Cortical Evolution Research Articles

Images in neuroscience. Brain development, VI: radial migration and cortical evolution.

Images in neuroscience. Brain development, VI: radial migration and cortical evolution.

The neural basis of cognitive development: a constructivist manifesto.

How do minds emerge from developing brains? According to "neural constructivism," the representational features of cortex are built from the dynamic interaction between neural growth mechanisms and environmentally derived neural activity. Contrary to popular selectionist models that emphasize regressive mechanisms, the neurobiological evidence suggests that this growth is a progressive increase in the representational properties of cortex. The interaction between the environment and neural growth results in a flexible type of learning: "constructive learning" minimizes the need for prespecification in accordance with recent neurobiological evidence that the developing cerebral cortex is largely free of domain-specific structure. Instead, the representational properties of cortex are built by the nature of the problem domain confronting it. This uniquely powerful and general learning strategy undermines the central assumption of classical learnability theory, that the learning properties of a system can be deduced from a fixed computational architecture. Neural constructivism suggests that the evolutionary emergence of neocortex in mammals is a progression toward more flexible representational structures, in contrast to the popular view of cortical evolution as an increase in innate, specialized circuits. Human cortical postnatal development is also more extensive and protracted than generally supposed, suggesting that cortex has evolved so as to maximize the capacity of environmental structure to shape its structure and function through constructive learning.

Exuberant development of connections, and its possible permissive role in cortical evolution

The callosal visual connections of the cat provide a model for studying the phenotypes of cortical axons and their differentiation. The terminal arbor of a callosal axon develops in several successive stages. At each stage, the arbor approximates the adult phenotype more closely. This is achieved through two mechanisms: (1) exuberant, but increasingly constrained, growth and (2) partial deletion of previously generated parts of the arbor. This differentiation is controlled by interactions of the axon with its cellular environment, and by visual experience. It might have played a permissive role in the evolution of the cerebral cortex by enabling adjustments of cortical connectivity to changes in the number, size, internal organization and cellular composition of cortical areas.

Organization of cortical afferent input to orbitofrontal areas in the rhesus monkey

Odorant signal processing takes place in a diverse group of primary olfactory areas which receive direct input from the olfactory bulb. Orbitofrontal cortices participate in olfactory functions, but the pathways through which they receive olfactory or other input have not been clearly defined. The retrograde tracers horseradish peroxidase and fluorescent dyes were injected in orbital cortices to study their afferent cortical connections. Labeled neurons in primary olfactory areas (prepiriform cortex, anterior olfactory nucleus and olfactory tubercle) were directed mainly to a posterior orbitofrontal region and to a lesser extent the neighboring caudal part of area 13. There was no evidence of direct projections from primary olfactory areas to the rostral parts of area 13, or to areas 12 or 11. Most labeled neurons in primary olfactory areas were directed to agranular cortices, fewer projected to dysgranular areas, and there was no evidence that any reached granular cortices. The areas which received the most robust olfactory projections showed the lowest degree of laminar organization among prefrontal cortices. Early processing in the olfactory system thus takes place in areas which differ sharply on structural grounds from “early” eulaminate post-Rolandic sensory cortices. In addition to olfactory cortical projections, numerous labeled neurons in transitional (limbic) cortices were directed to orbital areas, and fewer but still substantial numbers of afferent neurons were found in eulaminate cortices. Unlike post-Rolandic unimodal sensory areas, which seem to be committed to the processing of input from one sensory modality via sequential and/or parallel pathways, caudal orbital areas received highly distributed input from primary olfactory areas, and in addition, from gustatory, visual, auditory and somatosensory areas. The structural and connectional features of olfactory recipient orbital cortices thus differ markedly from those observed in other sensory association areas and suggest a mode of processing adapted early in cortical evolution.

Conservative Features of Neocortical Evolution in Dolphin Brain

A Golgi survey of the convexity cortex in the brain of the dolphin, Tursiops truncatus, has revealed many cellular characteristics which may be indicative of conservative cortical evolution. These include a high degree of pyramidalization, and an accentuation of layer II. The presence of an accentuated layer II in convexity cortex is a protoneocortical characteristic found in more 'primitive' cortical arrangements. The growth ring concepts of cortical development outward in concentric waves from archicortical and paleocortical origins are discussed. In that context we have not been able to identify cores of hyperspecialization in the dolphin cortex corresponding to koniocortex and gigantopyramidal areas. This leads us to suggest that the cortex of the dolphin reflects a condition of the paralimbic-parinsular stage of evolutionary development. Thus, the dolphin brain may serve as a model of the theoretical mammalian archetype brain and its study may shed light on the organization of the brains of the initial ancestors of modern mammals.

The learning skills of great apes

Through use of a refined measurement of learning-set capacity, a comparative study of 45 great apes, six gibbons, and representative monkey groups was conducted to determine the relationship between cortical evolution and cognitive skills. The measurement employed. termed the Transfer Index, entailed criterional achievement on each of a series of two-choice visual discrimination problems, then test trials in which the initial cue values were reversed. The pre-reversal criterional mastery served as an empirical basis for matching subjects of the diverse primate groups on a performance criterion prior to the procurement of the critical measures taken on the reversal trials to assess learning and transfer-to-training capabilities. The results reveal the following: (1) general superiority of the great apes over all other primate groups tested; (2) interactions between species, pre-reversal criterional level employed, and reversal performance; and (3) support for the conclusion that learning and transfer-of-training skills are positive correlates of cortical evolution.