Theories of brain localization from Flourens to Lashley.

Introduction A history of theories of localization of function in the brain can be found more or less briefly outlined in Boring (I929 and I950) and Lashley (I929). Both writers point out that all such theories can be broadly divided into two types-localization theories, which hold that specific functions are controlled by specific parts of the brain, and field theories, which hold that the brain acts as a single functional unit. It is said that, historically, a swing of the pendulum tends to occur between these two positions. At one period the majority of informed opinion holds a localization theory, but a generation later this tends to be considered distinctly unorthodox. Too much can be made of this distinction. Localization and field theories, as defined above, have not been held since the time of Gall. The first achievement of Flourens, usually regarded as the founder of field theory, was to show that the different parts of the brain have specific functions: he claimed that only the hemispheres act as a single functional unit. Later field theorists further restricted this claim. Nevertheless, the distinction between localization theorists and field theorists is broadly valid. The purpose of this article is to examine the factors responsible for the 'swing of the pendulum', that is, for the development and general acceptance of successive theories. Two factors are shown to be important. The development of a new theory of localization depends partly on the development of new and more refined techniques of investigation, and partly on the nature of current psychological preconceptions. This is because such a theory must involve assumptions about psychological processes and brain function, and the models of brain function are themselves determined by psychological preconceptions. In the past these assumptions have been explicit. It is argued that the implicit assumptions of contemporary localization theory could usefully be examined, and brought into line with contemporary psychology.