Summary

Lateralization is manifested in two main ways: (1) in individuals but with no common direction (bias) in the group or population, or (2) in individuals and in the same direction in most individuals so that the group or population is biased. The first is discussed in terms of evidence of efficiency of neural processing in a lateralized brain. The second is discussed mainly in terms of the hypothesis that population biases occur as evolutionarily stable strategies when lateralized individuals coordinate with each other. This hypothesis is supported both by recent evidence and by mathematical models.

Introduction

It is believed that bilateral symmetry evolved when organisms adopted an axial orientation to their direction of movement and it is usually agreed that the pathway to a bilateral nervous system led from radial symmetry. In addition to being bilaterally divided, however, the nervous system of vertebrates shows a pervasively contralateral organization in that afferent and efferent pathways cross the midline of the body so that each side of the brain connects to the opposite side of the body. Also, as we have already seen (Chapter 1), the nervous system has a certain degree of asymmetry between the left and the right sides, and this is seen in both function and structure. Before considering the function of such an asymmetrical organization let us discuss the problem of why the nervous system is organized contralaterally (Figure 2.1), given that we shall refer to such an organization almost continuously while describing experiments and observations on asymmetries in animal behaviour.