  Self-organization of orientation-wheels observed in the visual cortex is
discussed from the view point of topology. We argue in a generalized model of
Kohonen's feature mappings that the existence of the orientation-wheels is a
consequence of Riemann-Hurwitz formula from topology. In the same line, we
estimate partition function of the model, and show that regardless of the total
number N of the orientation-modules per hypercolumn the modules are
self-organized, without fine-tuning of parameters, into definite number of
orientation-wheels per hypercolumn if N is large.
