Part 2 book “An introduction to the visual system” has contents: Colour constancy, object perception and recognition, face recognition and interpretation, motion perception, brain and space, what is perception. | 7 Colour constancy The colour constancy problem One of the most important functions of the visual system is to be able to recognise an object under a variety of different viewing conditions. For this to be achieved, the stimulus features that make up that object must appear constant under these conditions. If stimulus parameters do not form a reliable ‘label’ for an object under different conditions, they are considerably devalued in their use to the visual system. For example, if we perceive a square shape on a video screen and the area it covers increases or decreases, we experience a sense of movement. The square seems to get closer or further away. The visual system assumes that the size of the square will not change, so that changes in its apparent size will signal changes in its relative distance from us. This is called object constancy. This is a sensible assumption, as under normal conditions, objects seldom change in size. Another example is lightness constancy. Over the course of a normal day, light levels change significantly, but the apparent lightness of an object will change very little. The visual system scales its measure of lightness to the rest of the environment, so that the apparent lightness of an object will appear constant relative to its surroundings. A similar problem exists with the perception of colour. Over the space of a day, the spectral content of daylight changes significantly (Figure ). This means that the spectral content of light reflected from an object changes too. One might expect that objects and surfaces acquire their colour due to the dominant wavelength of the light reflected from them, thus a red object looks red because it reflects more long-wave (red) light. However, surfaces and objects retain their colour in spite of wide-ranging changes in the wavelength and energy composition of the light reflected from them. This is called colour constancy, and is not only displayed by humans and primates, but by a wide range .
