Background

In The Importance of Contrast in Perception several illusions that result from how our visual system handles contrast in images were illustrated. These illusions are well understood to be there result of the way that the center-surround receptive fields of the ganglion cells and lateral geniculate nucleus work. To review how these receptive fields need and respond to contrast go to Center-Surround Receptive Fields as Contrast Detectors. To help you see more clearly how these receptive fields center in on contrast, you can see how a whole series of receptive fields respond to an image. Each receptive field will receive at a different part of the image so you can see how cells respond across a scene. You can try a version of each of the illusions you used in the previous activity.

In addition, there are a few photographs you can try to see how these cells respond to more realistic images. The photographs are black and white as the model does not handle color. Finally, you can upload your own images to see how these cells respond to them. As a recommendation, keep the image relatively simple as it will make it easier to understand the output. A black and white image is not necessary but the model will treat it as if it were black and white.

Use this activity to see how the nature of these center-surround receptive fields help explain the importance of contrast in our pereption.

Instructions

Full Screen Mode

To see the illustration in full screen, which is recommended, press the Full Screen button, which appears at the top of the page.

Illustration Tab

On the Illustration tab, you can stimulate a region of the retina and see the effect on the single cell you are recording from.

Settings

Below is a list of the ways that you can alter the model. The settings include the following:

Choose a Stimulus: select from a preset stimuli. Choices include the contrast illusions mentioned in the last activity, but also a small dot, an edge, a moving bar and a few images.
Direction of Stimulus: make the stimulus brighter or darker than the surround. Does not work on phhotographs.
Type of Center-Surround Receptive Field: select to have the receptive fields be either on-center or off-center.
Receptive Field Size: alter the size of the receptive fields to see how different size receptive fields can pick up different size features of the stimulus..
Cell Level: move the cells up and down the image to see how the cells to respond to different parts of the image.
Use Your Own Image: open an image of our own to interact with.

Reset

Pressing this button restores the settings to their default values. It also gets you a new cell which might have a different receptive field.