Evolution of neocortex
We began by reviewing the contribution made by comparative anatomists at the turn of the century to our understanding of neocortex. In lower mammals such as the hedgehog or opossum, almost all of the neocortex is devoted to three regions-visual, auditory, and sensory-motor. Each of these cortical areas receives sensory fibers from a thalamic relay nucleus. In higher mammals-man and species closely related to man-the neocortex has greatly expanded and these three regions occupy a relatively small proportion of the whole. Most of the remaining neocortex was called association cortex since it receives "association" or corticocortical fibers from the sensory and motor areas. These results clearly defined the task of comparative psychology: to demonstrate that the function of association cortex consisted in the formation of associations. But no learning task that could measure and reflect the size of association cortex was ever invented. Further, the method of ablation failed to localize the engram or uncover the neural mechanism of learning. These disappointments led to doubts about the basic assumption that species occupying diverse niches could be compared on a common abstract scale, and ultimately led to a bifurcation of comparative neurology and psychology. In our own experiments on the visual cortex of two "primitive" species, the hedgehog and the tree shrew, we have been able to take advantage of the considerable progress that has been made in anatomical, physiological, and behavioral ablation studies of neocortex. In particular, the continuing investigation of the thalamus has led to the conclusion that the association cortex of man and higher primates is not restricted to a reception of corticocortical fibers but, like sensory cortex, receives fibers from the thalamus. One thalamic nucleus in particular stands out-the pulvinar-both because of the extent of its cortical target and because of the degree of its internal differentiation. A precursor to the pulvinar, the lateroposterior nucleus, can be found in the lowly hedgehog, and its further development can be traced through comparisons between hedgehog and tree shrew. A clue to its function is provided by the fact that visual impulses project to it from the tectum. In the hedgehog we find that both lateral geniculate and lateroposterior nuclei relay visual impulses to an extensive posterior sector of neocortex comprised of two architectonic subdivisions - a core and a belt. The boundaries of the core coincide with visual area I as defined by evoked potentials; visual area II occupies the belt. Both cortical areas are implicated in pattern discrimination. We have concluded that the two visual projection systems may represent stages of phyletic development, arguing that the tectal-belt pathway may have come first in evolution and may have given rise to both the striate cortex and the association cortex found in advanced mammals. In the hedgehog the two visual systems have not achieved the degree of independence found in higher mammals, and we take this to be a sign of the hedgehog's primitive level of organization as well as support for our thesis. A complete separation of the cortical projections of the lateral geniculate and pulvinar nucleus is attained in the tree shrew. The ablation method can therefore tell us something about the function of each system: the geniculostriate system is not necessary for simple sensory tasks, but the striate cortex is required for a higher level of integration. A clue to this higher function is provided by the failure of the animals with brain lesions to distinguish a figure embedded in a larger pattern. On the other hand, the pulvinar system is implicated in learning visual patterns. Further behavioral evidence - the loss of reversal-learning sets in animals with lesions of the temporal area - tempted us to speculate that the temporal lobe has begun to assume primate-like functions. In carnivores, on the other hand, the temporal area expands in response to projections from the medial geniculate, and as a result the lateral surface of the neocortex is dominated by the auditory cortex and not the cortical target of the pulvinar. We now propose that the functions of this expanded auditory cortex may be considered as contributing to the highest behavioral capacities in the carnivore repertoire. Finally, we offer evidence that the primary and secondary visual systems of the squirrel and the tree shrew have evolved along similar lines, apparently in response to similar ecological niches. It is our hope that the continuing inquiry into the converging and diverging evolution of cortex in the several lines of mammalian descent will reunite comparative neurology and psychology - a union originally envisioned by Elliot Smith.
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