How the Brain Develops and How It Functions

For many models of the cerebral cortex, particularly developmental ones, knowledge of cortical structure is essential to a proper understanding of its functional capacities. We have studied the microscopic neuroanatomic changes of the postnatal human cerebral cortex during its development from birth to 72 mo. The microscopic structural changes we have identified to date are complex, yet well organized, and mathematically describable. The discoveries to date arising from analyses of the Conel data include: 1. That the total number of cortical neurons increases by 1/3 from term birth to 3 mo, then decreases back to the birth value by 15 mo, then increases by approximately 70% above the birth value from 15 to 72 mo. 2. Based on 35 cortical areas, the mean number of neurons under 1 mm2 of cortical surface extending the depth of the cortex decreases by 50% from term birth to 15 mo, then from 15 to 72 mo, increases by 70% above the value at 15 mo. Both of the previous findings provided the first evidence for postnatal mammalian (human) neocortical neurogenesis. These findings have received subsequent support from studies demonstrating cortical neurogenesis in adult macaque monkeys. 3. That changes in total cortical neuron number from birth to 72 mo inversely correlate strongly (ρ = −0.73) with the number of new behaviors acquired during this time. The correlation appears strongest when there is a time delay, suggesting that changes in cortical neuron number precede the appearance of newly acquired behaviors. 4. That each of 35 cortical areas analyzed show characteristic increases and decreases in neuron number in a wave-like fashion from birth to 72 mo, suggesting local regulatory control of both neuronal cell death and neurogenesis. The changes in neuron number appear to follow gradients that correspond to functional cortical systems, including frontal (motor, dorsolateral prefrontal, and orbitofrontal, separately), visual (ventral and dorsal streams, separately), and auditory systems. 5. That within any given cortical area from birth to 72 mo, there are functionally related shifts in the relative numbers of neurons in the six cortical layers. Since each of the six cortical layers has a specific function with specific communication to other layers, the neocortex can create 720 variations (6!) in its function just by changing the relative power of the six cortical layers in all possible permutations. The data clearly show that only a few of these permutations are actually used. It appears that the function of secondary association neocortical areas or higher are most developed when layers III and VI have the most neurons, and that the function of primary sensory, 1st order association, or transitional (i.e., cingulate) neocortical areas are most developed when layers III and IV have the most neurons. Layer III is primarily responsible for long distance cortico-cortical communication; layer IV is primarily responsible for receiving thalamic sensory information from the environment plus feedforward cortico-cortical communication; and layer VI is primarily responsible for sending cortical information back to the thalamus and receiving feedback cortico-cortical information. In this chapter, we present the data and studies that formed the basis for the above discoveries in postnatal human cerebral cortex from birth to 72 mo. These data have particular relevance to those interested in building computational models of cortical development and provide a basis for concomitant cortical electrophysiological and behavioral developmental changes. Computational models incorporating such knowledge may provide a mechanistic understanding of how the brain develops and how it functions.