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(Kermadi and Boussaoud, 1995; Kimura et al., 2004; Mechelli et al., 2004; Saenz et al.,
2003). Except for that, attention activates other BG nuclei, including the globus pallidus, as
well as the medial thalamic nuclei and superior colliculus (SC) connected with the BG
(Corbetta et al., 1991; Kermadi and Boussaoud, 1995; Kimura et al., 2004; Nakamura et al.,
2000). On the contrary, activity in the frontal cortical areas, striatum and globus pallidus are
suppressed during attentional deficiency (Booth et al, 2005). The role of dopamine in
attention is specified by the data that patients with attention deficit hyperactivity disorder
have abnormal dopaminergic function in multiple brain regions especially in the input BG
nuclei, accumbens and putamen (Forssberg et al., 2006). Low dopamine concentration in the
BG plays an important role in attentional deficits in patients with Parkinson's disease (Filoteo
et al., 1997), but deficit of attention in nigrostriatal lesioned rats could be improved by
dopamine receptor agonists (Nieoullon and Coquerel, 2003; Turle-Lorenzo et al., 2006).
Known data led to a hypothesis that dopamine-dependent modulation of cortico-striatal inputs
could participate in attentional effects (Miller, 1993).
Earlier we pointed out that dopamine-dependent modulation of cortico-striatal synaptic
inputs in the motor cortico - basal ganglia - thalamocortical (C-BG-Th-C) loop might
underlie a selection of a movement in response to conditioned stimulus (Sil'kis, 2006). Based
on the similarity of the functional organizations of motor and visual C-BG-Th-C loops
(Middleton and Strick, 1996), and taking into account that attention is a form of activity
directed to selection of a stimulus for processing (Naatanen, 1998), we assumed that visual
cue evoked dopamine release and subsequent activity reorganizations in visual C-BG-Th-C
loops may underlie visual attention. A goal of the present work was to determine the role of
C-BG-Th-C loops and dopamine in attentional modulation of visual processing.
A H YPOTHETICAL R OLE OF D OPAMINE -D EPENDENT M ODULATION
OF C ORTICO -S TRIATAL S YNAPTIC T RANSMISSION IN A TTENTIONAL
M ODULATION OF V ISUAL P ROCESSING
According to our hypothesis, visually evoked dopamine release and subsequent
dopamine-dependent reorganizations of neuronal activity in the C-BG-Th-C loops that lead to
amplification of firing in neocortical neuronal patterns representing diverse properties of
stimulus underlies the attentional enhancement of visual perception. Earlier we pointed out
that each visual C-BG-Th-C loops could include a thalamic nucleus connected with
corresponding visual cortical area that projects to one of striatal loci (Silkis, 2007). This
striatal locus projects to corresponding loci in different BG nuclei, including the globus
pallidus and SNr, which projects to the same thalamic nucleus (Fig. 1, 2). Limbic C-BG-Th-C
loop includes the mediodorsal thalamic nucleus (MDN) or the pulvinar of thalamus connected
with one of frontal cortical areas, which projects to the NAcc. This ventral part of the striatum
is connected with the ventral pallidum and dorsomedial part of the SNr that send projections
to the MDN or pulvinar.
Advances in neuroscience implicate reentrant signaling as the predominant form of
communication between brain areas and mechanism subserving conscious sensory perception.
According to the conventional view, this reentrance is the result of activity circulation in the
cortico-cortical and cortico-thalamocortical loops (Crick and Koch, 1995; Edelman, 2003).
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