I can’t take credit for this speculation, take it up with Carl Jung. Freud advocated an individual unconscious that repressed that which ego couldn’t handle. Jung went a step further and proposed a collective unconscious. This wasn’t people connected to each other by magic brain waves, it was that evolution left in all human beings an unconscious as a result of our history as a species. It was primitive and highly symbolic, and its inability to rise to the surface of human thought was not a disease process of repression but a reflection of how far down human consciousness went.
Recurring themes, myths, dreams, visions, and art are all a way to access this treasure trove of information. What else would disturb the ink of the poet, the tedious organizing of oral traditions, the slippery memory of a dream, the dissolution of reality of the visionary?
Perhaps a tangent of more empirical source: the degenerative brain disease called Huntington’s disease. Huntington’s disease affects many areas of the brain, including the basal ganglia, known to help modulate movement. The degeneration of the basal ganglia inhibits the afflicted’s ability stop random movement: hence the sudden jerking movements that develop in the afflicted. I bring this up because it suggests that a great deal of brain activity lies not in initiating electrical impulses, but inhibiting them. What lies behind that inhibition?
Perhaps clarity? A great tale in science comes to us from Kekule and the benzene ring. Vexed by his monumental difficulties, Kekule decided to rest. He began to imagine a snake eating its own tail. The metaphor (a common mythological creature) that pointed to the benzene ring that is one of the foundations of organic chemistry (carbon rings).
What lies in the collective unconscious is symbolic, universal truth–a truth undiluted from the lies, blocks, and misperceptions of the ego and its personal unconscious. The ability of the brain to inhibit is among its greatest achievements. That which would crowd or injure conscious space is pushed back. But the ability to go around the brain’s blockades (in fact to recognize the blockades as blockades) is a feat greater still, and requires the ability to let consciousness go (up to a point) and explore the unconscious truth of symbols, be they physical, surreal, or spiritual.