The postnatal development of clustered intrinsic connections in area 18 of the visual cortex in kittens

Retrogradely transported neuronal markers were injected into area 18 of the visual cortex in normal kittens of various ages and in animals that had been binocularly deprived of patterned visual stimulation by eyelid suture. In normal kittens aged 20 days or more, distinct clusters of retrogradely labelled cells were identified in area 18 surrounding the injection sites; these cells lay mainly in cortical layers II, III, and the upper part of IV, but with some in layers V and VI. In kittens younger than 10 days, labelled cells were observed around injection sites, although they were not organized into clusters. The results suggest that the typical clustered pattern of neurones forming intrinsic connections in area 18 emerges during the second week postnatally from an immature non-patchy distribution. Binocular deprivation did not prevent the appearance of these patches of cells.