| ISLE | Chapter 3
| Mach Bands
The center surround receptive fields that we find in our eye have both excitatory and inhibitory regions. Review how they operate in ISLE 3.11(a) and ISLE 3.11(b). As a result of having both excitatory and inhibitory areas, these receptive fields exhibit lateral inhibition. See the text for more information about lateral inhibition.
The goal of lateral inhibition is to facilitate edge detection. Edge detection is the process of
distinguishing where one object ends and the next begins. That is, it works to make edges
as clear as possible. Edge detection is important—think about trying to see a camouflaged
predator, perhaps a lioness—in the tall dry grass. The lioness's fur and the grass are roughly
the same color, important for the lioness as she slowly stalks her prey (you, in this case). However,
the fur and the grass are not identical, and you want your visual system to pick up where the grass
ends and where the lioness begins. This involves edge detection and Gestalt figure ground
discrimination. It is for this reason that edge detection evolved.
Lateral inhibition explains a famous visual illusion known as Mach bands, named after their discoverer, Physicist Ernst Mach (1838–1916). Lateral inhibition accentuates the edges of the stimulus. Mach noticed that when two bars, one dark and one bright, are next to each other, you see little bands of extra dark at the edge of the dark band and extra light at the edge of the light bar. These bands do not exist but are an illusion caused by lateral inhibition via our center-surround receptive fields. In this illustration, you will be able to manipulate some images to make Mach bands appear and disappear. Examine the figure to the right.
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 illustration. The settings include the following:
Stimulus Type: change stimulus to a series of bars you can add and remove or a stimulus with
a gradual change in luminance. Each stimulus has different ways you can change it.
Number of Bars: (BAR STIMULUS ONLY) change the number of
bars on the screen starting from the darkest bar on the left.
Center: (GRADIENT STIMULUS ONLY) change the intensity of the region in the
middle of the stimulus area from bright to dark.
Indicate Mach Bands: draw arrows to highlight where
the Mach bands may be found.
Pressing this button restores the settings to their default values.