Center-surround receptive fields have both excitatory and inhibitory areas. This observation has an interesting consequence for how our visual system operates. If you present light that covers the entire center-surround receptive field of the cell, the response will be small as the excitation and the inhibition will cancel each other out. However, if you present a pattern with edges (contrast of light or dark), the cell may respond maximally. That is, these cells respond to luminance contrast or differences in light intensity across the field. To make sense of this, think of the following—a uniform field in the visual world is usually not interesting or important—think of a blank wall or a uniformly blue sky. Contrast, however, is important, whether it is the change of lightness where the wall meets the door or the roiling patterns of clouds of an impending thunderstorm. Or think about our police officer on patrol at night. She wants to be able to make out the dim form of a prowler as d distinguished from the background night. Thus, our visual system should be looking for these contrasts, as they indicate interesting features of the visual world. Indeed, it appears that edge detection, such as what is seen in center-surround organization, starts very early in the visual system.
In this illustration, you can examine the contrast detection of center-surround receptive fields in two different ways. You can control the size of a dot stimulus from very small to filling up the center to filling up the entire receptive field. Watch how the response changes. You can also drag an edge across the receptive field and watch how the firing rate changes and the position of the edge moves across the receptive field.
To see the illustration in full screen, which is recommended, press the Full Screen button, which appears at the top of the page.
Below is a list of the ways that you can alter the model. The settings include the following:
Screen Area: the black area on the screen to the left is a screen that the
eye of your animal is seeing. The receptive field of the cell will be shown on this area: green =
excitatory and red = inhibitory.
Stimulus Type: select the type of stimulus to use, either a dot that will be centered, or an edge that will start in the middle of the screen. The dot cannot be moved, but you can move the edge by clicking on the Screen Area.
Dot Size (if stimulus is Dot): the dot starts out with no size. Use this slider to change the size of the dot.
Edge Direction (if stimulus is Edge): you can change the angle the edge approaches from with this slider. See if changing the direction changes the pattern of responses.
Receptive Field Type: change the cell between and on-center and off-center receptive field.
Pressing this button restores the settings to their default values. It also gets you a new cell which might have a different receptive field.