We made extracellular recordings from 1176 single units in area 18 of adult cats and kittens aged between 7 days and 10 weeks, and from 137 single units in area 17 in kittens aged between 12 days and 10 weeks. 2. All cells examined in area 18 of adult cats were visually responsive, 84% being orientation selective, 9% orientation biased and 7% non-oriented. Orientation columns and ocular dominance columns were identified. There was an over-all bias towards horizontal and vertical in the distribution of preferred orientations in the rostral part of area 18, where we were recording. 3. In area 18 of 7-day-old, visually inexperienced kittens the majority of cells (60%) were visually unresponsive; the remainder were either non-oriented (25%) or orientation biased (15%), and no orientation-selective units were found. Nevertheless there seemed to be a rudimentary columnar system, even in the youngest animals, in that orientation-biased cells tended to occur in clusters with neighboring neurones having similar orientation preferences. In normal animals of 3 weeks and younger we found that the distribution of preferred orientations of a sample of neurones in area 18 with receptive fields scattered over much of the left lower quadrant of the visual hemifield was biased to oblique orientations. 4. As in adult cats, most cells in area 18 in young kittens were binocularly driven, and periodic alternation of dominance along oblique penetrations, characteristic of ocular dominance columns, was sometimes seen, even at the earliest ages. 5. Many of the developmental changes that we observed in area 18 occurred during the first 4 post-natal weeks. Orientation selectivity, orientation tuning, directionality and responsiveness of neurones matured rapidly over this period. The proportions of simple and complex cells were similar in kittens aged 4 weeks or more to those in adult cats, whereas prior to this most neurones that displayed an orientation preference appeared to be immature simple cells. 6. A laminar analysis revealed that very few units in the superficial layers (I, II and III) in area 18 are visually responsive in kittens during their first and second weeks, but orientation selectivity rapidly develops during the third week. By contrast, even in very young, visually inexperienced kittens, the majority of neurones found in deeper laminae (IV, V and VI) are visually responsive and a few of them already show an orientation preference; however, the subsequent appearance of larger proportions of orientation-selective cells in these lower layers is a more prolonged process than in upper laminae.(ABSTRACT TRUNCATED AT 400 WORDS)
