THE CHANNEL MODEL

The Channel Model represents how different vision cells, or channels, handle different aspects of seeing such as color, size, shape, contrast and motion. Each visual channel collects different bits of information for these varying aspects of seeing and individually transmits them to the brain for processing and assembling into a complete picture. To illustrate this process, consider the high and low contrast situations in the picture below. Our visual system uses different channels to gather information from these different situations.

A letter chart, like you would see in an eye doctor's office (left), is composed of high contrast elements only. A common street scene (right) is composed of elements of varying contrasts -- high, low and everything in between.

The Contrast Sensitivity Curve :: Everything we see is broken down into a range of spatial frequencies or channels. Channels are size selective. Our visual system uses these different channels to see in high and low-contrast situations. Consider Figure 1 below.

Figure 1 - The Channel Model

Each size selective channel filters a limited range of information about the object as seen in the different images of the little girl portrait in Figure 1. The large size channels that filter the general shape of the face do not have any information about the details of the face, which are filtered by the small size channels. Our visual perception is the superimposition of all the channels as seen in the lower right portrait.

Research shows that only the small size channels determine the ability to see the letters used on the *20/20 visual acuity test -- this limited range is illustrated by the light green bar in Figure 1. The channels that are used to see the letters on the 20/20 visual acuity test are completely different from the channels that help us see objects in our everyday life, such as a human figure in a room. Because these channels are independent of each other, we need to test the sensitivity of each channel separately to determine how well different size objects are seen. (In references, see: Ginsburg AP. Forensic aspects of visual perception.)

*20/20 letter chart testing and scoring depicts how well a person can identify black letters on white background at a certain distance away from the test chart. See Vision Testing for more information.

 

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