[Orientation hypercolumns of the visual cortex: ring model]

A hypercolumn of the visual cortex is a functional unit formed of the neighbouring columns whose neurons respond to a stimulus of particular orientation. The function of the hypercolumn is to amplify the orientation tuning of visually evoked responses. According to the conventional simple model of a hypercolumn, neuronal populations with different orientation preferences are distributed on a ring. Every population is described by the frequency (FR) model. To determine the limitations of the FR-ring model, it was compared with a more detailed ring model, which takes into account the distribution of neurons of each population according to their voltage values. In the case of the leaky integrate-and-fire neurons, every neural population is described by the Fokker-Planck (FP) equation. The mapping of parameters was obtained. The simulations revealed differences in the behaviour of the two models. Contrary to the FR model, the model based on the Fokker-Planck equation reacts faster to a change in stimulus orientation. The Fokker-Planck ring model gives a steady-state solution in the form of waves of activity travelling on the ring, whereas the FR ring model presents amplitude instability for the same parameter set. The FR ring model reproduces the characteristic effects of the ring model: the virtual rotation and the symmetry breaking.