Institute for Biomedical Research1, The University of Sydney, NSW, 2006 and Division of Psychology2, The Australian National University, ACT 0200, Australia.
Department of Neurophysiology3, Nencki Institute of Experimental Biology, 3 Pasteur Street, Warsaw 02-093, Poland

Aims: Previous studies indicated that discrete retinal lesions made in adult cats induce a reorganization of visuotopic maps in the part of striate cortex (striate cortex, area V1) which represents the lesioned part of the retina1.2. The present study was designed to examine, following monocular retinal lesions, 1) the extent of reorganization of visuotopic maps in area 17; 2) the properties of the "ectopic" receptive fields; 3) the characteristics of cortical reorganization in cats with lesions made in adolescence. Methods: In eight adult cats and 5 adolescent 8 weeks old kittens intense, sharply circumscribed, monocular laser lesions were made to remove all cellular layers of the retina. The extents of the retinal lesions were subsequently confirmed with counts of a-ganglion cells in retinal wholemounts; in some cases these revealed radial segmental degeneration of ganglion cells distal to the lesion. Area 17 was studied electrophysiologically two to 24 weeks later in case of lesions made in adulthood and one-half to 2 years later in case of lesions made in adolescence. The animals were anaesthetized (a mixture of 0.5 - 0.7% halothane in 67/33% of N2O/O2), paralyzed (gallamine triethiodide; i.v. 7.5mg/kg/hr) and artificially respired. Single neurone or multiunit activity was recorded. Results: An extensive topographical reorganization within the lesion projection zone (LPZ) was revealed in cats with lesions made in adulthood or adolescence3,4. Thus, when stimulated via the lesioned eyes about 70% (lesions made in adulthood) and about 85% (lesions made in adolescence) of single neurones in the LPZ had "ectopic" visual discharge fields displaced to normal retina in the immediate vicinity of the lesion. The sizes of the "ectopic" discharge fields were not significantly different from the sizes of the normal discharge fields. Furthermore, binocular cells recorded from the LPZ when stimulated via the "ectopic" receptive fields exhibited orientation-tuning and preferred stimulus velocities, which were indistinguishable from those found when the cells were stimulated via the normal eyes. However, there was a clear difference when comparing the magnitude of the responses of binocular cells between cats with lesions made in adulthood and those made in adolescence. In cats with retinal lesions made in adulthood the responses to stimuli presented via the "ectopic" discharge fields were generally significantly weaker (P <0.05, Wilcoxon test) than those to stimuli presented via the normal discharge fields and were characterised by a lower-than-normal upper-velocity limit. However, in cats lesioned in adolescence the responses from each eye were very similar. Conclusion: 1) the properties of the "ectopic" receptive fields indicate that cortical mechanisms rather than a retinal "periphery" effect underlie the topographic Reorganization of area 17 following monocular retinal lesions; 2) in cats lesioned in adolescence the presumed cortical mechanism(s) underlying the topographic reorganization of area 17 following monocular retinal lesions, appears to be capable of virtually complete compensation for the loss of the retinal input relayed directly via the dorsal thalamus.