A supervised learning procedure was applied to individual cat area 17 neurons to test the possible role of neuronal co-activity in controlling the plasticity of the spatial 'on-off' organization of visual cortical receptive fields (RFs). 2. Differential pairing between visual input evoked in a fixed position of the RF and preset levels of postsynaptic firing (imposed iontophoretically) were used alternately to boost the 'on' (or 'off') response to a 'high' level of firing (S+ pairing), and to reduce the opponent response (respectively 'off' or 'on') in the same position to a 'low' level (S- pairing). This associative procedure was repeated 50-100 times at a low temporal frequency (0.1-0.15 s-1). 3. Long-lasting modifications of the ratio of 'on-off' responses, measured in the paired position or integrated across the whole RF, were found in 44 % of the conditioned neurons (17/39), and in most cases this favoured the S+ paired characteristic. The amplitude change was on average half of that imposed during pairing. Comparable proportions of modified cells were obtained in 'simple' (13/27) and 'complex' (4/12) RFs, both in adult cats (4/11) and in kittens within the critical period (13/28). 4. The spatial selectivity of the pairing effects was studied by pseudorandomly stimulating both paired and spatially distinct unpaired positions within the RF. Most modifications were observed in the paired position (for 88 % of successful pairings). 5. In some cells (n = 13), a fixed delay pairing procedure was applied, in which the temporal phase of the onset of the current pulse was shifted by a few hundred milliseconds from the presentation or offset of the visual stimulus. Consecutive effects were observed in 4/13 cells, which retained the temporal pattern of activity imposed during pairing for 5-40 min. They were expressed in the paired region only. 6. The demonstration of long-lasting adaptive changes in the ratio of 'on' and 'off' responses, expressed in localized subregions of the RF, leads us to suggest that simple and complex RF organizations might be two stable functional states derived from a common connectivity scheme.
