depends
not only on its own luminance but also on that of the surrounding stimulation.
The same gray square looks whiter against a dark background and blacker when
placed in a bright surround.One of the simplest instance of relational (or context) effects in perception
is that of brightness contrast. Thus, the apparent brightness of a stimulus depends
not only on its own luminance but also on that of the surrounding stimulation.
The same gray square looks whiter against a dark background and blacker when
placed in a bright surround.
Analogous context effects are evident in many commonplace experiences. A man of average height seems to be a runt when he is on a basketball court with much taller players; yet the same man looms like a giant when refereeing a game played by little boys. It is known that a typical winter's day seems delightfully balmy when temperatures rise after a week of subfreezing weather.
To the Gestaltist, contrast effects dramatize the relational nature of perception. They also play a significant role in a more recently developed adaptation-level theory, which also provides a general perceptual model. At the core of the model is the notion that the manner in which a stimulus is perceived depends not only on its own physical characteristics but also on those of surrounding stimuli and of stimuli previously experienced by the observer. In other words, the perceiver is said to be perceptually adapted to past sensory stimuli; his adaptation level forms a kind of zero point against which any new stimulus is perceived. An example is provided by the almost overwhelming silence one experiences when the sound of an air conditioner (to which he has adapted) suddenly ceases.
Note: Concerning the Hermann grid, its history and follow-up, see see Bernd Lingelbach and Walter Ehrenstein's recent review (in German) in: www.leinroden.de/304herfold.htm
