THE MICROSCOPIC ANATOMY AND PHYSIOLOGY OF THE MAMMALIAN SACCADIC SYSTEM

A central goal of the Neurosciences is to provide an account of how th e brain works in terms of cell groups organised into pattern generatin g networks. This review focuses on the neural network that generates t he rapid movements of the eyes that are called saccades. A brief descr iption of the metrical and dynamical properties of saccades is provide d first. Data obtained from lesion and electrical stimulation experime nts are then described; these indicate that the relevant neural machin ery spreads over at least 10 distinct cortical and subcortical regions of the brain. Each one of these regions harl;ors several distinct cla sses of saccade related cells (i.e. cells whose discharge encodes the metrical and often dynamical properties of saccades). The morphologica l and physiological properties of about 30 saccade related cell classe s are described. To generate the signals they carry, and therefore sac cades, distinct classes of cells influence each other in a non-random manner. Anatomical evidence is provided that indicates the existence o f about 100 distinct connections established between saccade related n eurons. The overall picture of the saccadic system that emerges from t hese studies is one of intricate complexity. In part this is due to th e presence of at least 3, multiply interconnected negative feedback lo ops. Several computational models of the saccadic system have been pro posed in an attempt to understand the functional significance of the s imultaneous operation of these loops. An evaluation of these models de monstrates that besides providing a coherent summary of the data that concern it, successful models of the saccadic system generate realisti c saccades (in precise quantitative psychophysical terms) when their e lements are stimulated, produce abnormal saccades, reminiscent of thos e encountered in the clinic, when their elements are disabled, while t heir constituent units display realistic discharge patterns and are co nnected in a manner that respects anatomy. Copyright (C) 1996 Elsevier Science Ltd.