ACTIVITY-DEPENDENT REGULATION OF ON AND OFF RESPONSES IN CAT VISUAL CORTICAL RECEPTIVE-FIELDS

1. A supervised learning procedure was applied to individual cat area 17 neurons to test the possible role of neuronal co-activity in contro lling the plasticity of the spatial 'on-off' organization of visual co rtical receptive fields (RFs). 2. Differential pairing between visual input evoked in a fixed position of the RF and preset levels of postsy naptic firing (imposed iontophoretically) were used alternately to boo st the 'on' (or 'off') response to a 'high' level of firing (S+ pairin g), and to reduce the opponent response (respectively 'off' or 'on') i n the same position to a 'low' level (X-pairing). This associative pro cedure 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' respon ses, measured in the paired position or integrated across the whole RF , were found in 44% of the conditioned neurons (17/39), and in most ca ses this favoured the S+ paired characteristic. The amplitude change w as on average half of that imposed during pairing. Comparable proporti ons 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 critic al period (13/28). 4. The spatial selectivity of the pairing effects w as studied by pseudorandomly stimulating both paired and spatially dis tinct unpaired positions within the RF. Most modifications were observ ed in the paired position (for 88 % of successful pairings). 5. In som e cells (n = 13), a fixed delay pairing procedure was applied, in whic h the temporal phase of the onset of the current pulse was shifted by a few hundred milliseconds from the presentation or offset of the visu al stimulus. Consecutive effects were observed in 4/13 cells, which re tained the temporal pattern of activity imposed during pairing for 5-4 0 min. They were expressed in the paired region only. 6. The demonstra tion of long-lasting adaptive changes in the ratio of 'on' and 'off' r esponses, expressed in localized subregions of the RF, leads us to sug gest that simple and complex RF organizations might be two stable func tional states derived from a common connectivity scheme.