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We assume that reentrance of information into the cortex could be also realized by diverse C-
BG-Th-C loops. In these loops, BG-thalamic influence is not only disinhibitory but also
excitatory since a large part of the SNr neurons projected to the thalamus is glutamatergic
(Kha et al., 2001). SNr neurons can receive excitation from the neocortex via the subthalamic
nucleus (Fig. 2). Some of the output SNr cells have visual receptive fields that are similar to
those of superior colliculus cells (Nagy et al., 2005). Thus, reentrant excitation of the
neocortex through the visual part of the BG and thalamus can participate in conscious visual
perception.
Under our assumption, realization of involuntary visual attention requires a fulfilment of
two conditions: release of dopamine in response to visual stimulus, and modulatory action of
dopamine on cortico-striatal synaptic efficacy. The fulfilment of the first condition is
supported by the data that dopaminergic neurons in the substantia nigra pars compacta (SNc)
and ventral tegmental area (VTA) are activated not only by conditioned stimulus (Schultz,
W., 1997) but also by non-conditioned visual stimuli (Domett et al., 2005; Horvitz et al.,
1997). The primary source of visual excitation of dopaminergic cells is the SC (Dommett et
al., 2005). However, dopaminergic cells become visually responsive only after disinhibition
of the SC, whereas disinhibition of the visual cortex was ineffective (Dommett et al., 2005).
We proposed that visual activation (via the thalamus) of GABAergic striatonigral cells
projected onto GABAergic SNr neurons could lead to SC disinhibition thus promoting
excitation of dopaminergic cells (Fig. 2) (Silkis; 2007). In addition, visual stimulus passing
through the SC and MDN to the PfC can lead to both direct excitation of dopaminergic cells
and descending prefrontal influence on dopaminergic cells through the NAcc. On the one
hand, PfC excites striatonigral cells of the NAcc, projected to the VTA (Fig. 2) (Berendse et
al., 1992). On the other hand, PfC acts on striatopallidal cells of the NAcc, projected to the
ventral pallidum, which GABAergic neurons also innervate the VTA (Fig. 2). Thus, the PfC
activating one group of dopaminergic cells and inhibiting others generates a pattern of firing
dopaminergic cells in response to a visual stimulus.
The fulfilment of the second necessary condition is supported by the data that visual
stimuli cause a greater than five-fold rise in the probability of burst firing of dopaminergic
cells (Horvitz et al., 1997). By this reason, visually evoked enlargement in dopamine
concentration might be sufficient for modulation of cortico-striatal synaptic transmission.
Earlier we pointed out that dopamine oppositely modulates the efficacy of cortical inputs
that "strongly" and "weakly" excite striatal spiny cells (inputs that allow and do not allow,
respectively, open postsynaptic NMDA channels) (Sil'kis, 2003). The character of dopamine-
dependent modulation of synaptic inputs to striatonigral and striatopallidal cells that mainly
express D1 and D2 receptors, respectively, is also opposite (Silkis, 2000; 2001). Due to such
character of modulation of striatal cell firing, signals passing through the BG could disinhibit
(via the SNr and thalamus) cortical neurons that initially were strongly excited by visual
stimulus (Fig. 2), and simultaneously inhibit activity of cortical neurons, that initially were
weakly excited by this stimulus. Thus in each visual cortical area, a contrasty amplified neural
pattern could be selected that represents a certain attribute of attended visual stimulus. In the
absence of dopamine, signals passing through the BG could inhibit activity of those cortical
neurons, which initially visual response was strong, and simultaneously disinhibit those
cortical neurons which response was weak. Such reorganization must disturb the initial neural
pattern representing non-attended visual stimulus (or its feature).
