In order to understand why and how the eye moves, we first need to characterise the image or signal registered by the eye, and how it is processed and organised in the brain. We will see that what is registered is not the same as what is perceived, and how eye movements relate to both these aspects. This chapter intends to give the reader a good general understanding of vision and the eye. It is not intended to give an exhaustive account of the highly complex structures and functions of the visual system; there are many excellent publications already available for that purpose. Rather, the intention is to outline the basics for a multidisciplinary audience who are interested and want to work with eye tracking and gaze-based communication.
As with any area of human anatomy, there is a regrettable amount of specialist terminology which can be daunting to the uninitiated. The reader is advised to bear in mind that long, complicated terminology does not necessarily translate to long, complicated concepts. This chapter will explain the concepts behind human vision sufficiently for the reader to understand later sections in the book on human perception and attention, and their relationship to (and potential measurement with) eye movements. We begin with a description of the path of light energy from the environment through the structures of the eye and on to the brain, as an introduction to the physiology of vision. We will then describe the image registered by the eye, and the types of movements the eye makes in order to perceive the environment as a cogent whole.