The human brain grows from a neural tube whose top, middle, and rear areas become the forebrain, midbrain, and hindbrain respectively (Figure 6.24). They then specialize in three basic brain functions:
1. Input. What is out there? (forebrain)
2. State. What is the body state? (midbrain)
3. Output. What actions can be done? (hindbrain)
Life requires all three, as animals must sense food or danger, know if the body is hungry or tired, and direct actions like biting or chewing, to survive. If early growth follows the same path as evolution, brains began as simple neural tubes that then specialized in analyzing sense input, evaluating body state, and directing muscle activity, so our brain has a cerebrospinal hindbrain, a limbic midbrain, and a cortical forebrain.
An engineer might design a feedback system to analyze input, assess internal state, and direct responses then add a control center but evolution doesn’t design ahead, so it just evolved three functions in parallel and didn’t bother with a control center. The basic brain-environment feedback loop then runs through three processing centers that specialized in sensory, state, and motor processing, as shown in Figure 6.25, any of which can drive the loop.
Locus of control theory is that the hindbrain, midbrain, and forebrain all evolved to control the feedback loop, so its locus of control can change as individuals evolve, grow, or live. The evolutionary advantage is flexibility, as what directs the body can shift as circumstances require to allow:
1. Sensory control: Based on sense input patterns.
2. State control: Based on body state feelings.
3. Movement control: Based on sensorimotor patterns.
Hence in life, an animal can respond differently according to the feedback locus of control. For example, given a chance to bite, it might do so under movement control, or it might freeze in place from fear under state control, or it might ignore the situation because sensory analysis suggests that there is no danger. Our brain then isn’t three overlaid sub-brains as Triune theory proposed, but three feedback-control centers, any of which can control behavior by itself alone.
Fish brains are then partitioned as ours are, as they have forebrain optical and olfactory areas to process sense data, a midbrain amygdala and pituitary to manage endocrine tasks, and a hindbrain cerebellum to direct movement (Figure 6.26). All three functions exist, but the fish cerebellum is far more evolved than its cortex (Montgomery et al., 2012), so it can control the feedback loop using data from the more primitive forebrain and midbrain. This is possible because the cerebellum projects excitatory and inhibitory nerves to the motor cortex, even in humans (Daskalakis et al., 2004).
The hindbrain control center then advanced before the other two because in evolution, actions are more critical to survival than sensations. It is better to act blindly than to see but be unable to act, so single-celled protozoa evolved the ability to move before they could see. Human embryo growth supports this conclusion, as motor nerves develop before sensory ones, so babies in the womb kick long before their eyes start working.
Locus of control theory suggests that as brains evolve or grow, the hindbrain runs the feedback loop while the midbrain and forebrain develop. In humans, the hindbrain bulges out from the base of brain as the cerebellum (Figure 6.27). The cerebellum doesn’t look big but it actually contains more nerves than the rest of the brain put together! Its two cross-linked hemispheres are about 80% of all nerves, so some call it the little brain. People with cerebellum damage struggle with walking, reaching, speaking, gaze, and balance, and their staggered walk, poor eye-control, slurred speech, and other features makes them appear drunk. They lose the hindbrain ability to handle fast sensorimotor sequences, so when a gymnast does a back-flip on a balance beam, it is managed by the super-fast processing of the hindbrain, not the slower motor cortex.
Our brains then contain a movement control center that once ran the body entirely, so can it still do so today? When we sleep, parts of our brain shut down to allow recovery but in parasomnia, people sleepwalk, to get up, walk, eat, and even cook a meal. In one case, a sleepwalker got up, rode her motorbike for 20 minutes, returned and parked it, then went to bed to wake up later with no memory of it at all. How then can sleepwalkers carry out complex tasks while still being largely asleep? Locus of control theory proposes that while the cortical and midbrain systems that govern rational thought and memory were dormant, the hindbrain woke up to control the body using sensorimotor connections that it evolved long ago. Clearly sleepwalking isn’t just reflexes, as cooking a meal and riding a motorbike are purposeful acts that use tools and navigation.
But how can the brain navigate if the visual cortex is dormant? Monkeys with no visual cortex can’t discern a circle from a triangle, as expected, but still move around like normal monkeys using vision (Humphrey, 1992). Humans also exhibit this blindsight, as people with visual cortex damage report seeing nothing but can still catch a ball, or insert a coin into a tilted slot whose angle they say they can’t see (Goodale & Milner, 2004). Hence if cortical vision fails, the hindbrain can still navigate in space by what is called implicit perception (Hannula et al., 2005), so the hindbrain has its own sensory input paths distinct from the midbrain and forebrain.
If advanced systems fail, older ones take over, so 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, so these older systems can also learn. For example, tasks like riding a bike are done badly by the voluntary motor cortex become until they automatic, and then they are easy. Essentially, the cerebellum learns a sensorimotor schema to ride a bike and remembers it, so when you get on a bike, it triggers to control movement and balance, like a sort of autopilot.
Note that the cortex doesn’t control the cerebellum but just lets it act, just as when driving we let a car’s automatic transmission monitor events and change gear as needed. How then do we know if we remember how to ride a bike if it is unconscious? The only way is to get on one and push off, and let the cerebellum take over to handle the body and balance as only it can. The hindbrain is then a movement control center with its own dedicated sensorimotor input-output, learning, and memories, that can act independently if needed because long ago, it was the senior brain system. Other parts of the brain can interfere with it, but they can’t do what it does, and have no need to.
We tend to forget that the evolutionary path to our brains was long and arduous. For example, in infant swimming, babies instinctively hold their breath underwater thanks to a diving reflex, and move their arms and legs 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 like frogs but lose the ability after four months suggests that our brain retraces its evolutionary ancestry as it grows.
To call the hindbrain primitive because it isn’t conscious is like calling a jet engine primitive because it has no video feed, when given what it does, that’s unlikely. Just as modern jets have the latest engines, our brain has the latest movement control that evolution can provide, so we don’t have an old reptile brain but a state-of-the-art movement center. It acts implicitly without fuss, so it’s easy to ignore, but the midbrain emotions of the next section are anything but unseen.