  Orientation selectivity is a remarkable feature of the neurons located in the
primary visual cortex. Provided that the visual neurons acquire orientation
selectivity through activity-dependent Hebbian learning, the development
process could be understood as a kind of symmetry breaking phenomenon in the
view of physics. The key mechanisms of the development process are examined
here in a neural system. Found is that there are at least two different
mechanisms which lead to the development of orientation selectivity through
breaking the radial symmetry in receptive fields. The first, a simultaneous
symmetry breaking mechanism, bases on the competition between neighboring
neurons, and the second, a spontaneous one, bases on the nonlinearity in
interactions. It turns out that only the second mechanism leads to the
formation of a columnar pattern which characteristics accord with those
observed in an animal experiment.
