Knowledge Effects in Colour Perception
Bottom-Up and Top-down Processes
When considering how the visual system works it is
customary to begin with the eye's response to electromagnetic energy and
following the ensuing neural events up to and within the brain. Such
processes are after referred to as "data driven" or
"bottom-up". And while they are a vital part of visual
functioning, bottom-up processes are only part of the story of visual
perception. Human observers are not only data-driven receivers of visual
information, but active seekers and shapers of that information. In
short, top-down cognitive processes also play an important role in vision,
including the perception of colour.
Knowledge is Created by Our Visual System
The transactional approach to vision maintains that the world is a product of perception, not the cause of it. It holds that the basic function of the perceptual process is to use the available information to resolve uncertainty or ambiguity. In this view, we don’t “see” the world, it is created by our visual system. In this approach, colour is not a physical property of wavelengths or objects; it is a manifestation of the visual system.
The Role of Light and Knowledge
The appearance of objects remain approximate despite widely varying lighting conditions. For example, an orange will continue to look orange in colour when viewed at different times during the day, indoor vs. outdoors, and under different lighting sources. Such colour constancy serves as an adaptive “biological signaling mechanism” to help us identify objects despite superficial changes in their appearance. Perceived constancy in the colour of objects is a function of both the physics of light and our knowledge about the object as well as the world around it.
The hue of an object or surface is determined in part by the dominant wavelengths of the light it reflects. If an objects spectral characteristics don't change greatly, it will continue to reflect many of the same wavelengths. So while a blue shirt under fluorescent lighting might look slightly different under incandescent lighting, it is still likely to look blue. The constancy of the shirt's appearance, however, is also a function of such top-down or knowledge factors as experience, complex judgment, and categorization. For example, our colour memory for an object based on past experience, influences how we see it. This has been shown by Delk and Fillenbaum, (1994), who had observers adjust a colour field to match the colour of orange-red cutout shapes of common objects. Objects that are typically red, such as the apple and the heart, were matched to deeper reds than were objects not normally red like the bell or mushroom. Presumably, this outcome reflects the influence of our prior experience - we have learned that apples and hearts are red in colour. Such colour memory can compensate to some degree for changes in lighting conditions in which objects are seen.