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I. The Chevreul Illusion

Updated: Nov 17, 2019

This is the first of a series of posts on Visual Processing

Our visual system processes input before presenting us with perception. Our sensory system can be thought of as a shell that alters input in many ways in order to deliver sensible information that is useful and actionable. Sensory reality is only what passes through that barrier.

Processing occurs on many levels. One of the first, most basic, and easily understood is lateral inhibition at the level of the inner retina. Lateral inhibition is a model for retinal processing that explains the enhancement of line, edge, and contour.

Lateral inhibition is evident in the Chevreul Illusion above. The vertical bands are created with a stepped luminance gradient. This as shown underneath the bands. Note that the left side of each band is noticeably brighter than the right side of the same band. If you look carefully you will notice a couple of other effects that won't be discussed here.

The effects you are noticing can largely be explained by lateral inhibition at the level of the inner retina. A photoreceptor, when it fires, inhibits the firing of the photoreceptors around it. Similarly, when a photoreceptor is surrounded by photoreceptors that are firing, it's firing will be inhibited.

Points surrounded by generally higher levels of lumance (such as the right side of each band) will be relatively inhibited, and will appear darker than a light meter would predict. Points surrounded by generally lower levels of luminance (such as the left side of each band) will be disinhibited and appear brighter than a light meter would predict. This is why the left side of each band is brighter than the right side of each band.

Note that this causes a pronounced increase in contrast at the border between each band. Lateral inhibition makes lines more obvious, increases contrast at the border between two surfaces, and enhances three-dimensional contour.

In the drawing below, the brilliance of the upper lip border is enhanced by the deep black just below it. Deep black, and brilliant white, are created with a self reinforcing juxtaposition.

Lateral inhibition also increases my sensitivity to form in dark places. This is one reason why so much detail can be perceived in the shadow spaces in the drawing above.

Lateral inhibition reminds me that whatever is next to what I am drawing is just as important as what I am drawing. This is why, if I want some detail to become brighter in a dark region, I can do so by darkening something next to it even further. The detail actually is perceived brighter without my ever touching it with a white pencil or an eraser.

Every time I make an object darker- even a little bit- the surroundings get brighter. Every time the surroundings get darker, the object gets brighter. In this way, the retinal response to a point of light might be compared to the ripples created in a still pond by a drop of water.

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