PLASTICITY IN THE TECTUM OF XENOPUS LAEVIS: BINOCULAR MAPS S. B. UDIN*% and S. GRANT$ *Department of Physiology and Biophysics, State University of New York, Bualo, NY 14214, USA and $Department of Sensorimotor Systems, Division of Neuroscience, Imperial College School of Medicine, Fulham Palace Road, London W6 8RF, UK (Received 29 October 1998) (Accepted 1 December 1998) AbstractÐXenopus frogs exhibit dramatic changes in the binocular projections to the tectum during a critical period of development. Their eyes change position in the head, moving from lateral to dorsal and creating an increasing region of binocular overlap. There is a corresponding shift of binocular projections to the tectum that keeps the two eyes' maps in register with each other throughout this period. The ipsilateral input is relayed via the nucleus isthmi. Two factors bring the ipsilateral projection into register with the contralateral projection. First, che- moanity cues establish a crude topographic map beginning when the shift of eye position begins. Ap- proximately 1 month later, visual cues bring the ipsilateral map into register with the contralateral map. The role of visual input is demonstrated by the ability of the axons that bring the ipsilateral eye's map to the tectum to reorganize in response to a surgical rotation of one eye and to come into register with the contralateral eye's map. This plasticity can be blocked by NMDA receptor antagonists during the critical period. In normal adults, reorganization is minimal. Eye rotation fails to induce reorganization of the ipsilat- eral map. However, plasticity persists inde®nitely in animals that are reared in the dark, and plasticity can be restored in normally-reared animals by treatment with NMDA. The working model to explain this plasticity posits that correlated input from the two eyes triggers opening of NMDA receptor channels and initiates events that stabilize appropriately-located isthmotectal connections. Speci®c tests of this model are discussed. # 1999 Elsevier Science Ltd. All rights reserved CONTENTS 1. Introduction 82 1.1. The unusual development and unusual plasticity of Xenopus tectum 82 1.2. The coincidence hypothesis 82 2. The tecto±isthmo±tectal pathway 83 2.1. Anatomy 83 2.1.1. The uncrossed tecto±isthmo±tectal projection 83 2.1.2. The crossed isthmotectal projection 84 2.2. Electrophysiology 85 3. Normal development of the ipsilateral map 86 3.1. Expansion of the binocular ®eld and the ipsilateral projection 86 3.2. Remodeling of crossed isthmotectal axons and arbors 87 3.3. Retinotectal shifting connections 89 4. The contribution of visual activity to crossed isthmotectal mapping 89 4.1. The eects of dark-rearing 89 4.2. The eects of early eye rotation 90 5. The critical period 92 5.1. The process of isthmotectal reorganization during normal development 93 5.2. Limitations on plasticity after eye rotation 93 5.3. Dark-rearing 93 6. Vision-independent mechanisms of crossed isthmotectal mapping 94 6.1. Cell production and ®ber ordering in the mediolateral axis 94 6.2. Chemoanity and the retinotectal projection 94 6.3. Cell death 94 7. Mechanisms underlying the eects of visual activity on map topography 95 7.1. Interactions among isthmotectal synapses in maintaining map topography 95 7.2. Correlated activity and rearing with stroboscopic illumination 95 7.3. The role of NMDA receptors 95 Progress in Neurobiology Vol. 59, pp. 81 to 106, 1999 # 1999 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0301-0082/99/$ - see front matter PII: S0301-0082(98)00096-3 % Corresponding author. Tel.: 001 716 829 3571; Fax: 001 716 829 2569; e-mail: sudin@bualo.edu 81