Those of you who are familiar with my work know that I have a thing for metaphors. Those of you who have been to my workshops know that I have a thing for the brain. I have been delighted to read Giorgio Ascoli’s book, Trees of the Brain, Roots of the Mind (MIT Press). In this illustrated book, Ascoli presents the primary metaphor of the brain: Trees. Neurons branch like trees. The photographs of trees and the colorful plates of actual neurons provide a compelling way for understanding the structure of the brain.

Like trees, neurons have a trunk, roots. and branches. Unlike real forests, the forests of the brain are densely packed with neurons–unimaginably dense. Also, neurons are so fine that if they were trees, they would reach miles into the sky.

There are two basic types of neurons in the brain: principle neurons and interneurons. These neuron types lend themselves to a metaphor on the differences between introverts and extroverts.

The fact that pyramidal cells (principle neurons) communicate with other brain regions to stimulate activity, whereas interneurons only talk to neighbors to keep them quiet, suggests a division of labor between protagonists and supporters. In this view excitatory neurons would be the protagonists of cortical computation, representing, processing, and transmitting the content of information. Inhibitory neurons would provide a supporting role, maintain the proper tempo of activity and fine-tuning network dynamics.

While admittedly oversimplified, this distinction is a metaphor for the differences between extroverts and introverts and why both aspects are necessary for function. There are exceptions to the rule that interneurons are always inhibitory, but mostly they are.

Introverts have always provided a heedful, inhibitory function in groups. Within an individual, introverted qualities provide the quiet tendency to pause and reflect and not to act (when action would not be appropriate).

The extroverted neurons are the stars of the show–flashy, active, and brash. The introverted interneurons quietly go about their business making the show run behind the scene.

While a metaphor, it is interesting to speculate how the basic personality difference between introverts and extroverts is mirrored in the basic building blocks of mental life (indeed all of life).

This is also an equalizing metaphor. Neither type of neuron is better than the other. They are both required and we can’t function without both the noisy and quiet aspects. Within ourselves, too, we can see both action and reserve as necessary parts of life. Extreme imbalance in either direction will result in the extroverted circus on the one hand or immobilization on the other.

As with all metaphors, not every aspect fits perfectly. Introverts can and do provide supportive roles, working behind the scenes. However, we can also take the lead role, like the principal neurons, engaging in quiet leadership, thoughtful action, and wise resolve.