Anatomical organization of forebrain circuits in the primate

The primate forebrain is a complex structure. Thousands of connections have been identified between cortical areas, and between cortical and sub-cortical areas. Previous work, however, has suggested that a number of principles can be used to reduce this complexity. Here, we integrate four principles that have been put forth previously, including a nested model of neocortical connectivity, gradients of connectivity between frontal cortical areas and the striatum and thalamus, shared patterns of sub-cortical connectivity between connected posterior and frontal cortical areas, and topographic organization of cortical-striatal-pallidal-thalamocortical circuits. We integrate these principles into a single model that accounts for a substantial amount of connectivity in the forebrain. We then suggest that studies in evolution and development can account for these four principles, by assuming that the ancestral vertebrate pallium was dominated by medial, hippocampal and ventral-lateral, pyriform areas, and at most a small dorsal pallium. The small dorsal pallium expanded massively in the lineage leading to primates. During this expansion, topological, adjacency relationships were maintained between pallial and sub-pallial areas. This maintained topology led to the connectivity gradients seen between cortex, striatum, pallidum, and thalamus.

Dynamics of Goldfish Subregional Hippocampal Pallium Activity throughout Spatial Memory Formation

The teleost fish hippocampal pallium, like the hippocampus of tetrapods, is essential for relational map-like spatial memories. In mammals, these relational memories involve the dynamic interactions among different hippocampal subregions and between the hippocampus-neocortex network, which performs specialized operations such as memory encoding and retrieval. However, how the teleost hippocampal homologue operates to achieve comparably sophisticated spatial cognition capabilities is largely unknown. In the present study, the progressive changes in the metabolic activity of the pallial regions that have been proposed as possible homologues of the mammalian hippocampus were monitored in goldfish. Quantitative cytochrome oxidase histochemistry was used to measure the level of activation along the rostrocaudal axis of the ventral (Dlv) and dorsal parts of the dorsolateral division (Dld) and in the dorsoposterior division (Dp) of the goldfish telencephalic pallium throughout the time course of the learning process of a spatial memory task. The results revealed a significant increase in spatial memory-related metabolic activity in the Dlv, but not in the Dld, suggesting that the Dlv, but not the Dld, is comparable to the amniote hippocampus. Regarding the Dlv, the level of activation of the precommissural Dlv significantly increased at training onset but progressively declined to finally return to the basal pretraining level when the animals mastered the spatial task. In contrast, the commissural Dlv activation persisted even when the acquisition phase was completed and the animal's performance reached an asymptotic level. These results suggest that, like the dentate gyrus of mammals, the goldfish precommissural Dlv seems to respond nonlinearly to increments of change in sensory input, performing pattern separation under highly dissimilar input patterns. In addition, like the CA3 of mammals, the commissural Dlv likely operates in a continuum between two modes, a pattern separation or storage operation mode at early acquisition when the change in the sensory input is high, probably driven by the precommissural Dlv output, and a pattern completion or recall operation mode when the animals have mastered the task and the change in sensory input is small. Finally, an unexpected result of the present study is the persistent activation of the area Dp throughout the complete spatial task training period, which suggests that the Dp could be an important component of the pallial network involved in spatial memory in goldfish, and supports the hypothesis proposing that the Dp is a specialized part of the hippocampal pallium network.

Spatial learning-related changes in metabolic brain activity contribute to the delimitation of the hippocampal pallium in goldfish

Comparative neuroanatomical, developmental and functional evidence suggests that the lateral division of the area dorsalis telencephali (Dl) of the teleost fish is homologous to the hippocampus of tetrapods. Nonetheless, some important aspects of the organization of the hippocampal pallium of teleosts are still under discussion and conflicting hypotheses regarding the extension and demarcation of this region have been proposed. Thus, whereas some authors suggest that the entire Dl region, including its dorsal (Dld) and ventral (Dlv) subdivisions, is homologue to the mammalian hippocampus, others claim that only Dlv should be considered as such. To further elucidate this debate, we investigated the role of Dld and Dlv in one of the most unambiguous functions of the hippocampus, spatial learning. We trained goldfish in a spatial constancy task and mapped the activity of Dld, Dlv, and the medial division of the area dorsalis telencephali (Dm) by means of cytochrome oxidase (CO) histochemistry. The results revealed that training goldfish in the spatial constancy task significantly increased the metabolic activity in Dlv, but not in Dld or Dm, suggesting that only Dlv is critically involved in spatial learning and in this regard comparable to the hippocampus. These data provide additional functional support to the hypotheses that consider Dl as a heterogeneous pallial region and propose that Dlv, but not Dld, might be homologous to the hippocampus.