#158 - 10 Small Brain Quirks

0:02Matt Walker: Hi there, it's Matt here and welcome back to the podcast. Today, I want to wander through some of the small, odd things your brain does every day, starting with an object that may well be in sight of you right now. If there's a clock nearby with a second hand, try something for me. Look away from it for a few seconds, then look back. Chances are, the second hand seemed to hang there, motionless, for a moment after your eyes landed on it, as if the clock had briefly stopped. It's called the stopped clock illusion. The stopped clock is one of 10 small glitches I'd like to look at today. Brain freeze, the fact that you can't tickle yourself, hiccups, the strange sound of your own voice on a recording.

0:48Matt Walker: These things happen to lots of people, and usually they're not a sign of anything wrong. They're quirks of a healthy brain. I've grouped them four ways. Prediction, filtering, the body's odd wiring, and memory and feeling. First, we'll start with the brain that predicts. Think of your eyes as a camera that blurs every time it swings, and your brain as the film editor. The swings are the very fast movements your eyes make when you look from one thing to another, and they're so quick that they blur everything you'd see along the way. So the editor cuts them out. It takes the image from wherever your eyes land and stretches it backwards to cover the gap, which means you see that first frame for slightly longer than you really did.

1:37Matt Walker: On a clock, the first frame happens to include a second hand. That's why it seems to hang. The evidence comes from a neat bit of laboratory trickery. Fix your eyes on a cross, move them to a counter on a screen and then say how long you saw the first number. Usually the answer comes back too long by around a tenth of a second and sometimes nearly two tenths. The editor is also fussy about what it will backfill. Shift the counter unpredictably while the eyes are still moving and the illusion disappears. The same kind of bookkeeping, the brain keeping track of its own movements explains the next puzzle.

2:19Matt Walker: You can't tickle yourself. At the end of the last century, it was still a mystery. Since the late 1990s, one line of research has followed a single idea. Every time your brain sends a movement command to your muscles, it keeps a copy, like the carbon copy of a letter you've posted. The cerebellum uses that copy to predict what the movement is going to feel like. When the touch arrives, it's checked against the prediction, and whatever matches gets turned down. Behavioral studies going back about 50 years show the result of that check. A touch you produce yourself feels less intense and less ticklish than the same touch applied by someone or something else.

3:02Matt Walker: Two recent studies have looked under the bonnet. In the first, 15 healthy men and 15 women had their brains scanned while they received a touch to the finger that was either self-generated or external. Self-generated touch led to less activity in sensory regions and in the cerebellum. And the more someone turned down their own touch, the stronger the coupling between the cerebellum and those sensory regions. The second study takes the copy away altogether. The same touch was delivered either actively with people moving their own finger or passively when a platform dropped away and the finger simply fell onto a sensor.

3:44Matt Walker: Only the active touch felt weaker than external touch. The passive touch felt just as strong as a touch from outside. No movement command meant no carbon copy, and no carbon copy meant nothing to cancel it. There's a reasonable objection to all this. Maybe tickling simply needs another person, and you don't count. To test that, one team blindfolded 35 people and tickled each of them twice, once when they were told the experimenter was doing the tickling and once when they believed a machine was doing it. In fact, both times, the tickling came from a research assistant hidden under a table. People smiled, laughed, and squirmed about as much for the machine as for the human and rated themselves just as ticklish.

4:34Matt Walker: Tickle laughter, it seems, doesn't need you to believe a person is involved. It needs a touch you didn't start. Tickle laughter even has its own sound. Gather 887 spontaneous laughs from online videos of everyday situations. The 241 that came from tickling sound different from laughs at jokes, at funny scenes, or at someone else's bad luck. A machine learning program sorting the laughs identified the tickle laughs more accurately than any other type. Human listeners could pick them out too, and the tickle laughs showed a reduced amount of vocal control. Next, we'll move to the brain that filters.

5:18Matt Walker: Think of your attention as a doorman at a busy club with a very short guest list. Most of the noise around you is turned away at the door unheard, but a few names are on the list. More than 60 years ago, people wore headphones playing a different message into each ear and had to repeat one message aloud while ignoring the other. In general, they couldn't report anything from the ignored ear, even words repeated many times over. The one exception was their own name. Four of the 12 participants reported noticing it. A replication study more than 30 years later found that 34.6 % noticed their name, and the two studies are summarized together as about a third.

6:03Matt Walker: A recent study tested 80 people, and 23 of them reported hearing their name. Hearing your name can come at a cost, though. Participants made more errors repeating their target message, especially around the moment their name was played. The doorman isn't simply waving through anything odd, either. In the same replication, a word that made no sense in its sentence was sometimes slipped into the ignored message. People rarely reported hearing it, and it didn't cause errors. One account, proposed in a review of the area, is that sound from the ignored ear is held briefly in pre-attentive memory. Think of it as the cue outside, and check there against the things that matter to you.

6:48Matt Walker: Your name, it seems, is high on that list. Some people's doormen are a little more relaxed than others. The best clue so far to who notices their name and who doesn't is working memory, your ability to hold things in mind. Those who notice their name tended to have a lower working memory capacity, apparently because they had a harder time filtering out distracting information.

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8:33Matt Walker: The next glitch runs the other way. Instead of something slipping through the filter, a word you know perfectly well stops meaning anything at all. Say it over and over and it starts to sound like gibberish. This is semantic satiation and it has been studied for more than a century, starting with people who stared at a word until it lost its meaning for them. Ask people to repeat a category word 30 times rather than three and their decisions about that category slow down. A brainwave called the N400 is sensitive to whether a word makes sense in its context, and its relatedness effect varies with satiation, even when the repeated word is written in different letter cases or spoken at different pitches.

9:24Matt Walker: So it isn't just tired eyes or ears. Some studies had produced conflicting results, so one was designed to settle it, and it found a smaller N400 after 30 repetitions than after 3. The best explanation seems to be that what tires is the link between the word and its meaning. Across 4 studies, people got slower over time at matching a category like fruit to an example like apple when they first repeated the category word. They didn't slow down if they repeated different examples of the category instead, or repeated the same example in a simpler word matching task. The researchers conclude that satiation is a temporary failure of the link between a word and its meaning.

10:11Matt Walker: The word is still there, and so is the meaning. It's the link between them that's tired. The third filtering glitch has a silly sounding name, the doorway effect. Think of your brain as filing your day into folders, one for each scene. A doorway is where it closes one folder and opens the next, and whatever you were holding in mind gets filed in the one that just closed. In the original experiments, people picked up objects in a virtual room, moved around, set some down, and carried others with them, then answered questions about them. Information about an object was less available once they'd moved into a new room, particularly if they were still carrying it.

10:55Matt Walker: The effect worked with pictures as well as words, and with pairs of words that had nothing to do with the rooms. It turned up on smaller screens, and in real rooms with real doors. The researcher's own explanation is close to the folders. The brain breaks experience into events, and walking into a new room updates the current event in a way that interferes with remembering the last one. A doorway is just one kind of event boundary. Then, another group tried to replicate it in four experiments. The first two were in immersive virtual reality rooms, with one of them using an extra memory task and the other one without using an extra task.

11:38Matt Walker: The next one was with people watching a video of someone walking down a real hallway, and finally, one with people walking down that hallway themselves. In none of the four experiments did they find evidence that doorways lead to forgetting. Next, we'll look at the body's odd wiring. Brain freeze has a wonderfully formal name, headache attributed to ingestion or inhalation of a cold stimulus. That means it has its own entry in the international classification of headache disorders. The classic trigger is ice cream, but any cold food, drink, or even cold air passing over the palate or the back of the throat can do it.

12:20Matt Walker: Think of it as a burglar alarm with its sensor in the roof of your mouth and its siren in your forehead. Trip the sensor quickly over a large enough area and the siren goes off. A review of the research calls brain freeze very common but under-researched and suggests that the speed of cooling and the amount of palate exposed may affect how severe it gets. In some people, the alarm is set more sensitively than in others, and children seem to get it more than adults. In a study of 283 German school children aged 10 to 14, 62 % had brain freeze. Among the students' parents and teachers, it was less common at 36 % of women and 22 % of men.

13:09Matt Walker: In the same study, a child was more likely to get brain freeze if a parent had it and less likely if they'd never had a headache. Migraine seems to turn the sensitivity up. Of 669 women at a breast screening center, 51 or about 1 in 13 got brain freeze after drinking 150 milliliters of ice-cold water through a straw. Women who'd had a migraine in the previous year were twice as likely to get it as those who'd never had one. The researchers concluded that active migraine seems to make people more sensitive to forehead pain set off by cold on the palate. When you purposefully set the alarm off in 77 volunteers, some of the wiring shows.

13:53Matt Walker: 39 got a headache from gulping ice water and 9 from an ice cube pressed on the palate. And the ice water headaches came on faster, at a median of 15 seconds. For most, the pain was in the forehead and temples. Those who got brain freeze were more likely to have watery eyes, and the researchers propose that a reflex involving the trigeminal nerve, the main sensory nerve of the face, might be involved. They suggested the first headache might come from cold, stimulating that nerve at the palate, and about a quarter of those with an ice water headache got a second later one. What brain freeze really does for us remains unknown.

14:36Matt Walker: Next, let's discuss hiccups. Hiccups are a reflex. The diaphragm muscle contracts itself, driven mainly through the phrenic nerve, and the glottis, the space between the vocal cords, closes during the in-breath. Familiar as they are, a systematic review still describes hiccuping as poorly understood. Whatever hiccups are for, they start early. Follow 45 low-risk pregnancies with serial ultrasound scans, and hiccuping turns out to be the most common movement of the diaphragm before 26 weeks. Between 24 and 26 weeks, it drops off sharply, and breathing movements take over in the weeks that follow.

15:18Matt Walker: In premature babies, hiccuping is one of the major motor patterns, taking up an estimated 1 % of the time. Researchers in London recorded brain activity in 13 pre - and full-term babies and analyzed the brain's responses to 1 ,316 hiccups in all. Those responses were there from as early as 10 weeks preterm. The researchers suggest that hiccuping may give the developing brain useful sensory input. It's also possible that hiccuping, like brain freeze, is simply a reflex with no real purpose. There's another published theory, and it's one I rather the like. Think of an old app on your phone that you never open, but that survives every upgrade because something newer depends on it.

16:06Matt Walker: Animals that breathe two ways, like frog tadpoles with their gills and lungs, show a breathing pattern you can also see in a hiccuping human. The hiccup is a sudden closure of the airway on an attempted in-breath. It appears early in development, It's inhibited by raised carbon dioxide, and it can be calmed down by baclofen, a drug that acts on the brain and spinal cord. The idea is that the hiccup is an old motor program for gill breathing. The app nobody opens kept because other jobs such as suckling use some of the same circuitry.

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18:06Matt Walker: Then there's the sound of your own voice on a recording, which can feel as though it belongs to somebody else. It sounds quite different from what you hear as you speak. And the reason is that you have, in effect, two speakers when you talk. One is out in the room, carried through the air, and others hear it. The other is carried through your skull, and only you hear that one. Bone conduction of sound has been recognized since at least the 16th century. Separate the two roots during speech using a large, specially built earmuff, and the bone-conducted part dominates between about 1 and 2 kilohertz for most of 10 speech sounds tested.

18:47Matt Walker: Decades ago, researchers who played people recordings of their own voices noted what they called an effective disturbance. Part of it was the gap between how people expected to sound and how they actually sounded, and part was a defensive reaction to the whole business. The researchers took it as evidence that we monitor our own voices and adjust how we use them. In a follow-up, only 38 % of people with moderate familiarity with their own voice recognized it when five-second recordings were played back up to three months later. Familiarity was the critical factor. The odd part comes when people aren't told it's them.

19:27Matt Walker: 80 men and women rated the attractiveness of a set of recorded voices without being told that their own was among them. They tended to rate their own voice as more attractive than other people's and more attractive than other listeners rated it, an effect dubbed vocal implicit egotism. Put that beside the older studies and it seems we like our own voice just fine. It's recognizing it as ours that upsets us. Lastly, we'll take a look at memory and feeling. You know exactly what the word means, and you're certain it's right there. You just can't say it. Psychologists have been exploring this tip of the tongue state for about 60 years, and it works a bit like a phone call on a bad line.

20:13Matt Walker: You know who's calling, and you can make out parts of the name, but the connection is a little too crackly for the whole name to come through. That feeling of nearness is built from real evidence the brain has about the word, like partial information about it, related words, and how familiar the question feels. The most influential theory, the transmission deficit model, grew out of studies of word-finding failures in younger and older adults. And it fits the crackly line rather well. Your brain can reach the meaning of the word. the connection from there to the word sound is too weak to get through.

20:52Matt Walker: Brain imaging suggests that tip-of-the-tongue states involve monitoring word retrieval using a network centered on the anterior cingulate and the prefrontal cortex. Tip-of-the-tongue states do become more common with age. Older adults have them almost twice as often as younger adults, especially with less common words and proper names. If that has you worried that your memory is slipping, There's some good news in the detail. Take 718 adults aged 18 to 99. The older ones did have more tip-of-the-tongue states for descriptions of people and places and for the names of famous people, but not for definitions of everyday words.

21:34Matt Walker: And when the researchers controlled for episodic memory, the kind most often tested when checking for memory problems, age was still linked to tip of the tongue states. And now the last quirk we'll discuss today almost sounds like a confession of mine for my dog. Cute aggression is the urge to squeeze, crush, or bite something adorable with no intention of doing it any harm. It belongs to a family researchers call dimorphous expressions of emotion. Strong positive feelings coming out in ways we usually reserve for negative ones, like crying tears of happiness. Think of the release valve on a pressure cooker.

22:13Matt Walker: When things get a little too intense, the steam finds a way out, even if it's a surprising one. That image fits what the original study found. The link between how cute something looked and how much cute aggression people felt ran through how overwhelmed they were by positive feelings. Cuteness isn't only about looks either. A review proposes that the sounds and smells of babies can evoke it too, and that cuteness in general gets rapid preferential processing in the brain. When you record brainwaves from 54 adults as they look at pictures, cute aggression lines up with signatures of reward processing and of the picture's emotional importance.

22:55Matt Walker: And once again, the effect ran through feeling overwhelmed by positive emotion, just as in the original study. So when we put all of that together, the glitches start to look less like mistakes. Several of them seem to be the brain doing its ordinary jobs just a little more visibly than usual. Part of what your brain does is predict what comes next. It covers the blur of each eye movement by stretching the next image back in time, and it keeps a carbon copy of each movement command so it can turn down the touch it expects. Part of it is filtering. Your own name gets past the bouncer for roughly a third of people.

23:38Matt Walker: A word repeated 30 times loosens its grip on its meaning for a while. And doorways may close one mental folder and open another, though that effect has largely failed a careful replication. Part of it is internal wiring. Brain freeze is an alarm with its sensor in the roof of your mouth and its siren in your forehead. and what it does for us is still unknown. Hiccups begin before birth, and they may be a leftover from gill-breathing ancestors, sensory input for a developing brain, or a reflex with no purpose at all. Your recorded voice sounds odd because you normally hear yourself through two speakers, one of them being your own skull.

24:21Matt Walker: And some quirks are about memory and feeling. On the most influential theory, a word on the tip of your tongue is a crackly line between meaning and sound, and cute aggression may be a release valve for an overload of good feeling. So back to the clock. The second hand never really stopped. Your brain stretched the first thing you saw after your eyes landed back in time to cover the gap, and it does that every time you shift your gaze. The clock just made the editing easy to see. Now, before I sign off, I have a quick favor to ask of you. If you would like a more in-depth view of any of these human quirks we've discussed today, please say so in the comments.

25:06Matt Walker: And if you have a sleep or brain topic that we have not covered yet, drop that in the comments. As always, thank you so much for listening to today's episode. I really do appreciate you giving me your time and attention. Please take care of yourself, enjoy your waking days, and sleep a sound sleep this night. Bye for now.