ABSTRACT
Perception is the basis of all action. Without perceptual systems to provide us with knowledge of the outside world we are impotent. The job of perceptual systems is to provide us with representations of what is happening in the outside world, representations based on information gathered from receptors based in different parts of the body that are tuned to specific classes of physical events. This process of building up internal representations turns out to be considerably more complicated than was initially thought. Our eyes are not cameras, our ears are not tape-recorders, faithfully but passively recording what is in the outside world. The physical events that activate our sense organs are already imperfect versions of the properties of objects that we wish to know about. For example, the perceived colours of objects are only loosely related to the wavelengths of light that they reflect into our eyes. Two objects may reflect the same wavelengths into our eyes yet be seen as having different colours. The same object may reflect different wavelengths at different times yet be seen as having the same colour. This is the stuff of colour constancy. Our sense organs are not passive relays, but encode the constantly fluctuating patterns of stimulation impinging on the receptors into fluctuating patterns of neural activity. These patterns are related to the patterns of input but already selective recoding has begun, since some aspects of the input will activate the nervous system more than others. For example, edges tend to activate the visual system more than areas of uniform brightness. Changes of pressure across the fingertips produce much more intense input from touch receptors than steady pressure. You can verify that for yourself by running your fingertips over a slightly rough surface and then stopping. The roughness that you feel while moving the fingers suddenly disappears. The job of perceptual systems is to take these fluctuating patterns of activity occurring at the receptors and interpret them in terms of what is going on in the outside world.
