You might be thinking of your brain all wrong.
Key points
- The cerebrum, especially the cerebral cortex, has long been thought to be the important part of our brain.
- However, the cerbrum contains less than 20 percent of the neurons in our brain.
- Eighty percent of our brain cells are in the cerebellum, which performs functions linked to the unconscious
Your mental image of your brain probably looks something like what's pictured here.
The cerebrum, shown here, is where all the really important stuff is supposed to happen in humans: consciousness, executive function, perception, motor control, a lot of memory, and, of course, the infamous amygdala where “fear” reputedly lives. The large size of the cerebrum—especially the cerebral cortex—relative to the rest of the brain, is a uniquely human trait that conventional wisdom says makes us smarter than animals.
Indeed, in most conversations, you could substitute "cerebrum" for "brain" and no one would quibble with your choice of words. And "cerebral" is synonymous with "smart" in common parlance.
There’s just one problem with all of this: the human cerebrum contains less than 20 percent of the brain’s roughly 100 billion neurons. [1,2] If, as Yale neuroscientists Robert Williams and Karl Herrup assert [3], the number of neurons in a brain region is a measure of the importance of that region for behavior, shouldn’t we pay more attention to a single brain region that contains over 80 percent of the neurons in the human brain? And what is that region anyhow?
An Overlooked Brain Structure
The answers to the above questions are "yes" and "the cerebellum": the small, football-shaped structure tucked in behind the cerebrum, almost as an afterthought. You can see it in the image here.
The cerebellum is so inconspicuous that if one omitted it from an image of the brain, most people wouldn’t notice. Indeed, cerebellum is Latin for “little brain,” implying that the structure has correspondingly minor importance.
When I learned neuroanatomy—and later taught it in a medical school—neuroscientists used to tell students that people born without a cerebellum (a rare condition called cerebellar agenesis) can lead relatively normal lives, as evidenced by case studies [11]. But according to neuroanatomist Dr. Herculano-Houzel, the human cerebellum contains 80.2 percent of the neurons in the human brain, compared to a meager 18.6 percent in the cerebrum (with 1.2 percent residing in the brainstem, midbrain, and other subcortical structures such as the hypothalamus). [1,2].
So what are all those cerebellar cells doing?
Traditionally, the cerebellum was thought of primarily as a motor structure, critical to fine movements, balance, and eye movement. [4]
More recently, the importance of the cerebellum for speech (especially the incredibly complex coordination of tongue, lips, jaw, and throat muscles of speech), emotional regulation, memory, cognitive function, and prediction and planning has come to light. [5-10]. Cerebellar cognitive affective syndrome (CCAS), a condition which results from cerebellar stroke, tumor, or trauma, underscores the importance of the cerebellum for non-motor functions, by highlighting deficits in executive function (e.g. planning), language (grammar deficiencies, word choicem atonal speech), spatial abilities (orientation vs disorientation) and mood (flat affect). [6]
The Cerebellum and Our Unconscious
One reason the cerebellum may get so little respect is that much of what it does occurs quickly, automatically, and without conscious thought.
When we speak, we don’t consciously plan where we’ll place our tongue with each upcoming syllable. For that matter, most of us don’t know which word we will utter next until we hear ourselves say it. Nor do we notice the incredible precision with which we predict how much the image of the world will move on our retinas when our gaze is about to shift, so that we can “zero out” that retinal movement to keep the world from seeming to jump around when we move our eyes.
The reason is that the cerebellum is built for speed and precision, not considered thought. The majority of cerebellar cells, called granule cells, are small, densely packed neurons with relatively few synapses per neuron (<10), which collectively act as blindingly fast parallel processors. In contrast, the mainstay of the cerebral cortex, more loosely packed pyramidal cells, typically have tens of thousands of synapses per neuron, operate more deliberately, and are associated with conscious experience (perception, memory, thought).
Although many unconscious processes also occur in the cerebral cortex [11] and in subcortical and brainstem areas (heart rate regulation, blood pressure, hunger, thermoregulation) [12-16], a significant portion of what we think of as “the unconscious” takes place in the 80 percent of our brain cells that get no respect.
Your cerebellum already knew that; now your cerebrum does too.