  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.
