Kaas (1990) notes an evolutionary trend towards increasing numbers of specialized areas in mammalian neocortex in animals of increasing size. For example, while the hedgehog has only 8 clearly defined regions in its cortex, the cat has at least 24. Along with this increase in structural complexity is an increase in behavioral sophistication. This trend continues in primates. Felleman and Van Essen (1991) identified at least 32 distinct areas in Macaque monkey visual cortex, interconnected by as many as 300 reentrant fiber projections. Whether or not this upward trend continues in man is not completely clear at this time. Brodmann's cytoarchitectonic maps of humans and monkeys (reproduced in Pandya et al, 1988) show similar areal divisions in visual cortex, but an increased level of complexity in areas related to hearing and language in the human. Brodmann's map of monkey visual cortex is much simpler than Felleman and Van Essen's map, which is based on electrophysiological measurements and connection studies as well as cytoarchitectonics. Many of these same studies cannot be done on human subjects.
A number of theories have been proposed to account for the trend of evolution of the mammalian brain. These theories are reviewed in Deacon (1990). An early 20th century synthesis held that the primary sensory cortical areas appeared first in mammalian brain evolution, followed by association areas (including language areas) which were inserted into the spaces between primary sensory cortex and limbic cortex. The emergent association areas were viewed as the highest, most complex and most powerful products of the evolutionary process. This view was based on arguments from homology ("primitive" mammals have primary sensory areas, but little if any associative areas) and from ontogeny (association cortex was seen as maturing later because it myelinates later.)
Pandya et al. (1988) and Sanides (1969), while accepting an additive view of the evolutionary process, proposed that the early 20th century view of the sequence of development of primary, secondary and associative cortex was inverted. Their argument was based on an alternative view of the significance of evolutionary trends in myelination: since primitive mammals exhibit little myelination, they argued that greater myelination (such as exhibited in the primary sensory and motor areas of higher mammals) is a sign of later evolutionary development. Thus, according to Sanides and Pandya's model, sensory systems evolved out of a process of differentiation of neocortical areas arising from limbic cortex. Furthermore, the neocortex is seen as originating from one of two primordial moieties: either archicortex (hippocampal cortex) or paleocortex (olfactory and pyriform cortex.) According to this model, as successive new cortical areas were elaborated during the evolutionary process, they became enervated by new projections from thalamic sensory centers. Primary and secondary auditory, sensorimotor and visual cortex were built as superstructures arising out of limbic cortex; they exist as islands of neocortex arising out of the primitive limbic structures, with few if any interconnections at primary input/output levels between the modalities. In this model, polymodal cortex exists primarily in limbic paleocortex. Thus, coupling between the visual system, auditory system, somatosensory system and the motor systems must also exist primarily at the level of limbic cortex.
As an alternative to both the early 20th century synthesis and the mechanisms suggested by Pandya et al. (1988) and Sanides (1969), Deacon proposes a symmetrical differentiation process, followed by parcellation of connections. According to this "displacement hypothesis", parcellation and invasion events are caused by competitive local factors. In this scenario, many events in the development of neocortex in mammalian evolution are the result of a general growth in the size of the cortex, followed by parcellation of connections according to conservative genetic rules which are determined as the outcome of a morphoregulatory process involving cell adhesion molecules (CAMs), substrate adhesion molecules (SAMs) and cell junctional molecules (CJMs), which regulate the development of the growth cones of axonal and dendritic projections (Edelman, 1987, 1992).
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