Attentional modulation in layer 4 of the visual cortex could be mediated by interneurons with complex receptive field characteristics
Many neurons in the visual cortex are orientation-selective, increase their firing rate with contrast and are modulated by attention. What is the cortical circuit that underlies these computations? We examine how synchrony can be modulated by the excitability of interneurons, in a model layer 4 network displaying contrast-invariant orientation-tuning. We did not find parameter settings for which the standard ring model (Somers et al, 1995), which contains only cells with simple receptive fields (RF), behaved appropriately. Only when interneurons with complex receptive fields were included, similar to those found recently in cat primary visual cortex (Hirsch et al, 2003), did the network behave appropriately. A critical feature in the model was that complex interneurons projected to simple interneurons but the simple interneurons did not project back to them. The network was switched from the non-attended state to the attended state by increasing the depolarization of the complex interneurons. In addition to contrast-invariant orientation tuning, the model reproduced the following experimental results: (1) the gamma-frequency range coherence between the estimated local field potential (eLFP) and spike trains of excitatory cells was higher in the attended state than in the non-attended state, but the firing rates of the excitatory cells did not vary between states; (2) the gamma-frequency-range power in the eLFP increased with contrast. The model predicts that there are two populations of inhibitory cells, one with complex RF characteristics whose firing rate increases with attention and the other with simple RF characteristics whose firing rate decreases with attention.