Functional anatomy of the basal ganglia: limbic aspects.

INTRODUCTION: The basal ganglia (BG) have been implicated in different processes that control action such as the control of movement parameters but also in processing cognitive and emotional information from the environment. Here, we review existing anatomical data on the interaction between the BG and the limbic system that support implication of the BG in limbic functions. STATE OF THE ART: The BG form a system that is fairly different from the limbic system, but have strong ties, both anatomical and functional, to the latter. Different models have been proposed. In the parallel model, five segregated circuits from the frontal cortex are individualized and terminate in different regions of the BG and thalamus, before projecting back to their cortical area of origin. Based on the extrafrontal cortical projections, another model has been proposed. It subdivides the cortico-striatal projection into three functional territories: limbic, associative and sensorimotor. In a third spiral model, propagation is possible between limbic information processed by the most medial striatal neurons to motor information processed by the most lateral neurons. PERSPECTIVES: Three main levels of interaction between the BG system and the limbic system are considered. (1) The BG receive direct afferences from several structures associated with the limbic system. Limbic cortical areas project to the striatum, of which the internal architecture is particularly complex, with significant cross-species differences: a compartmental striosome/matrix subdivision described mainly in primates, and a core/shell topographic subdivision of the nucleus accumbens more sharply marked in rodents. (2) Projections from the amygdala form a patchy dorso-ventral progressive gradient in the nucleus accumbens and ventral caudate. (3) Both shell and striosomes receive limbic information from cortical and subcortical limbic structures and project to the dopaminergic neurons of the substantia nigra pars compacta, which in turn modulates their activity. (4) There is a significant overlap between the ventral portions of the BG, nucleus accumbens and ventral pallidum, and the ventral subcortical structures of the limbic system, extended amygdala and nucleus basalis.
CONCLUSION: Important interactions exist between the limbic system and the BG system but questions remain about the role that this information plays in the functional organisation of this system. Is limbic information processed separately in the BG, or is it integrated to motor and cognitive information? Do pathological conditions such as obsessive-compulsive disorders or Tourette syndrome result from abnormal afferent limbic input to the BG or abnormal processing within the BG?