The more data we deal with on a daily basis - whether that data's related to websites or social relationships - the higher our mental and physical hierarchies climb.
A one-horse town might only need a single sheriff to protect the populace, but a city of twenty million people may benefit from maintaining a staff of police sergeants to supervise junior officers and report large-scale trends to their superiors. There's no need for the city's chief commissioner to directly supervise every arrest - but if a drug bust turns out to involve the son of a congressman, even a high-ranking officer might take a sudden interest in the case's details.
And it's this same pattern of hierarchical organization that dominates our brains: Lower-ranking areas tend to handle the trivial details of shapes, scents and so on; these regions pass only the "general idea" of what they perceive up to higher-ranking areas. When an unexpected event comes along, though, those lower regions can alert higher-ranking areas to take notice of the minutiae. This is one reason why the details of exciting and traumatic moments tend to stand out so sharply in contrast to our other memories: Those moments attract a boost of brainpower, literally giving our brains more attention to devote to each sight, sound and smell.
On one hand, your brain's organizational hierarchy isn't really reflected in its physical structure - regions that sit higher in rank aren't always physically "higher" in your brain; nor are they always the most recently-evolved. The hippocampus, for example, seems to play a central role in regulating conscious attention, but it's a deeply buried brain structure whose evolutionary origins lie in the time before amphibians first crawled on land.
Even so, scientists have known for centuries that brain areas toward the front of the cerebrum tend to juggle input from a wide variety of other brain areas - a trait that enables these frontal regions to deal with elaborate multisensory tasks like recognizing faces or ignoring distractions. Areas toward the rear of your brain, meanwhile, tend to specialize in a particular medium: Sight, sound and so on. You might develop visual problems if you injure the back of your brain; but damage the area behind your forehead and you could lose your ability to make decisions.
As you might expect from a system like this, brain areas that deal with simpler tasks - sorting light from shadow, for example - tend to look and behave pretty similarly across populations. If you've got healthy vision, my visual cortex probably looks and works almost exactly the same as yours. But in frontal brain regions that evolved more recently and deal with more complex tasks, our brains start to behave more differently from one another.
Just how different can this brain behavior get? Well, one recent study found some clues about which areas of our brains are most likely to vary from one person to the next - and how those differences may play out in our mental processes.
As the journal Neuron reports, a team led by Dr. Hesheng Liu of the Massachusetts General Hospital scanned the brains of 23 volunteers on five separate occasions over the course of six months. Using an imaging technique known as functional magnetic resonance imaging (fMRI), the team watched each volunteer's brain activity unfold over time, then compared each patient's set of brain videos to those from the other volunteers.
In every volunteer's brain scans, the regions most expanded by recent evolution - areas involved in impulse control and attention-shifting, for example - communicated with areas all throughout the brain. But here's the thing: Those recently-evolved communication networks never looked or acted the same from one patient to another. In fact, the team found that the more a given brain region had expanded in our recent evolutionary past, the more differently that region was wired in each person.
Evolution, in other words, has given us each a processing network that adapts uniquely to its own experiences. Today's human brains are like custom-tailored computer networks, each tuned to understand and perform a single individual's most-practiced tasks with maximum precision. What's amazing is that those tasks can vary from the sensory - recognizing species of trees or musical keys - to the social - knowing what to say and what not to say in specific company - to the highly abstract, like proving mathematical theorems or appreciating beautiful writing.
This isn't to say, of course, that our human brains are "more evolved" than those of any other species. Our brains have evolved toward adaptability and complexity, but sharks (for example) have dominated the oceans for hundreds of millions of years with brains far simpler and less adaptable than ours.
No, what makes our species unusual is that we adapt to radically new circumstances with stunning speed - not over hundreds of generations, but often within a single lifetime. Our hierarchical understanding of the world can accommodate anything from visual images to social structures to websites that rank other websites - as well as concepts we haven't even dreamed up yet. We may not be able to breathe underwater, but we've built machines that swim deeper than most fish. We can't survive in deep space, but we've seen more of the universe than any other species in our planet's history. We can't see molecules, but we know which ones our neurons use to communicate.
And if your brain wasn't so different from mine, none of the above would be possible.
