ABSTRACT

In the course of evolution, along with the emergence of the eyes arose the extraocular muscles to move them about. Initially, the retina itself was not highly specialized; the density of photoreceptors was similar throughout the eye. Subsequently, a major specialization has occurred in numerous species: To improve acuity, rather than increasing the density of photoreceptors throughout the eye, which would have required an inordinately large number of neurons, the increase in density was limited to just a small region that we call the fovea. As the basic mechanisms were already present to move the eyes about, their capacity was increased to extend the range and velocity of eye movements, thereby enabling organisms to shift their center of gaze rapidly from location to location and to maintain the center of gaze on objects when there is motion. The two distinct systems that have emerged are saccadic eye movements that rapidly shift the center of gaze from one location to another and tracking eye movements that can stabilize the eye on the visual scene, keep the two eyes in register, and allow for keeping the center of gaze on selected

CONTENTS

5.1 Introduction .......................................................................................................................... 97 5.2 Brainstem Control of Eye Movement .............................................................................. 100 5.3 Superior Colliculus and Saccadic Eye Movements ....................................................... 102 5.4 Cortical Control of Saccadic Eye Movements ................................................................ 106

5.4.1 Effects of Inactivating Area V1 on the Superior Colliculus ............................. 107 5.4.2 Effects of Cortical Electrical Stimulation on Target Selection ......................... 109 5.4.3 Effects of Blocking Various Brain Regions on Saccadic Eye Movements ...... 109

5.4.3.1 Hand-Eye Coordination Task ............................................................... 111 5.4.3.2 Generation of Express Saccades ............................................................ 113

5.5 Role of Frontal Cortical Areas in the Generation of Saccadic Eye Movements ............. 115 5.6 Role of Excitatory and Inhibitory Circuits in the Generation

of Saccadic Eye Movements .............................................................................................. 119 5.7 Tracking Eye Movements for Image Stabilization ........................................................ 128 5.8 Eye Movements That Keep the Two Eyes in Register ................................................... 132 5.9 Smooth-Pursuit Tracking of Moving Visual Objects .................................................... 133 5.10 Summary ............................................................................................................................. 133 References ..................................................................................................................................... 134

targets when there is movement. One may pose the question then as to why the extraocular muscles have evolved in the first place in animals that have photoreceptors of similar density throughout the eye. The prime reason seems to be that movement of living organisms is pervasive; unless the eyes can be stabilized on the visual scene, images would become blurred. Two major systems have evolved to combat such blurring: a specialized set of retinal ganglion cells selective for direction of motion called the cells of Dogiel, and the vestibular system that makes extensive connections with the eye muscles through the brainstem oculomotor complex. Any movement made results in compensatory counterrotation of the eyes, thereby stabilizing them with respect to the visual scene. The major neural mechanism involved in this stabilization process is the accessory optic system that will be discussed in Section 5.7 of this review.