Clues to Understanding the Phylogeny of Behavioral Control

Abstract

Any living organism has, by definition, successfully adapted to its environment. And, broadly speaking, success leaves behind its clues. Reviewing the “duties” of the vertebrate brain provides these clues. According to Ito [149], the vertebrate brain has five basic duties. These duties are common to all vertebrate organisms, from fish to humans. First, there are autonomic and somatic reflexes and second, there are compound movements such as posture, breathing, saccadic eye movements, and locomotion. Many aspects of these functions are mediated by neural activity within the spinal cord and the brainstem. Third, functions such as nutrient intake or feeding, drinking, “fight or flight” mechanisms, and sexual behaviors are all essential to survival and are mediated through the activity of the hypothalamus and limbic system. Many of these behaviors are innate and automatic, demonstrating the adaptive value of automaticity and the lack of the requirement of conscious cognitive control. The speed with which these basic functions operate is quite impressive. The fourth function of the vertebrate brain concerns cerebral, or cortical, sensorimotor activity, as observed in the development of birds and mammals. The sensorimotor regions occupy significant areas within the neocortex, while the activities of mammals, including humans, require the integrated interactions between the cerebral hemispheres, the basal ganglia, and the cerebellum. In this regard, large regions of the cortex are recruited when engaging in activities that are not “cognitive.” Fifth, cerebral association functions developed in primates. These functions, which include thought, are represented within approximately three-fourths of the human neocortex. While the cortex significantly expanded during the course of evolution, a parallel expansion is observed in the cerebellum, with increasing and new cerebro-cerebellar circuitry systems observed as we ascend the phylogenetic scale [150–154]. The expansion of the striatum is considered to be proportional to that of the neocortex in all vertebrates, but in the human brain, the progression of the neocortex greatly outweighs striatal expansion [155]. However, the striatum receives direct projections from all major cortical association regions with the exception of primary visual cortex. The ratio of inputs are as high as 20,000 cortical association neurons to 1 medium spiny cell striatal neuron [156]. This input anatomy provides a substrate for a wide variety of contextual information to be made available to the striatum, enabling the basal ganglia to generalize by recognizing patterns [157]. Finally, it is also notable that higher-order control functions which are presumably dependent upon the neocortex operate much more slowly [149, 158–162].