QR6.2.5 Three-center Theory

Figure 6.23 Embryo brain divisions

The human brain grows from a neural tube with forebrain, midbrain and hindbrain areas that later form the cortex, limbic system and cerebellum (Figure 6.23). This basic division reflects the three types of processing:

1. Input processing. What is out there?

2. State processing. What is the body state?

3. Output processing. What actions can be done?

Life involves all three, as animals must sense food or danger, know if the body is hungry or tired and coordinate actions like biting to survive. The basic parts of the neural tube then evolved analyze input sense patterns, evaluate body state feelings and coordinate muscle habits, and this evolution occurred in parallel.

Figure 6.24 Brain processing centers

An engineer might design a feedback system to analyze sense input, then body state and then check available muscle responses before combining them in an integrated way (Figure 6.24), but evolution works in parallel not in sequence, so given three types of useful brain processing, it developed them all at once. The original neural tube areas evolved independently because the system has no single center.

Three center theory is that the hindbrain, midbrain and forebrain evolved as independent feedback control centers that allow three types of feedback control:

1. Sensory control: Based on input sense patterns.

2. State control: Based on body state feelings.

3. Movement control: Based on muscle habits.

It implies that how an animal responds depends on which control center drives the feedback loop. In a given moment, an animal with a chance to bite might do so under movement control, or might freeze in place from fear under state control, or might decide that the threat isn’t really dangerous and ignore it under sensory control.

Rather than sense processing developing first, it seems that motor processing  did, perhaps because action is generally better than inaction. Single-celled life moved before it saw and motor nerves develop in embryos before sensory ones, as babies kick in the womb before their eyes even start working.

Figure 6.25 The fish brain has three parts

For example, fish brains have forebrain optical and olfactory areas to process sense data, a midbrain amygdala and pituitary to manage endocrine tasks and a hindbrain to handle movement activity (Figure 6.25). All three basic brain processes exist but the hindbrain cerebellum of fish is far more evolved than its cortex (Montgomery et al., 2012), so it is more likely that it controlled the feedback loop by accessing data from the primitive forebrain and midbrain.

As it happens, the forebrain area that receives sense data from the muscles is next to the area that directs movement in fish, as it is for us, perhaps because using the same paths for both was easier and having sensory and motor areas nearby helps sensorimotor timing. The motor cortex directs movement for us but for fish, a simpler assumption is that the cerebellum controls movement, as it projects both excitatory and inhibitory nerves to the motor cortex (Daskalakis et al., 2004). If the fish hindbrain was the first autonomous control center to evolve, the cerebellum can initiate motor acts using links to the still developing forebrain and midbrain.

Figure 6.26 The hindbrain

In humans, the hindbrain bulges out from the base of brain as the cerebellum (Figure 6.26). It may be ancient but the cerebellum has more nerves than the rest of the brain put together! Its two cross-linked hemispheres are known to control complex movement and it was certainly the most advanced part of the brain when reptiles ruled the earth. If this movement control center drove the brain-world feedback loop at that time, does it still exist in us today?

In infant swimming, babies instinctively hold their breath underwater thanks to a diving reflex and move their arms and legs in parallel to propel them through the water by an amphibian reflex that flexes same-side hip and knee kicks. These instinctive actions disappear later, as the child learns to swim as people do, by moving limbs alternately. That babies “swim” as reptiles do but lose the ability after about four months suggests the brain retraces its reptilian ancestry as it matures.

 In parasomnia, sleepwalkers can get up, walk, eat, cook dinner or ride a motorbike while asleep but awake later with no recall. With the cortex and midbrain dormant, the hindbrain moves the body by itself. There is no recall because the  midbrain doesn’t lay down memories. Sleepwalking behavior isn’t just reflexes, as cooking a meal is a purposeful act that requires constant situational adaptation. It follows that the hindbrain can act like a brain in itself that requires neither intellect nor episodic memory help to control the body entirely.

An autonomous movement center also explains blindsight, where people with visual cortex damage report seeing nothing but can still catch a ball and insert an object into a tilted slot whose orientation they say they can’t see (Goodale & Milner, 2004). When cortical systems that identify objects fail, the hindbrain uses older subcortical paths to handle spatial location and direct motor acts by implicit perception (Hannula et al., 2005). It can still use that primitive circuitry to direct motor output because it evolved when the cortex was still in its infancy.

When cortical systems fail, older ones take their place, so aphasic subjects who can’t speak due to cortical damage can still swear and sing. Amnesic patients given the same jigsaw every day say: “I have never seen this before” but still solve it faster each day. Research confirms that a monkey with no visual cortex can’t discern a circle from a triangle but can still move under visual guidance like a normal monkey (Humphrey, 1992). Brain systems that evolved millions of years ago still operate in our brain if the systems that evolved after them fail.

If the brain was built in a factory and then put to work, the cortex might run the brain but evolution didn’t have that luxury, as some center had to run the body-world loop at every stage of evolution. The fish cortex wasn’t ready to take charge so the cerebellum, as the first independent brain center to evolve, ran the feedback loop at the time.

By some estimates, the cerebellum or “little brain” contains about 80% of the nerves of adult brains so despite its ancient origin, its role today isn’t just backup. People with cerebellar damage struggle with movement in a wide range of activities, like walking, reaching, speaking, gaze and balance. They have staggered walking, inability to maintain eye-gaze, slurred speech and other features associated with being drunk. The common feature is an inability to relate moment-to-moment muscle actions to sense input.

The cerebellum can’t depend on a neocortex that evolved long after it and it still acts the same way in us today. For a gymnast to back-flip on a balance beam takes super-fast processing that the cortex just can’t do. Even simple tasks like riding a bike are done badly by the cortex until the hindbrain takes over as we automate the task.

In a decentralized brain, the cortex doesn’t control the cerebellum but triggers acts for it to carry out. The cerebellum learns schema that control sensorimotor sequences, like riding a bike, by experience. When the senses trigger a schema, the cerebellum acts as needed without direction, just as a car’s automatic transmission monitors events and changes gear as needed. To ride a bike, we just push off and let the hindbrain take over to handle balance as only it can. The movement center can act alone because it was once the senior brain system and it retains that ability to act autonomously in us today.

To call the hindbrain primitive because it can’t speak is like saying a jet engine is primitive because it has no video feed, when that’s impossible given what it does. Just as modern jets have the latest engines, our movement center is the latest version evolution can provide. We don’t have an old reptile brain but a state-of-the-art moving center. The cerebellum acts implicitly without fuss, so it’s easy to ignore, but the midbrain and the emotions it adds are anything but unseen.