Figure 4. Possible cortical circuits underlying contrast gain control.

A, contrast gain control may be implemented within layer 4, by either shunting inhibition or synaptic depression, and these changes may be inherited subsequently by pyramidal cells in supragranular and infragranular layers. Further gain control may also occur within these non-granular layers. B, alternatively, a subpopulation of thalamorecipient layer 6 cells may drive gain changes in all other cortical layers via feedforward inhibition (adapted from Bortone et al. 2014). C, vasoactive intestinal polypeptide (VIP)-expressing interneuron activity may modulate pyramidal cell gain by controlling the activity of parvalbumin (PV)-expressing interneurons. The VIP interneurons may be recruited by cholinergic inputs during attention or by corticocortical inputs during multisensory or sensorimotor integration. Abbreviation: MGB, medial geniculate body.