The proposed experiments will utilize neuroanatomical and electrophysiological methods to investigate the effects of partial removal of visual cortex during infancy upon the projection of the intact cortical remnant to the ipsilateral colliculus in the adult animal. This paradigm is of particular interest because it creates conditions which should permit the reorganization of one topographically organized projection to the colliculus (that from cortex) while leaving the topographic order of the dominant visual input to this structure (that from the retina) relatively undisturbed. Cortical lesions will be carried out during the second week of life and the animals will recover until adulthood at which time the extent of the cortical scotoma be delineated electrophysiologically. The topographic relationship between the damaged cortex and the ipsilateral colliculus will be investigated by two methods. In some hamsters the autoradiographic technique will be employed. Injections of tritiated amino acids will be made into the cortical remnant and the distribution of label in the colliculus will be determined. Other animals will be utilized in electrophysiological experiments in which microelectrode stimulation from physiologically defined loci in the remaining cortex and single unit recording in the ipsilateral colliculus will be used to assess the topographic relationship between the two structures. The results of the anatomical and electrophysiological experiments will allow the determination of whether or not be corticotectal projection from the damaged hemisphere expands to innervate the entire colliculus or remains restricted to that portion of the tectum with which it is retinotopically matched. An expanded projection would suggest that "interaxonal sorting" may be the dominant mechanism involved in ordering the corticotectal projection in these animals while a restricted pattern of innervation would suggest that cell-cell affinities are the dominant factor.