Development of local circuits in mammalian visual cortex.

Most of the synapses in the mammalian cerebral cortex are components of local circuits. Not only do these local, or intrinsic, connections dominate in numerical terms, but such synapses are directly responsible for gen­ erating neuronal codes that convey sensory data and elicit behavior. In the mammalian primary visual cortex, the precise arrangement of local circuits in both vertical and horizontal dimensions is crucial for such fundamental properties of cortical neurons as detecting the orientation and direction of movement of visual stimuli. The elaboration of local connections, therefore, is probably a key factor in the emergence of com­ putationally competent neural circuits. Despite the central role of local circuits in neuronal processing, until recently several factors conspired to limit insight into how precise local excitatory and inhibitory interconnections emerge during development. First, the patterns of intrinsic circuitry in many brain areas were either unknown or known only in crude outline, which greatly hindered interpre­ tation of developmental studies. Second, techniques used to elucidate local circuits were limited, as they relied almost exclusively on Golgi staining. In recent years, advances in techniques have improved our understanding of adult local circuits and have provided powerful tools for detailed inves­ tigations of local circuit development. These studies have been particularly