The key long-range model results depend on the properties of the local cortical circuitry, so we begin by reviewing and expanding on these results. This model requires only weakly oriented thalamic inputs and utilizes recurrent, short-range excitatory connections between neurons to amplify and sharpen orientation selectivity.
Inhibition contributes indirectly to tuning by balancing excitation within the columnar population.
Several differential properties of excitatory and inhibitory neurons and synapses contribute within a local feedback circuit to generate this gain regulation as a network property. Inhibitory neurons due to their lower surface areas appear to receive fewer synaptic inputs and thus may have higher contrast thresholds. Other possible differences between excitatory and inhibitory neurons may also exist, but have not been considered here. Such a variable gain control mechanism appears well suited to contribute to other reported cortical response nonlinearities within the classical receptive field. Cortical gain control of local circuitry.