THE SUPERIOR COlliCUlUS NEI APPROACHES FOR STUDYING SENSORIMOTOR INTEGRATION Edited by William c. Hall Adonis Moschovakis CRC PRESS Boca Raton London New York Washington, D.C. Library of Congress Cataloging-in-Publication Data The superior colliculus : new approaches for studying sensorimotor integration 1edited by William C. Hall and Adonis Moschovakis. p. ; cm. (Methods & new frontiers in neuroscience) Includes bibliographical references and index. ISBN 0-8493-0097-5 (alk. paper) I. Sensorimotor integration. 2. Superior colliculus. I. Hall. William C., Ph. D. II. Moschovakis, Adonis. nI. Methods & new frontiers in neuroscience series. [DNLM: I. Superior Colliculus-physiology. 2. Saccades. WL 310 S959 2003] QP454.S87 2003 612.7'6-dc22 2003058471 This book contains infonnation obtained from authentic and highly regarded sources. Reprinted muterial is quoted with permission, and sources are indicated. A wide variety of references are listed. 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Visit the eRC Press Web site at www.crcpress.com © 2004 by CRC Press LLC No claim to original U.S. Government works International Standard Book Number 0-8493-0097-5 Library of Congress Card Number 2003058471 Printed in the United Stutes of America I 2 3 4 5 6 7 8 9 0 Printed on acid-free paper 3 Concurrent, Distributed Control of Saccade Initiation in the Frontal Eye Field and Superior Colliculus Douglas P. Munoz and Jeffrey D. Schall CONTENTS 3.1 Introduction .................................................................................................... 55 3.2 The Time to Initiate a Saccade ...................................................................... 56 3.3. Overview of the Saccade Generation Network ............................................. 56 3.3.1 Brainstem Saccade Generator ............................................................ 57 3.3.2 Superior Colliculus ............................................................................ 58 3.3.3 Cerebral Cortex - Frontal Field ............................................... 60 3.4 Role of SC and FEF in Saccade Initiation .................................................... 62 3.4.1 Reflexive Saccades The Gap Saccade Task ................................. 62 3.4.2 Express Saccades: A Special Class of Visually Guided Saccade ..... 65 3.4.3 Control of Saccade Production - The Countenuanding Task ......... 67 3.4.4 Voluntary Saccade Production - The Antisaccade Task ................. 70 3.5 Conclusions .................................................................................................... 74 Acknow!edgn1ents .................................................................................................... 75 References ................................................................................................................ 75 3.1 INTRODUCTIOI\J A central problem in neuroscience is the localization offunction. Significant progress on this has been made in the oculomotor system. With progress has come the realization that function is usually not localized within a structure but rather it is distributed across a network of brain areas. In this review, we contrast two brain areas that play a critical role in the planning and initiation of saccadic eye move­ ments: the superior colliculus (SC) and the frontal eye fields (FEF). We describe the O·B493·Q097·5104/$ii.oo+$i.50 © 1004 by CRC Press LLC 55 56 The Superior Colliculus role of these structures in the control of visual fixation and initiation of saccadic eye movements by contrasting neural discharges recorded from neurons in each area in a variety of oculomotor tasks. 3.2 THE TIME TO INITIATE A SACCADE In flight, saccades are incredibly predictable. The velocity and duration of a saccade is mechanistically related to the amplitude of the saccade. I However, the time of initiation of the movement is exceedingly unpredictable. Numerous experiments have measured the time that elapses from presentation of a visual stimulus until initiation of a saccade. Each experiment finds that this saccadic reaction time (SRT) ranges from rarely less than 100 m5 to as much as 500 ms or more. Moreover, SRT can vary over a wide range across a block of trials even within a single task with constant stimuli and unchanging instructions. The origin of the delay and variability of SRT is a central problem that has received increasing attention with, it is fair to say, notable progress toward its elucidation. Many models have been developed to explain the stochastic variability of reac­ tion time.2 Accumulator models have been evaluated in terms of brain function. Accumulator models suppose that in response to a stimulus, some signal grows until it reaches a tln'eshold thereby triggering a movement in response to the stimulus. Models of this sort include three sources for the stochastic variability evident in reaction times (Figure 3.1). One type of accumulator model (Figure 3.1A), assumes that the threshold is constant, but that the average rate of growth of the accumulator is random across trials. 3,4 This architecture can account for a broad range of reaction times measured in a variety of tasks. 5•6 Another type of accumulator model (Figure 3.1B) supposes that the vmiability in reaction time arises from randomness in the level of the threshold.7·8 A third scenmio (Figure 3.1 C) could employ a fixed threshold but a variable baseline at target onset.9 Although this latter model is the mathematical equivalent of vmiable threshold, it would be implemented very differ­ ently in the brain. The alternative models cannot be distinguished on the basis of performance data alone. As a matter of fact, it has been shown that random accu­ mulator and random threshold models generate equivalent predictions. 1O How are aspects of these models instantiated at the level of the single cell, a single brain area, and the entire saccadic generating circuitry? We first review the saccade generating circuitry including the connectivity between the FEF and the Sc. We then review neurophysiological experiments that provide information that seems to differentiate the possibilities illustrated in Figure 3.1. 3.3 OVERVIEW OF THE SACCADE GENERATION NETWORK A network of cortical and subcortical structures is required for the accurate and timely control of saccadic eye movements (Figure 3.2), including regions within the cerebral cortex, thalamus, basal ganglia, cerebellum, SC, and brainstem reticular ll ls formation. Details of this network are reviewed elsewhere. - Concurrent, Distributed Control of Saccade Initiation 57 Target Eye / •••••• """""""'" 'ihiiiih""'hlhiiiiiiii""iiiiiii"""".r............",· A Variable Rate of Rise .............................................................·.. ··7·········~·~·;;;*~~fr~~tl~:t~::~·;····..·..··............·..·..Th·~~·~h·~i·d ------,.. ..­ .~.... I!.?:!.!.?.P../~...J3..§./.$..f!llfJ.~..................~..,..."'...,.,.,".!............................................................ FIGURE 3.1 Accumulator models of saccade initiation. (A) Variability in saccadic reaction time (SRT) could be the result of a variable rate of rise towtU'C1 the saccadic threshold. (B) Variability in SRT could be the result of a variable threshold. (C) Variability in SRT could be the result of variability in baseline activity before accumulation begins. 3.3.1 BRAINSTEM SACCADE GENERATOR 13 The saccadic burst generator circuit is housed in the brain stem reticular fOI1l1ation. ,17 Burst neurons (BNs) in the reticular formation innervate the extraocular muscle motoneurons (MNs) to provide the high-frequency burst of spikes necessary to move the eyes. BNs are silent during fixation and discharge action potentials for saccades in a specific direction. Excitatory burst neurons (EBNs) monosynaptically excite the one-direction motolleurons, while inhibitory burst neurons (IBNs), which receive their input from the EBNs, inhibit the antagonist MNs. The EBNs and IBNs for hOJizontal saccades are located in the pontine and medullary reticular formation, while the BNs for