This chapter provides an overview of the projections from the retina to the brain in vertebrates and reviews the key terms used in describing the pathway. The major components of the pathway and their functions are examined in greater detail in subsequent chapters.

The Visual Fields

The central projections of the two eyes map the visible world onto the brain. To understand this process, it is important to know how the visual field of each eye is described and how the projections from the two eyes are combined in the central pathways. The retina of each eye is conventionally divided into nasal and temporal parts, on the basis of proximity to the nose or temporal bone, respectively. Similarly, the visual field of each eye is divided into nasal and temporal parts, and because of the inversion of the retinal image by the eye's optics, the nasal visual field is imaged on the temporal retina, and the temporal field on the nasal retina. Figure 4.1 schematizes the projections of the visual fields in an animal whose eyes are located at the sides of its head. In such lateral-eyed animals, the axons from one retina generally cross completely in the optic chiasm, so that the input from that eye is directed at the contralateral hemisphere of the brain.

Figure 4.2 illustrates diagrammatically the monocular visual fields as they would appear to a frontal-eyed human observer, left eye (oculus sinister, O.S.) on the left, right eye (oculus dexter, O.D.) on the right.