.

Lateral inhibition is also responsible for Mach bands. The bright halo you can see around a blurred shadow is a Mach band. You can create a blurred shadow on a bright sunny day (winter days are best) by holding your hand about 5 or 6 feet from a sunlit area of the floor. Many people notice a bright halo around the edge of the blurred shadow. You can enhance the halo by moving your hand makes back and forth at about 6 inches per second. The bright halo is not present in the physical stimulus. It is created in the retina by lateral inhibition. The bright halo has the effect of sharpening the edge of the blurred shadow. The same process occurs with a sharp edge, bit it is not easily noticeable.

You can find an illustration and further explanation of Mach bands on the Web by clicking HERE.