Lateral Inhibition
A critical role for vision is to be able to detect boundaries between objects so that we can distinguish objects from other objects and also from the background. Not surprisingly our visual system contains a number of properties that make edge detection easier. One such mechanism is called lateral inhibition.
In the first demonstration in this lab activity, you will be shown two rectangles, one darker and one a lighter shade of gray. To the right of these two rectangles are empty boxes with green dashed lines. Your job is to drag the rectangles so that they fit inside the green dashed boxes and create a border.
After you drag the rectangles into the empty boxes, look at the border between the black and gray rectangles. The border area on the black rectangle side appears slightly darker than the rest of the rectangle and the gray rectangle side appears slightly lighter than the rest. The reality is that the rectangles are uniform in shading, but your visual system enhances boundaries through lateral inhibition. It is through lateral inhibition that those darker and lighter bands are created.
How Does Lateral Inhibition Work?
The second demonstration shows that neurons are cells arranged next to each other such that each cell inhibits the activation of adjacent cells. Inhibition is related to strength of activation. More activation means more inhibition. A lighter shade of gray will activate a cell more strongly such that a cell responding to light gray will inhibit a neighboring cell activated by dark black more so than a cell on the other side activated by light gray. The result of inhibition is a decrease in the net activation of the cell. The result of lateral inhibition is that cells with little activation (black cells) give less inhibition than cells with more activation (gray cells). A black cell receives the most inhibition since next to it is a gray cell and a black cell (compared to two black cells). A gray cell on the border is the most active since it receives the least inhibition because it is next to a black cell and a gray cell (instead of two gray cells). Lateral inhibition is a highly efficient mechanism for detecting edges, or contrast, between objects that reflect light differently.
Example of Lateral Inhibition at Work
The third demonstration in this lab activity is an example of lateral inhibition known as a Hering Grid. This is a 3-by-3 grid of black rectangles spaced apart on a white background. Just looking at the grid demonstrates a powerful illusion caused by lateral inhibition. There appear to be faint smudges at the intersections where areas flanked on four sides by white space appear darker than areas flanked by black.
Lateral inhibition causes the apparent smudges in the white spaces of the Hering Grid. White areas produce lateral inhibition to all adjacent areas because the areas surrounded on four sides by white will receive more inhibition than other white areas. The end result of inhibition is that it reduces activation in these areas and makes them appear darker.