The Origins of Neocortex: Connections and Lamination as Distinct Events in Evolution

Abstract

In the nonmammalian telencephalon, there are neuronal populations corresponding to cell groups in the neocortex of mammals in terms of connections, single unit-responses, chemical content, and functions. Some of these populations in nonmammals, however, are organized in a nonlaminar, rather than laminar fashion. These observations may prompt a reassessment of the functional roles of lamination and the evolutionary origins of the mammalian neocortex. Thus, the role of neural circuits and laminar organization can be differentiated in order to understand the cognitive functions of the neocortex. Moreover, the origins of neocortex can be separable into the precursors of nonlaminar and laminar regions.

A Dissociation of Motion and Spatial-Pattern Vision in the Avian Telencephalon: Implications for the Evolution of “Visual Streams”

“…A much simplified schematic of the mammalian tectofugal pathway is shown in Figure 5 (right). The homolog of nRT in mammals is those areas in the thalamus that receive input from the superior colliculus (i.e., tectum) and/or project to the telencephalon (Nauta and Karten, 1970;Karten, 1979;Karten and Shimizu, 1989;Husband and Shimizu, 2001). This includes the lateral posterior nucleus of the thalamus and the adjacent pulvinar in cats and squirrels (Robson and Hall, 1977;Chalupa et al, 1983;Kelly et al, 2003) and parts of the medial, lateral, and inferior pulvinar subdivisions in primates (Soares et al, 2001).…”

“…There are many similarities with respect to the neuroanatomical organization and physiology of the avian and mammalian subcortical visual systems but marked differences with respect to telencephalic structures (Karten and Shimizu, 1989;Engelage and Bischof, 1993;Shimizu and Bowers, 1999). For example, although the avian visual wulst is thought to be the homolog of mammalian visual cortex (Medina and Reiner, 2000), lesions to the visual wulst result in only minor impairments in visual tasks (Hodos et al, 1984).…”

“…For example, although the avian visual wulst is thought to be the homolog of mammalian visual cortex (Medina and Reiner, 2000), lesions to the visual wulst result in only minor impairments in visual tasks (Hodos et al, 1984). Furthermore, in birds, it appears that the tectofugal pathway is highly developed compared with the thalamofugal pathway, but the opposite is true in primates (Macphail, 1982;Karten and Shimizu, 1989;Butler and Hodos, 1996;Husband and Shimizu, 2001). …”

“…In the tectofugal system, the avian nucleus rotundus (nRT) receives a robust tectal input (Karten and Revzin, 1966) and is comparable with parts of the pulvinar nuclei and the lateral posterior nucleus of the thalamus in mammals (Nauta and Karten, 1970;Karten, 1979;Karten and Shimizu, 1989;Husband and Shimizu, 2001). On the basis of neurochemistry, cytoarchitecture, tectal innervation, and physiology, nRT has been divided into several subnuclei (Benowitz and Karten, 1976;Martinez-dela-Torre et al, 1990;Mpodozis et al, 1996;Marin et al, 2003).…”

“…The nRT projects to a telencephalic structure, the ectostriatum (Ec) , which is possibly homologous to the thalamo-recipient cells in mammalian extrastriate cortices (Karten and Shimizu, 1989;Butler and Hodos, 1996;Shimizu and Bowers, 1999). The nRT-Ec projection is topographic: the caudal and rostral Ec receive input from subnuclei in the caudal and rostral nRT, respectively (Benowitz and Karten, 1976;Nixdorf and Bischof, 1982).…”