Introduction

Patterns and Tiles

In this chapter, we “zoom out” from the study of individual retinal neurons and circuits to a consideration of how these individuals are organized across the retinal surface. In particular, we focus on an almost universal tendency for these neurons to lie in regular patterns. Although regularity can be a consequence of uniform size and tight packing, it also occurs among neurons that are widely spaced. In either case, it fulfills a behavioral requirement that each class of functional circuits should sample the visual scene systematically and completely, so that vital features (such as prey and predators) do not go undetected.

Regular arrays of retinal neurons often extend dendrites in a competitive, territorial manner that minimizes the overlap of their dendritic fields and causes them to tesselate, “tiling” the retina like the individual pieces (Latin: tesserae) of a ceramic mosaic. By convention, regular arrays of neuronal somata are often called neuronal mosaics even if precise dendritic tiling has not been shown directly, and the term mosaic is used in this sense throughout the chapter. It is important to be aware that geneticists and developmental biologists exploit the analogy of the tiled mosaic in another way, using it to describe mixtures or clusters of cells with distinct histories or genotypes and emphasizing the potential for abrupt contrasts at the boundaries of adjacent tesserae; these two senses of mosaic may confront each other uneasily in studies of retinal developmental genetics.

Neuronal mosaics are informative in many different ways. The following sections briefly review their implications for the function of the mature retina and the elucidation of its many neuronal types, consider what they can tell us about retinal development and evolution, summarize the methods used for their quantitation, and look ahead to questions that remain.