Your Brain: The Missing Manual

Ignoring Things

Your brain has another skill that's just as important as finding patterns in the chaos. Not only can your brain imagine new objects into existence, it can also block out the things it wants to ignore.

As you learned earlier, your brain is hard-wired to focus attention on threatening sights and sounds. In order to better separate these potentially dangerous cues, the brain filters out repetitive, unchanging stimuli like a whirring air conditioner or the rocking motion of a boat at sea.

Note

There are many different neurological processes supporting this "tune-out" behavior. At the lowest level, constantly stimulated neurons temporarily stop firing. (For this reason, your eyes jitter imperceptibly back and forth even when you hold your gaze steady. If they didn't, the same neurons would always be stimulated by the sight in front of you. They'd get tired out, stop firing, and everything would fade out into blackness until you looked somewhere else.) The brain also has higher-level processes that adapt to constant stimuli and direct attention away from things that aren't changing in favor of those that are.

Most of the time, your brain's tune-out feature is exactly what you want. After all, who wants to be bothered thinking about the sound of air rushing by your ears, the feeling of weightiness as you sit in your couch, or the tactile sensation of clothes rubbing against your skin? Instead, your brain notices each one of these phenomenon briefly when they first appear, and then quickly adapts to ignore them. However, sometimes this effect can lead to some interesting illusions.

You're no doubt keenly aware of the way the brain adapts itself to different levels of brightness. (If not, try walking from a darkened room into a bright summer day without getting run over.) However, the following version is more fun:

Figure 4-18. 

1. Stand in a doorway, with your arms down at your sides.

2. Place the back of both hands against the door frame on either side.

3. Push up with as much strength as you can muster. Keep this up for a couple of minutes.

4. Now relax and walk away from the door.

For the next few minutes, you'll have the sensation that your arms are drifting up, weightless—in much the same way that your brain might adjust to a stronger gravitational field on another planet. After only a couple of minutes in the doorway, your brain becomes accustomed to the fact that it needs to exert more effort to keep your arms up than at your sides.

As this experiment shows, the brain's tendency to ignore things is really a remarkable ability to adapt itself to its current environment. There are dozens of do-it-to-yourself experiments that show similar adaptations at work. For example, if you scatter your living room furniture haphazardly, you'll spend the first few hours bumping into sofas, the next few hours steering yourself effortlessly (and subconsciously) through the chaos, and the following weeks wondering why everyone looks at you so oddly when you have them over for tea. A similar automatic adjustment and eventual ignoring happens with smells, but even more quickly. If you want to know if the scent from last night's curry cook-off is still around, you'll need to step outside your house and then come back in, because your nose tunes out even the strongest smells after just a few sniffs. And if you want to answer the age-old question "Do I smell OK?" you'll need the help of a friend, because your brain is perpetually filtering out the familiar odor of your own body.

Lastly, your brain also adjusts itself automatically to pleasure, making sure you don't get too much of it no matter how many triple-chocolate sundaes you down in a single sitting. Chapter 6 has more about this frustrating fact.