Light is the stimulus for vision and the eye is the organ which responds to light energy. This chapter will examine two contemporary developments from the heritage of perception that are actively being pursued. One concerns advances in our understanding of the nature of the stimulus-visual optics. The other is about how the visual system responds to light-visual neurophysiology. The image-forming properties of the eye are quite well understood, and most aberrations of the eye can now be corrected optically. The performance of the human eye is remarkable considering that its optical parts are so simple. One of the ways of measuring the performance of the visual system is to treat it as a physicist would a lens, by determining how faithfully it can transmit patterns of light incident upon it. One outcome of this approach has been the suggestion that the visual system is most sensitive to sharply defined contours and also to spatially periodic patterns of parallel lines (referred to as gratings).
Knowledge about the neuroanatomy and neurophysiology of vision has advanced because it has proved possible to examine the structures in the visual system in greater detail. Techniques have been devised to examine structure and function at and below the cellular level. Thus, there is now better understanding of how light is absorbed by pigments in the receptors, how these chemical changes modify the electrical potentials of the receptor cells, how a nerve impulse is eventually generated, and how the patterning of light is processed. The methods of recording the electrical activity of individual nerve cells have proved particularly productive, and have demonstrated that the cells do not respond solely to the presence or absence of light, but also to its patterning. Single cells in the visual cortex of animals closely related to humans (like monkeys) respond most strongly to oriented edges. Accordingly, similar conclusions are being reached from quite different lines of enquiry: The results from both visual psychophysics and neurophysiology suggest that the early stages of vision involve the extraction of simple features from the light stimulating the eye. Indeed, it has been said that the two most significant ideas in visual science are that the activity of single visual nerve cells can influence perception and that their characteristics can be inferred from perception (Lennie, 1998). It is not surprising to find that visual