Why contrast is crucial to our visuality

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The chapter about the basic properties of our visual system has shown that it constructs and organizes our world and the objects in it based on the edges between things. The objects in a scene are not fully detected but are constructed one at a time based on their perceived borders. This process is elaborate and its exact sequence is still controversial among scientists.

Without the registration of the object boundaries, no visual perception could arise. The following experiment practically simulates the truth of this statement. Imagine, for example, a red square in the middle of which there is a smaller green square. If you artificially stabilize the border between the two areas on your retina, you will first lose the perception of the green square and only the red area of the background will remain. After about a second without any movement relative to the retina, this impression also dissolves and you see nothing. This is the case because the photoreceptors only report potential differences to us, not absolute potential levels, which serves to increase efficiency. To ensure that we do not lose perception when our gaze lingers on one point for a long time, the eyes perform unconscious movements that are random in direction several times per second, so-called microsaccades.

The answer to why our visual system structures and distinguishes objects based on the borders between areas of different color and brightness is simple: economy, effectiveness, and low energy consumption. It is highly efficient for the visual system to process objects based on light pattern interruptions. In this way, the visual system only needs to encode the parts of the image where something changes, rather than the entire image. Edges and borders are the only important information that the apparatus in our heads needs to construct the shapes of things in our environment. It is unnecessary to define brightness and color at every single point of, for example, a solid red object. Instead, it is sufficient to define brightness and color at the exact point where something changes. And this is the case at an edge or boundary. This significantly reduces the amount of information required for transmission and processing.

We can see exactly how much by comparing two graphics in .tif or .jpeg format. Let’s assume that the image shows a red circle in the middle of a green area in 10×15 cm format. In .tif there are 1715 KB. In .jpeg only 13 KB – 132x less. The reduction is due to the fact that .jpeg, just like the visual system, only defines those pixels where something changes. The file only contains the border position and color on the inside or outside. The image processing program automatically fills in the pixels in between.

The nervous system as a whole greatly benefits from this reduction in information because nerve cells require energy to fire, and the body must use this raw material as sparingly as possible. – Consider that the brain has a particularly high demand for oxygen and energy. Despite being only 2% of the body’s mass, it uses 20% of oxygen and 25% of glucose. So fewer active nerve cells are better for the body.

The presence of contrast is crucial to our way of seeing.

In order to capture as many edges as accurately as possible, the eye and brain must rasterize the field of view in as much detail as possible and then isolate the borders. This is quite an ambitious task, and our visual system accomplishes it in several stages. For precise scanning, it uses a large number of photoreceptors. Their distance from each other, along with a few other factors, determines the resolving power of the visual apparatus. The visual system, like a computer, processes the nerve impulses from the exposure of the photoreceptors to reliably isolate the object edges in the image. The visual system utilizes a specific type of ganglion cell, physiologically split into the center and periphery, for this task. They interconnect in a way that mutually inhibits each other. We refer to this cell structure as the center/surround organization, which filters out irregularities such as object boundaries. The greater the contrast at an edge, the greater the output potential of such a center/surround cell. Klick here to learn about the function and place of these cells in the hierarchy of the visual system.

Given this objective, it is obviously important for us to be able to distinguish the contrast at an object edge over as wide a range of brightness as possible. This chapter explores the size of the visual system’s contrast capacity and how it bridges the dynamic range.

Next The dynamic range of the visual system

Main Contrast

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