The present study grew out of an investigation into the projection of the visual fields on the lateral geniculate nucleus (LGN) in the cat. The new methods we have developed for studying this projection using single-unit recording and the precision we have found in the projection itself directed our attention to many of the basic problems inherent in the idea of topographical localization in the visual system. The present paper is concerned with an examination of these problems particularly as they pertain to the eye. The nature of the projection of the visual fields on the LGN will be described in the following paper (Bishop, Kozak, Levick & Vakkur, 1962).In order to describe a direction in the visual field a suitable system of co-ordinates is required, the direction being defined in terms of angles from a reference axis and plane. Under experimental conditions the visual field will consist of a tangent screen or perimeter. In addition, the reference axis and plane of the visual field co-ordinate system must be defined in relation to the projection of retinal landmarks into the visual field. Unless this is the case the nature of the projection of the visual fields on to the brain centres will vary with the position of the eyes. Thus the orientation of the eyeballs should be known and for this purpose an axis and plane of reference for the eye must be defined in relation to appropriate retinal landmarks.Even before the development of vision the direction of gravity provided the vertical co-ordinate as the basic reference for the orientation of the organism in its environment. The development of vision, particularly binocular vision, has added a second fundamental reference, namely the horizontal co-ordinate determined visually from the horizon. The direction of gravity and the plane of the horizon are the axis and plane of reference used by the animal in its interpretation of the visual world. If our system

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Henschen (1897, 1898) was the first to provide direct evidence in favour of a retinotopic projection in the lateral geniculate nucleus (LGN). He described a woman suffering from blindness of the lower left quadrants of both visual fields who was subsequently found to have a lesion limited to the dorsal half of the right LGN. Actually these early observations are not in accord with the later experimental studies on primates by Brouwer and subsequent workers (see below). Further early human clinico-pathological observations were made by Wilbrand & Saenger (1904), Ronne (1913Ronne ( , 1914 and Winkler (1912), but the extreme rarity of sufficiently restricted lesions, particularly in the nucleus itself, made it unlikely that the details of the retinotopic projection in the LGN would be unravelled in this way.By enucleating the eye of a cat and studying the distribution of the secondary degeneration in each LGN, Minkowski (1913) provided the first direct experimental evidence that the crossed and uncrossed optic fibres were distributed differently in the nucleus (cf. Minkowski, 1920) and that the binocular part of a visual field projected only to the medial part. The first detailed study of the retinotopic projection in the LGN was, however, carried out by Brouwer and his colleagues (Brouwer, Zeeman & Houwer, 1923). By making small retinal lesions and subsequently studying the degenerating fibres in Marchi preparations they established the quadrantic projections of the retina in the rabbit and cat and the quadrantic and intraquandrantic projections of the central and peripheral retinal segments in the monkey (Brouwer & Zeeman, 1925Overbosch, 1927). Further details in respect to the monkey were added by Brody (1934), Penman

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