The Superior Colliculus: New Approaches for Studying Sensorimotor Integration PDF

Preface

To explore the world, the brains of humans and other animals orchestrate movements of several effectors (eyes, head, trunk, upper and lower limbs) to orient the direction of gaze toward interesting features of the visual field. Such orientation is one example of general neural mechanisms that allow organisms to acquire information about the external world and to process the information so that it can be utilized by motor systems to generate adaptive behavior. What are the biological mechanisms, that is, the sequences of neural events, that allow sensory systems to interface and meaningfully engage their motor counterparts? To answer this fundamental question, experts have focused for more than 30 years, on a small, phylogenetically ancient midbrain structure, the superior colliculus. The close proximity within its layers of sensory cells that encode the spatial loci of external stimuli and premotor cells that command corresponding movements of the eyes and head toward these stimuli provides investigators with an almost unique opportunity to study how sensory signals are translated into adaptive motor commands within a single structure. Because of these features and also because of the simplifying benefits of the fixed relationship between stimulus location and the metrics of an appropriate orienting movement, progress in understanding the structure and function of the superior colliculus has reached the point that sophisticated quantitative models can be proposed and tested experimentally. For all of these reasons, investigators in this area appear to be on the threshold of unveiling, for the first time in the mammalian brain, the neural mechanisms responsible for a specific example of sensorimotor integration. The time, therefore, seems opportune for offering, in a single volume, a summary of the progress accomplished to date and a view of the vistas opened up for future exploration.

For this volume, authors were chosen to represent the wide variety of approaches that have contributed to this level of understanding of neural mechanisms in the superior colliculus. To encourage perspectives on what has been accomplished during the past 3 decades as well as the expression of views concerning the future course of research in this field, the volume includes contributions from outstanding scientists at different stages of their careers. In all cases, the authors are recognized experts on the frontier of research in this productive field. The first three chapters discuss the role of the superior colliculus in the control of gaze shifts and, more specifically, the rapid eye movements called saccades. Authors Platt, Lau, and Glimcher consider the place of the superior colliculus in the complex hierarchy of nuclei that control the direction of gaze. Stanford summarizes classical experiments that implicate the superior colliculus in the control of saccades, while Munoz and Schall compare its role to that of the frontal eye field in saccade preparation and initiation.

In the first of several chapters devoted to a detailed description of structural features of the superior colliculus as they relate to its functional properties, Harting describes certain medium-scale morphological features that have intrigued anatomists over the years. Then, Alexej Grantyn and Adonis Moschovakis describe the morphology of collicular neurons and the nature of the signals they carry to their targets both inside and outside of the colliculus. Next, Özen, Helms, and Hall describe the properties of circuits intrinsic to the deeper, premotor layers of the superior colliculus, and the means by which they generate behaviorally relevant signals. Isa, Kobayashi, and Saito consider the interaction of the deeper layer cells with information originating in the superficial, visuosensory layers.

The next two chapters are devoted to developmental issues; Rosemarie Grantyn, Jüttner, and Meier emphasize the development of the visual layers while Mize and Salt focus on mechanisms responsible for the development of the retinal input to these layers. The next two chapters emphasize computational issues. Guitton, Bergeron, and Choi argue in favor of a transcollicular feedback loop implementing a gaze controller, while Keller summarizes progress in understanding the operation of the highly distributed neural processing that takes place inside the superior colliculus.

The next two chapters emphasize computational issues. Guitton, Bergeron, and Choi argue in favor of a transcollicular feedback loop implementing a gaze controller, while Keller summarizes progress in understanding the operation of the highly distributed neural processing that takes place inside the superior colliculus.

In a fitting epilogue to this volume, Sparks summarizes certain methodological and conceptual problems of the field; he considers how collicular efferent signals could be processed by the structures that receive input from the superior colliculus, whether feedback signals must be rerouted to it, and the types of external physical variables that each structure could encode.

The editors of this volume are grateful to the series editors, Sid Simon and Miguel Nicolelis, for providing us with the opportunity and the encouragement to undertake this venture. We also are indebted to the publisher of the life sciences group at CRC, Barbara Norwitz, for her constant attention and expert advice and especially for her persistent, and necessary, prodding in motivating us to complete this project in a timely fashion.