Figure 1.

Schematic of the thalamocortical model with, on the left, a primary thalamocortical circuit including a 3-layered primary visual cortical area (Vp), thalamic reticular nucleus (Rp), and dorsal thalamus (Tp) and, on the right, a secondary visual area, Vs, with its associated thalamic sectors Rs and Ts. Visual inputs (left), including spontaneous random optic nerve firing, excite inhibitory (black) and excitatory (white) neurons in Tp. (1) Thalamocortical loops: excitatory Tp and Ts neurons project to L4 (corresponding to cortical layer 4), to infragranular layer L5-6 cortical neurons and, via collaterals, to Rp and Rs. (2) Reticular nucleus networks: Rp and Rs neurons are part of a dense inhibitory network that sends diffuse inhibitory projections to thalamocortical neurons in Tp and Ts. (3) Cortical interlaminar (vertical) loops: columnar projections are made from L4 to supragranular L2-3, from L2-3 to L5-6, and from L5-6 back to L4 and L2-3. (4) Cortical intralaminar (horizontal) connections: each layer contains excitatory projections (shown only for L2-3 in Vp) forming connections between patches of cells with similar response selectivity (for horizontal or vertical bars). (5) Interareal corticocortical loops: forward projections from L2-3 of Vp to L4 of Vs; backward projections from L5-6 of Vs to L2-3 of Vp. (6) Excitatory projections from L5-6 to thalamocortical neurons in Ts. (7) Diffuse neuromodulatory (cholinergic, noradrenergic, serotonergic, etc) systems project throughout the entire thalamocortical network. Not drawn to scale.