The nerve cells are believed to have such ability of self-organization that, given a number of input patterns, each cell tunes itself to become responsive to only one of the patterns, or to one subset of patterns having some features in common. The detectors of patterns or pattern subsets are formed in this manner. A simple but plausible mechanism of self-organization is proposed based on the two hypotheses: 1) Synaptic modification process is non-linear, activated when the output of a cell is positive. 2) Not only excitatory but also inhibitory synapses are modifiable. A rigorous mathematical analysis is given to elucidate the characteristics of modifiable synapses to form these detectors. The present model fits well most of the experiments on the developmental plasticity of the visual cortex such as the formation of orientation detecting cells, monocular and alternate monocular deprivation in normal and abnormal environments.