Telencephalic Components A The Pallium

Recently, the pallial cover of the telencephalic vesicle has been shown to be divided molecularly and structurally into four pallial territories, that are common to all vertebrates: the medial, dorsal, lateral, and ventral pallial domains (Fig. 8). The medial and dorsal pallial parts mature structurally into purely cortical centers. The medial pallium primarily forms the hippocampal cortex (allocortex; three-layered), although parts of the surrounding transitional cingu-late/entorhinal cortex (mesocortex; four- or five-layered) may have the same origin. This functionally connected complex is subsumed under the term ''limbic lobe'' and is important in the evaluation of experiences, motivation of behavior, emotions and memory.

The dorsal pallium forms the isocortex (six- or seven-layered; predominant in relative surface extent). This domain specializes into separate sensory or motor cortical areas. Primary sensory cortex processes relatively simple sensory data; secondary cortex generates more complex perceptual constructs of the external world and the body. Associational cortical areas are where higher mental functions are believed to occur (Figs. 9B and 9C). The ancestral dorsal pallium appears as a variably sized island surrounded by the rostrally and caudally interconnected medial and lateral pallia; the reduced sensory/motor and associative representations partly overlap (Fig. 9A). The dorsal pallium is the portion that expands more in mammalian evolution so that the isocortex represents the largest and most variably subdivided cortical domain in mammalian species (Figs. 9B and 9C). Major axonal outputs of the isocortex (apart from the massive layer II/III corticocortical projections, both within the same hemisphere and projecting into the contralateral hemisphere using the corpus callosum) are the corticoclaustral, corticostriatal, corticothalamic, corticopretectal-collicular, corticoreticular, cor-ticopontine, corticonuclear, and corticospinal fiber pathways. They jointly constitute a potent means for

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Figure 9 Drawings of flattened cortical surface, with hypothetical three stages of cortical evolution in mammals. The progressive expansion and diversification of specialized and associative fields within the isocortex is put in the context of its invariant topology, both externally [with respect to the surrounding limbic and piriform (PIR) cortexes] and internally (with respect to each area's topological relationship to the other isocortical areas). Dashed lines indicate imprecise boundaries. The thick black lines in B and C indicate the boundary between frontal lobe (action planning and motor functions) and the parietal, occipital, and temporal lobes (sensorial analysis and abstraction). The different patterns of lines in C symbolize associative multimodal intermixing of analysis properties. In man, the number of identified visual areas is approximately 20, and similar numbers may exist for the other specialized regions; this indicates the relative complexity of the functions performed in these domains. See Table I for abbreviations.

Figure 9 Drawings of flattened cortical surface, with hypothetical three stages of cortical evolution in mammals. The progressive expansion and diversification of specialized and associative fields within the isocortex is put in the context of its invariant topology, both externally [with respect to the surrounding limbic and piriform (PIR) cortexes] and internally (with respect to each area's topological relationship to the other isocortical areas). Dashed lines indicate imprecise boundaries. The thick black lines in B and C indicate the boundary between frontal lobe (action planning and motor functions) and the parietal, occipital, and temporal lobes (sensorial analysis and abstraction). The different patterns of lines in C symbolize associative multimodal intermixing of analysis properties. In man, the number of identified visual areas is approximately 20, and similar numbers may exist for the other specialized regions; this indicates the relative complexity of the functions performed in these domains. See Table I for abbreviations.

descending modulation of subjacent alar and basal centers, and they arise primarily from layer V pyramidal neurons (except those to claustrum and thalamus, which arise from layer VI neurons). The acquisition of direct cortical innervation of motoneurons seems to be a relatively recent evolutionary addition since it is absent in nonmammals.

The lateral and ventral pallial domains both give rise to portions of the olfactory cortex. In addition, each develops underlying pallial nuclei, which have been classified tentatively according to their molecular profiles (Figs. 8A and 8B). The lateral pallial nuclear formation consists rostrally of the dorsolateral claustrum and caudally of the endopiriform nucleus and basolateral parts of the amygdala (plus associated cortical amygdala domains). The ventral pallial nuclear formation is represented rostrally by the ventrome-dial claustrum and caudally by the lateral and basomedial parts of the amygdala (and associated cortical parts). Little is known of the functions of the claustrum, although the dorsolateral part has a bidirectional topographically ordered projection with the whole isocortex. The ventromedial claustrum may receive multimodal thalamic connections. The pallial nuclei of the amygdala are known to integrate multiple inputs from the isocortex, apparently integrating recognition of contextual Gestalts (characteristic sense data configurations) with entrainment of appropriate instinctive or learned (associated) behavior through motivational (limbic) and direct descending efferents onto extratelencephalic circuitry. It should be noted that the amygdala was long held to be only a part of the basal ganglia, but it has recently emerged as a heterogeneous complex of interactive pallial and subpallial elements (Fig. 8C). The same can be said for the telencephalic septum, found at the interhemi-spheric or medial wall, which also shows pallial and subpallial portions, although in this case the pallial part (the dorsalmost part of the septum) is much smaller than its subpallial constituents.

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