Most of the biologically important information about the environment provided by our senses remains essentially constant from one instant to the next. Animal nervous tissue is designed so that its response diminishes, even stops, with repeated stimulation, this process is called adaptation. Adaptation occurs not only in the retinal tissue, but also at higher levels in the brain.
Most sensory tissue (retina of the eye, cochlea of the ear, pressure sensitive nerves of the skin) and even portions of the brain, is organized so that stimulation of any given location produces inhibition of the surrounding nerve fibers, the effect, called lateral inhibition, is to (mathematically) differentiate the signal being processed (or high pass filter).
In the case of visual information, such (spatial) differentiation causes gradual changes in the contrast between an object and its background to be enhanced, i.e., to become more visible. In human perception, this contrast enhancement produces what is known as Mach Bands: between two regions of different intensity a thin bright band appears at the lighter side and a thin dark band appears on the darker size. These bands are not physically present are "overshoot" and "undershoot" caused by our neural circuits in processing a step discontinuity in illumination.
|Figure Lateral Inhibition: Intensity step wedge used to demonstrate lateral inhibition and the Mach Bands (adapted from Fischler and Firschein, 1988)|
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Last modified on April 05, 1999, G. Scott Owen, firstname.lastname@example.org