Cortical Architecture

The evolution of the structure of the neocortex is one of the most important events in the chain that led to the human brain. The paleontological evidence shows that the human brain expanded two-fold in size over three million years, while modern chimpanzees still have brains about the size of the earliest hominids. The brains of chimpanzees and modern humans have a similar anatomy, so the vast difference in their size (400ml vs 1400ml) is due to an expansion of the cerebral cortex, rather than the development of entirely novel brain structures. Here we explore in what way the neocortical circuits are common to all mammalian species. We define a canonical structure that can be identified in all cortical areas and in all land-based mammalian species where data are available. This structure has recurrent excitatory and inhibitory loops formed by local neurons as a feature of its design. Quantitative studies from our laboratory show that the input from the sensory periphery forms less than one percent of the total input to the primary visual cortex in the cat. Thus the major synaptic input to a cortical neuron comes from its neighbors. We provide a conceptual model that offers an operational view of how the canonical circuit of the neocortex might operate.