It has been shown in a number of previous studies that in adult cats discrete retinal lesions induce a reorganization of visuotopic maps in that part of striate cortex (area 17; V1) in which the lesioned part of the retina was represented (Kaas et al. 1990;Chino et al. 1992Chino et al. , 1995Gilbert & Wiesel, 1992;Darian-Smith & Gilbert, 1995;Schmid et al. 1995Schmid et al. , 1996Calford et al. 1999). Projected onto area 17, these lesions represented an area (lesion projection zone; LPZ) typically 3 to 5 mm in diameter, within which the visual receptive fields of many neurones were displaced to normal retina immediately adjacent to the lesioned retina. So far the studies of this issue have covered three experimental protocols: (1) matched homonymous retinal lesions have been made in the two eyes (Gilbert & Wiesel, 1992;Chino et al. 1995;Darian-Smith & Gilbert, 1995); (2) a retinal lesion has been placed in one eye while the other has been enucleated (Chino et al. 1992;Schmid et al. 1996); and (3) a retinal lesion has been made monocularly while the retina in the other eye remained intact (Chino, 1992;Schmid et al. 1996). The topographical extent of the retinal lesioninduced plasticity in the dorsal lateral geniculate nucleus (LGNd) is very small and therefore presumably incapable of providing a substrate for the extensive reorganization of the visuotopic map in the cortex (Eysel et al. 1981; DarianSmith & Gilbert, 1995). It is therefore generally agreed that the apparent topographic reorganization observed in studies with the 1st and 2nd experimental protocols, is based on plasticity at the cortical level and that the most likely source Journal of Physiology (2000) 1. In eight adult cats intense, sharply circumscribed, monocular laser lesions were used to remove all cellular layers of the retina. The extents of the retinal lesions were subsequently confirmed with counts of á_ganglion cells in retinal whole mounts; in some cases these revealed radial segmental degeneration of ganglion cells distal to the lesion. 2. Two to 24 weeks later, area 17 (striate cortex; V1) was studied electrophysiologically in a standard anaesthetized, paralysed (artificially respired) preparation. Recording single-or multineurone activity revealed extensive topographical reorganization within the lesion projection zone (LPZ). 3. Thus, with stimulation of the lesioned eye, about 75% of single neurones in the LPZ had 'ectopic' visual discharge fields which were displaced to normal retina in the immediate vicinity of the lesion. 4. 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 their ectopic receptive fields, exhibited orientation tuning and preferred stimulus velocities which were indistinguishable from those found when the cells were stimulated via the normal eye. 5. However, the responses to stimuli presented via ectopic discharge fields were generally weaker (lower peak discharge rates) tha...

We have examined the influence of 'feedback' projections from extrastriate visual cortical area 21a on the responses of neurons in area 17 of the cat, by cooling area 21a to 5-10 degrees C while the temperature over the recording sites was kept at 36 degrees C. Orientation, direction and length selectivities as well as contrast sensitivity were tested before and during cooling and after rewarming of area 21a. Overall, for the sample of cells recorded from the part of area 17 visuotopically corresponding to area 21a, the 'spontaneous' activity (at background illumination of 1 cd/m(2)) and the responsiveness to visual stimuli of standard contrast (15) were significantly reduced by inactivation of area 21a. In about half of the cells inactivation of area 21a affected substantially the sharpness of orientation-tuning. However, only in a minority of the cells were the direction or length selectivities significantly affected by inactivation of area 21a. Thus (i) the feedback projections from area 21a appear to exert mainly an excitatory influence on the background activity and responsiveness of area 17 cells and (ii) only in subgroups of area 17 cells does the feedback activity originating from area 21a appear to modulate specifically certain receptive field properties.

The receptive fields of neurons in primary visual cortex that are inactivated by retinal damage are known to 'shift' to nondamaged retinal locations, seemingly due to the plasticity of intracortical connections. We have observed in cats that these shifts occur in a pattern that is highly convergent, even among receptive fields that are separated by large distances before inactivation. Here we show, using a computational model of primary visual cortex, that the observed convergent shifts are inconsistent with the common assumption that the underlying intracortical connection plasticity is dependent on the temporal correlation of pre- and postsynaptic action potentials. The shifts are, however, consistent with the hypothesis that this plasticity is dependent on the temporal order of pre- and postsynaptic action potentials. This convergent reorganization seems to require increased neuronal gain, revealing a mechanism that networks may use to selectively facilitate the didactic transfer of neuronal response properties.