What Makes the Human Brain “Human?” Part 2

The first part of of What Makes the Human Brain “Human?” (April 16, 2017) introduced the question of how human brain structure differs from the brains of other animals. It was emphasized that brains are genuine complex systems, producing dynamic patterns and sub-patterns of electrical and chemical activity at multiple levels of organization (spatial scales). In order to better understand these patterns, brain science advances in small steps—early experimental results lead to new hypotheses, new experiments, new results, and the process is repeated. In this manner, scientific truths are approached in a series of successive approximations. But, here we turn this process upside down by speculating that brains share some basic features with known complex physical and social systems that may serve as helpful analogs. The accompanying figure indicates that for this process to be scientifically useful, such speculations should be checked to see if they are consistent with established brain science. If such ideas do not seem to violate known science, we may label them “brain friendly” and take them more seriously.

Source: Paul Nunez

Essential questions about the behavior of nearly any complex system involve a distinction between the extreme states of localization (isolation) versus global states. In other words, we ask if the system in question behaves mostly like a single integrated entity, implying strong on-going interactions between all of its smaller parts. Or does the system consist of many little sub-systems that act more or less independently? This idea is nicely illustrated by fans interacting in a football stadium; the fan’s collective behavior is analogous to various brain states. Before the game begins, most interactions occur between persons sitting close together; the individual conversations are largely unrelated. We may call this condition a football fan state of functional localization. The opposite state occurs when a touchdown is scored and the home team’s fans are all cheering together; let’s call this condition global coherence. But even in this global state, local pockets of disgruntled visitors and tipsy drinkers remain embedded in the global system.

The local-global issue is closely related to brain health. Brain science suggests that the most complex brain states occur between the extremes of fully local and fully global behavior, thereby suggesting a correspondence between brain complexity and healthy consciousness. Chemical (neurotransmitter) systems may act to move the brain to different places along the local-global gamut of dynamic behaviors. Different chemicals may alter the coupling strength between distinct cortical areas by selective actions at different cortical depths. A healthy consciousness is associated with a proper balance between local, regional, and global mechanisms. The actual brain states that appear analogous to these metaphorical football states range from schizophrenia and autism (extreme localization) to healthy states (moderate localization) to coma or anesthesia (extreme global coherence).

How might human brain physiology have evolved to generate complex patterns, and also allow brains to transition easily between their localized and global states? In particular, the preponderance of small world connections between distant regions of the human cerebral cortex provides one plausible means for producing a broad range of complex brain patterns. Each pair of neurons in the cerebral cortex of mammals may be separated by a path length of only two or three synapses. This suggests that an action potential from a neuron targets a secondary neuron which, in turn, targets a third neuron, and so forth. But, only two or three such steps may be required for influences from one region to spread to distant cortex. Such cortico-cortical path lengths are analogous to the global human social network with its so-called six degrees of separation between any two humans, meaning a path length of no more than six social contacts world-wide. Pick someone at random, say a man from Taiwan. You probably know someone, who knows someone, and so forth who knows this man, with perhaps only six acquaintance steps needed to complete the path. We humans live in a small world social network. Several other properties are often associated with small-world networks. Typically there is an over-abundance of hubs—nodes in the network with a high number of connections. By analogy, the small-world networks of airline flight paths have short travel distances (path lengths) between any two cities because many flights are routed through hub cities.

Source: Paul Nunez

The cerebral cortex forms an outer brain layer. How do our arguments about small world cortical connections pertain to the brains of different mammals? Suppose we count the number of axons entering and leaving a patch of the underside of cortex. Some of these fibers connect cortex to cortex (cortico-cortical fibers); others connect cortex to deep (midbrain) structures, especially the thalamus. In the human brain only about 2 percent to 5 percent of human fibers entering or leaving the cortex are connected to midbrain structures. That is, the vast majority of these fibers (axons) in humans are cortico-cortical. By contrast, the relative density of cortico-cortical axons is much lower in lower mammals, perhaps only about 50 percent in rats, for example.

It appears that the relative density of cortico-cortical fibers becomes progressively larger as mammals become capable of more complex behavior. This seems to makes intuitive sense—I claim that I am smarter than my dog Savannah, but in what sense am I smarter? My olfactory cortex is a moron compared to Savannah’s. Somehow our humanness seems to originate from global interactions of multiple cortical neurons and columns at different scales within the nested hierarchy of cortical tissue. Dynamic feedback linking cortex to cortex may be relatively more important in humans than in lower mammals.