The reorganization of the adult hamster's lateral posterior nucleus after neonatal superior colliculus lesions was studied using primarily light and electron microscopic degeneration techniques. Two types of experiments were conducted. First, the distributions of the remaining afferents from the contralateral superior colliculus, the contralateral retina, and the ipsilateral posterior neocortex were determined using the Fink‐Heimer ('67) technique. Normally the projections from the contralateral superior colliculus and retina are sparse and restricted to small areas in the rostrolateral subdivision. After neonatal lesions of the ipsilateral colliculus, however, these two minor projections greatly increase in density and expand to share a common border. In contrast, the normal projection from the posterior neocortex is dense throughout the rostrolateral subdivision. After a neonatal colliculus lesion, however, this projection is greatly decreased in the region occupied by the optic tract terminals. Second, the ultrastructural organization of the rostrolateral subdivision was studied in adult animals which had received neonatal colliculus lesions. Normally, this region is characterized by synaptic clusters in which numerous medium‐sized terminals (M‐terminals), almost all from the ipsilateral colliculus, synapse around the shaft of a large central dendrite. The contralateral colliculus and retina normally contribute only a few M‐terminals. After a neonatal colliculus lesion, typical clusters still form, but now the expanded projections from the contralateral colliculus and retina contribute numerous M‐terminals. The cortex does not contribute M‐terminals in either normal or experimental animals. These results suggest that the afferents to the rostrolateral subdivision normally compete for synaptic space. The various factors that might be involved in determining the outcome of such competition are discussed.
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