The balance between excitation and inhibition not only leads to variable discharge of cortical neurons but also to contrast invariant orientation tuning

The orientation tuning width of the spike response of neuron in layer V1 of primary visual cortex does not change with contrast of input signal. It is also known that cortical neurons exhibit tremendous irregularity in their discharge pattern which is conserved over large regions of cerebral cortex. To produce this irregularity in responses the neurons must receive balanced excitation and inhibition. By a modeling study we show that if this balance is maintained for all levels of contrast it results in variable discharge patterns of cortical neurons at all contrast and also in contrast invariant orientation tuning. Further, this study supports the role of inhibition in shaping the responses of cortical neurons and we also obtain changes in circular variance with changing contrast, similar to what is observed experimentally.