Human motion (walking, running, even crawling or hopping) produces a very specific set of signals. There’s the steady up–down rocking motion inherent in bipedal walking, the general velocity and the external forces such as the movement of air passing you and your shifting internal fluids that this produces. All of these are detected by proprioception and the vestibular system.
The image hitting our eyes is one of the outside world going by. The same image could be caused either by us moving or by us staying still and the outside world going past. At the most basic level, both are valid interpretations. How does the brain know which is right? It receives the visual information, couples it with the information from the fluid system in the ear and concludes “body is moving; this is normal,” and then goes back to thinking about sex or revenge or Pokemon, whatever it is you’re into. Our eyes and inner systems work together to explain what’s going on.
Movement via a vehicle produces a different set of sensations. Cars don’t have that signature rhythmical rocking motion that our brains associate with walking (unless your suspension is well and truly shot), and the same usually goes for planes, trains and ships. When you’re being transported, you’re not the one actually “doing” the moving; you’re just sitting there doing something to pass the time, such as trying to stop yourself from throwing up. Your proprioception isn’t producing all those clever signals for the brain to comprehend what’s going on. No signals means you’re not doing anything to the reptile brain, and this is reinforced by your eyes telling it you’re not moving. But you are actually moving, and the aforementioned fluids in your ear, responding to the forces caused by high-speed movement and acceleration, are sending signals to the brain that are saying you are traveling, and quite fast at that.
What’s happening now is that the brain is getting mixed signals from a precisely calibrated motion-detection system, and it is believed that this is what causes motion sickness. Our conscious brain can handle this conflicting information quite easily, but the deeper, more fundamental subconscious systems that regulate our bodies don’t really know how to deal with internal problems like this, and they’ve no idea what could possibly be happening to cause the malfunction. In fact, as far as the reptile brain is concerned, there’s only one likely answer: poison. In nature, that’s the only likely thing that can so deeply affect our inner workings and cause them to get so confused.
Poison is bad, and if the brain thinks there’s poison in the body, there’s only one reasonable response: get rid of it, activate the vomiting reflex, pronto. The more advanced brain regions may know better, but it takes a lot of effort to alter the actions of the fundamental regions once they’re under way. They are “set in their ways” after all, almost by definition.
The phenomenon is still not totally understood at present. Why don’t we get motion sickness all the time? Why do some people never suffer from it? There may well be many external or personal factors, such as the exact nature of the vehicle in which you are traveling, or some neurological predisposition to sensitivity to certain forms of movement, that contribute to occurrence of motion sickness, but this section sums up the most popular current theory. An alternative explanation is the “nystagmus hypothesis,”3 which argues that the inadvertent stretching of the extra-ocular muscles (the ones that hold and move the eyes) due to motion stimulates the vagus nerve (one of the main nerves that control the face and head) in weird ways, leading to motion sickness. In either case, we get motion sickness because our brain gets easily confused and has a limited number of options when it comes to fixing potential problems, like a manager who’s been promoted above his or her ability level and responds with buzzwords and crying fits when asked to do anything.
Seasickness seems to hit people the hardest. On land there are many items in the landscape to look at that reveal your movements (for instance, trees going past); on a ship there’s usually just the sea and things that are too far away to be of any use, so the visual system is even more likely to assert that there’s no movement happening. Traveling on the sea also adds an unpredictable up–down motion that gets the ear fluids firing off even more signals to an increasingly confused brain. In Spike Milligan’s war memoir Adolf Hitler: My Part in His Downfall, Spike was transferred to Africa by ship during World War II, and was one of the only soldiers in his squad who didn’t succumb to seasickness. When asked what the best way to deal with seasickness was, his reply was simply, “Sit under a tree.” There’s no supporting research available, but I’m fairly confident this method would work to prevent airsickness too.
Room for pudding?
(The brain’s complex and confusing control of diet and eating)
Food is fuel. When your body needs energy, you eat. When it doesn’t, you don’t. It should be so simple when you think about it, but that’s exactly the problem: us big smart humans can and do think about it, which introduces all manner of problems and neuroses.
The brain exerts a level of control over our eating and appetite that might surprise most people.* You’d think it’s all controlled by the stomach or intestines, perhaps with input from the liver or fat reserves, the places where digested matter is processed and/or stored. And indeed, they do have their part to play, but they aren’t as dominant as you might think.
Take the stomach; most people say they feel “full” when they’ve eaten enough. This is the first major space in the body in which consumed food ends up. The stomach expands as you fill it, and the nerves in the stomach send signals to the brain to suppress appetite and stop eating, which makes perfect sense. This is the mechanism exploited by those weight-loss milkshakes you drink instead of eating meals.5 The milkshakes contain dense stuff that fills the stomach quickly, expanding it and sending the “I’m full” messages to the brain without you having to pack it with cake and pies.