A hypothetical role of cortico-basal ganglia-thalamocortical loops in visual processing.

The goal of the present work was to define the mechanisms underlying the contribution of sensory and limbic cortico-basal ganglia-thalamocortical loops to visual processing and its attentional modulation. We proposed that visual processing is promoted by dopamine-dependent long-term modifications of synaptic transmission in the basal ganglia that favour a selection of neocortical patterns representing a visual stimulus. This selection is the result of the opposite sign of modulation of strong and weak cortico-basal ganglia inputs and subsequent activity reorganization in each loop. Reorganization leads to disinhibition/inhibition of cortical neurons strongly/weakly excited by stimulus during dopamine release. Recruitment of the thalamo-basal ganglia-collicular pathway is proposed to be necessary for stimulus-evoked dopamine release that underlies bottom-up attentional effects. Visual excitation of the prefrontal cortex and hippocampus (via the thalamus), their cooperation in control of the basal ganglia and dopaminergic cell firing, and simultaneous modulation of activity in diverse cortico-basal ganglia-thalamocortical loops is proposed to underlie top-down attentional effects. It follows from our model that only those components of cortical responses can be modulated by attention, whose onset exceeds the latency of visual responses of dopaminergic cells (50-110 ms). This and other consequences of the model are in accordance with known experimental data.