Fingertips To Hair Follicles: Why 'Touch' Triggers Pleasure And Pain

Feb 3, 2015
Originally published on February 4, 2015 4:39 pm

The rate at which someone strokes your hair can cause feelings of pleasure or annoyance — too slow is repulsive, too fast is annoying, and just right soothes.

There's a scientific explanation for this: People have special nerve endings (wrapped around the base of hair follicles) that detect the deflection of the hairs.

"In turns out, remarkably ... that hairy skin has a special caress sensor," neuroscientist David Linden tells Fresh Air's Terry Gross. "If you actually record electrical nerve endings long before they get to the brain, they send more signals to the caresses that feel the best."

In Touch, his latest book, Linden writes about this and the "weird, complex and often counterintuitive system" of touch circuits involving the skin, nerves and brain that create pleasure and pain.

The two sensations are inextricably linked, Linden explains.

"Part of what we know is that when pain ends, that is pleasurable," he says. "Think of taking off your ski boots after a day of skiing — it feels really good. But there's something more complicated than that: What I think it is, is that both pain and pleasure are emotionally salient. They mark experiences that are important for your life and, in terms of memory, they're the signal that says, 'This is important; write this down and underline it. Don't forget it.' "

So what else can be emotionally salient? Chili peppers.

"They're a bit painful," Linden says. "Why should we want to put something painful in our food? I think it is because it is rewarding to eat something that is a little bit of a threat."


Interview Highlights

On why life without touch is so problematic

Touch is so central to our humanity that it's hard to even imagine [life without] it. For example, if a child is born blind, they can grow up and have a completely full and normal life. They will be cognitively normal, psychiatrically normal and not have profound problems — the same if a child is born deaf. However, if a child is born into a situation, like a Kurd in Romanian orphanages in the 1980s and '90s, where social touch is deprived because there are not enough caregivers around, then that child will develop terrible psychiatric problems, attachment disorders, mood disorders, and also physical problems — problems with the digestive system and immune system, higher incidences of diabetes. And, amazingly, these problems are not just problems of childhood, but persist throughout life.

On how pain can actually protect people

There are a number of different relatively rare cases where you can lose different aspects of touch, and one of them ... is called "congenital insensitivity to pain." Folks who have this inherited syndrome — if they whack themselves on the thumb with a hammer — they'll feel the pressure and the thumb will swell up, but they won't feel any pain at all.

There's a famous case of a boy in Pakistan who jumped off of a high roof to impress his friends. He hit the ground, got up and said, "I feel fine," and he went home and he promptly died because he had no pain to realize that he had broken bones and sustained massive internal injury.

So we think, "Oh great, a life without pain! That would be idyllic!" But if you don't have pain, then you don't have the protective reactions that are so crucial.

On why a caress against the grain of hair can feel strange

There are sensors that wrap around the base of the hair follicles that only wrap around one half of the base. If you looked in a microscope, you would see that they don't make a full 360-degree circle; they cover about 180 degrees of the base. And that means that they are tuned to detect deflection in one direction versus the other.

When we think about the way we speak in our lives, if we had interaction with someone we might say, "He or she rubs me the wrong way." Someone who is socially clumsy we call "tactless" — literally they lack touch. I think it's important — it's telling us something that so many of our common expressions in English refer to the tactile sense.

On why fingertips are so sensitive

Fingertips are endowed with a number of different sensors for mechanical stimuli. There's one sensor called a "Merkel ending," which is good for feeling texture and fine little bumps.

There's a different one that is good for vibration. So if you are driving your car and there are subtle vibrations that come from the road up through your tires up through the steering, through the wheel that you use to detect how slick the road is ... or what the texture of it is — if it's pebbly or smooth — that is being detected by what are called "Pacinian sensors" in your fingertips and in the palms of your hands.

On why you can't read Braille with your genitals

The key point is that the word "sensitive" is really too broad. So it can mean two different things: areas like the fingertips and the lips and the tongue not only can respond to tiny deflections, but they can precisely localize fine special features on objects or sense textures, the sort of thing you would need to read Braille.

Other parts of the body, like the cornea of the eye or the tip of the clitoris or the tip of the penis are very sensitive in the sense that they can detect very fine deflections of the skin, but they're not very discriminative. They lack the Merkel-type nerve ending. And, as a consequence, it will fail if you attempt to read Braille with your genitals.

On the connection between anxiety and pain

We don't entirely understand why depression leads to chronic pain, but it's part of a larger phenomenon in which pain perception is modulated by all kinds of situational factors having to do with mood and expectancy and surprise.

So it turns out that the emotional pain centers are richly interconnected with regions of our brain having to do with cognition and anxiety and anticipation. So this is why many people who suffer from chronic pain can get partial relief from anti-anxiety medication. It's not that the anti-anxiety medication directly affects pain-perception — what it does is it breaks this horrible positive feedback loop between anxiety and chronic pain. So if you have chronic pain, then you become anxious about, "When is it going to stop? When is it going to recur?" And that anxiety seems to trigger more chronic pain. If you can interrupt that ... then often times that can bring at least partial pain relief.

Copyright 2015 NPR. To see more, visit http://www.npr.org/.

Transcript

TERRY GROSS, HOST:

This is FRESH AIR. I'm Terry Gross. The pleasures of sex and food and the pain of bruises and other injuries - you experience them because of your sense of touch. In the new book "Touch," my guest, neuroscientist David Linden, explains how pleasure and pain result from the weird, complex and often counterintuitive system of touch circuits involving the skin, nerves and brain. Pleasure also figures into Linden's previous book, "The Compass Of Pleasure: How Our Brains Make Fatty Foods, Orgasm, Exercise, Marijuana, Generosity, Vodka, Learning, And Gambling Feel So Good." Linden is a professor of neuroscience at Johns Hopkins University School of Medicine, and is a former chief editor of the Journal Of Neurophysiology. Note to parents of young children, our conversation will include some questions related to sex in the context of neuroscience.

David Linden, welcome back to FRESH AIR.

DAVID LINDEN: Thank you very much for having me back.

GROSS: So your book kind of starts with this game that people often play - the what-if game - and one of the turns the what-if game plays in your book in this conversation you're having is, what if you had to lose all your senses except one. Which would you keep? And people always want to save their sight or save their hearing, and you think they're not worried enough about saving their sense of touch. What don't we realize about how important touch is?

LINDEN: Well, I think the reason that touch doesn't come up is because we can't easily imagine losing it. We can close our eyes and imagine being blind. We can plug our ears or plug our nose and imagine losing hearing or our sense of smell, but there's no way to turn off touch. It's always there, and because it's always on, we take it for granted.

GROSS: And what's its importance?

LINDEN: Touch is so central to our humanity that it's hard to even imagine it. For example, if a child is born blind, they can grow up and have a completely full and normal life. They will be cognitively normal, psychiatrically normal and not have profound problems - the same if a child is born deaf. However, if a child is born into a situation like occurred in Romanian orphanages in the 1980s and '90s, where social touch is deprived because there are not enough caregivers around, then that child will develop terrible psychiatric problems, attachment disorders, mood disorders and also physical problems - problems with the digestive system and the immune system, higher incidences of diabetes. And amazingly, these problems are not just problems of childhood, but persist throughout life.

GROSS: Can you explain why?

LINDEN: We don't entirely know why social touch - in particular, traditionally, mother's touch - is so crucial during this time, but it seems as if that bond with the caretaker is not formed early, that there are profound problems manifest well beyond the functioning of the brain and the nervous system.

GROSS: And of course, there's the issue if you don't have touch and you don't experience pain, you don't know that you're hurting yourself, and you can kill yourself that way, accidentally.

LINDEN: Well, so there are a number of different relatively rare cases where you can lose different aspects of touch, and one of them that you're alluding to is called congenital insensitivity to pain. And so folks who have this inherited syndrome, if they whack themselves on the thumb with a hammer, they'll feel the pressure and the thumb will swell up, but they won't feel any pain at all. And there's a famous case of a boy in Pakistan who jumped off of a high roof to impress his friends. He hit the ground, got up, said, I feel fine, and he went home and he promptly died because he had no pain to realize that he had broken bones and sustained massive internal injuries. So we think, oh, great, a life without pain - that would be idyllic, but if you don't have pain, then you don't have the protective reactions that are so crucial. You will put hot food into your mouth and burn your esophagus. You'll wear clothes that are too tight and cut into your skin and then get infections. It's actually a terrible thing.

GROSS: If you're just joining us, my guest is David Linden. He's a professor of neuroscience at Johns Hopkins University and the author of the new book, "Touch: The Science Of Hand, Heart, And Mind." Why does it feel so good to be caressed? Why do we get such pleasure of being touched in a loving way?

LINDEN: That is a wonderful question, and it gets into a line of research that has really only become evident in the last 15 years. It turns out, remarkably, that in our skin, particularly in our hairy skin, that is to say, the skin on all parts of our body except a few places like the lips and the palms and the soles and the tips of the fingers - all that hairy skin has a special caress sensor. This is a nerve ending that wraps around hair follicle bases and detects the deflection of hairs. And remarkably, these nerve endings are tuned precisely to the kinds of caresses that we find most appealing. So just imagine that someone is stroking you on the arm. If they stroke really slowly, it doesn't feel loving. It feels like a crawling bug. It's repellent. And if someone strokes very quickly, it doesn't feel loving either. There is a range of speeds in which that caress feels good and prosocial and affiliative. And you might think, oh, well, that's just because the nerves in the skin will detect the whole range of caresses, and it'll send that information to the brain, and then the brain will be programmed to make the decision of what feels good. And remarkably, that's not the case. If you actually record electrical activity from these nerve endings long before they get to the brain, they're - they send more signals to the caresses that feel the best.

GROSS: So these little hairs are very important in communicating sensation and in communicating pleasurable sensation, and there's - you think - there's parts of our bodies that are hairy that we don't think of as hairy. What are some of those parts?

LINDEN: Well, so for example, the skin of a woman's cheek, we would think of, typically, as being smooth, or the skin of the inner thigh, we would think of as being smooth. But if you look very carefully, there are lots of tiny hairs there called vellus hairs. And those hairs can convey the caress sensation very well, whereas the palms of the hands or the soles of the feet or the lips or parts of the genitals - those are also what we call glabrous, or hairless skin, which has a fundamentally different structure.

GROSS: Does it feel different to be stroked against the grain of the hair? Like, when you pet a cat, you pet it with the grain of the hair. You don't pet it the opposite way.

LINDEN: That's absolutely right. So it does feel different, and it feels different because there are sensors that wrap around the base of the hair follicles that only wrap around one half of the base. If you looked in a microscope, you would see that they don't make a full 360-degree circle. They make - they cover about 180 degrees of the base. And that means that they are tuned to detected deflection in one direction versus the other. When we think about the way we speak in our lives - if we have a bad interaction with someone, we might say, he or she rubs me the wrong way. Someone who is socially clumsy, we call tactless - literally, they lack touch. I think it's important - it's telling us something that so many of our common expressions in English refer to the tactile sense, even though we don't think about them that way when we use them.

GROSS: So different parts of the body experience sensation in different ways. Like, there are things the fingertips are equipped to feel that other parts of the body aren't equipped to feel. So give us some examples of what fingers are designed to experience through touch.

LINDEN: So fingertips are endowed with a number of different sensors for mechanical stimuli. So there's one sensor called a Merkel ending that's very good for feeling texture and fine, little bumps. There's a different one that is good for vibration. So if you are driving your car and there are subtle vibrations that come from the road through your tires, up through the steering, through the wheel, that you use to detect how slick the road is, for example, or what the texture of it is - if it's pebbly or smooth - that is being detected by what are called Pacinian sensors in your fingertips and in the palms of your hands. The Merkel endings...

GROSS: Can I just stop you there? Even though your fingertips aren't on the ground - they're just on the steering wheel - it's your fingertips that are communicating that information?

LINDEN: They are. Remarkably, these sensors seem to be really good at detecting sensations at the ends of tools. Now, just imagine that you have a shovel in your hand, and you're out in your garden. And you are - you make a motion to embed the shovel in a pile of sand versus a pile of loose dirt versus a pile of rocky dirt. You can feel the difference in your hands even though the difference is only at the end of the shovel, which might be five feet away. Those sensations are being largely transmitted by your Pacinian sensors. The same thing is happening with a violinist sensing the tension of the strings with the bow or a sculptor with a chisel or a surgeon with a scalpel. A car is just another tool, and so it's a tool, in part, to feel the road through this linkage from the tires up to your fingers.

GROSS: I should say here that we're going to talk a little bit more about the sense of touch as it pertains to sexuality and the genitals. We'll be discussing it in a scientific and clinical way. Nevertheless, if it's a conversation, parents, that you don't want your children to hear, now would be a good time to duck out for just a few minutes. All right, let's proceed (laughter).

You mention this odd experiment in your book. People who are blind read Braille with their fingertips, so the question that was raised was, well, the genitals are very sensitive. Could the genitals read Braille? It's a weird question to ask in first place, but somebody you know actually conducted the experiment on herself and...

LINDEN: That's right.

GROSS: ...On her male companion (laughter). So - and they found out that the genitals cannot read Braille, in case people want to know the answer to that (laughter) - to that never-before-asked question. But anyways, it's just odd to think about - that as sensitive as the genitals are, they're not equipped to feel certain things.

LINDEN: Right, and I think that the key point is that the word sensitive is really too broad. So it can mean two different things. Areas like the fingertips and the lips and the tongue not only can responded to tiny deflections, but they can precisely localize fine spatial features on objects or sense textures, the sort of thing you would need to read Braille. Other parts of the body are very sensitive in the sense that they can detect very fine deflections of the skin, but they're not very discriminative. They lack the Merkel-type nerve ending, and as a consequence, it will fail if you attempt to read Braille with your genitals. Now, if your readers want to experiment, they can wait until nobody's around and go down to the ATM and try it themselves, but trust me, it won't work.

GROSS: (Laughter) I don't think it's a very good idea (laughter). So do you think the kind of science that you're talking about - the kind of, like, neurology of touch and sensation - is explaining some of the differences between male and female arousal?

LINDEN: I think the differences in the way nerves are routed does not explain the differences in male or female arousal. Part of the problem here, you have to realize, is that these nerves are very small. They're tiny, little threads. They're below the resolution of medical scanning machines, so we can really only study them in cadavers. And in cadavers, we can't ask them what their sex lives have been like, so we really don't have a lot of the information to really make a strong correlation there.

There are well known differences in sexual function between average - on average between men and women. But what is rather remarkable is that if you ask men and women to describe their orgasms - to write a little essay - a few paragraphs - and then you hire someone to take out all the words that give it away whether this person were male or female and then ask people to read the descriptions, it turns out that nobody can tell the difference - not doctors, psychologists, sex therapists, anyone. And this correlates with what happens if you have someone in a brain scanner. Just imagine how unsexy that really is, right? You've got your head fixed so that it doesn't jiggle in this giant clanging tube and your nether regions sticking out into the room where there are geeks like me in white coats with clipboards, taking notes. But when this is achieved, what we find is that the pattern of brain activation in men and women is remarkably similar.

GROSS: Well, have you been in that position of watching and taking notes while looking at the MRI scan?

LINDEN: I have personally been in this position. It turns out that - maybe not surprisingly - most of these experiments have been done in the Netherlands.

(LAUGHTER)

GROSS: Let me reintroduce you here. My guest is David Linden. He's a professor of neuroscience at Johns Hopkins University and author of the new book, "Touch: The Science Of Hand, Heart, And Mind." Let's take a short break here, then we'll talk some more. This is FRESH AIR.

(SOUNDBITE OF MUSIC)

GROSS: This is FRESH AIR. And if you're just joining us, my guest is David Linden, author of the new book "Touch: The Science Of Hand, Heart, And Mind." He's a professor of neuroscience at Johns Hopkins University.

We've been talking about pleasure and the neuroscience of that. Let's get to pain. You write that pain isn't a single, discrete sensation even when you experience it in a single moment in time. What do you mean that it's not a single, discrete sensation?

LINDEN: Well, what we know - and we know this from our own experience, we don't need a neuroscientist like me - is if you whack your big toe against the wall - right? - you have an immediate, sharp pain sensation that you feel nearly instantly. And then you can count one, 1,000, two - or as I like to count in-a-gadda-da-vida, baby - and then, only then, does the second wave of pain arrive. And this is because there are two separate pain systems in your skin and in your muscles. There is a very fast one that travels at over a hundred miles an hour, and then there is this very slow one that moves at sidewalk-strolling speed of about 2 miles an hour. And the fast one has a lot of detail - where exactly is the pain? And this is the one that subserves protective reflexes - jerking your hand away from a hot stove, for example. And then the slow one is the lingering pain that drives decisions like, do I need to attend to this wound? Do I need to adopt a posture or a mode of walking to protect this injured area from further damage? So there's a fast and a slow sensation.

But I think just as importantly - perhaps more importantly - there are separate sensory aspects of pain and emotional aspects of pain. There are separate places in the brain - the primary somatosensory cortex for the facts and the posterior insula for the emotional aspect. And these areas are completely different, and as a result, when people sustain damage to one or the other, they can feel pain in a way that is very odd. So, for example, if you sustain damage to your brain's emotional pain center, then you will have a syndrome called pain asymbolia. And so if you whack yourself on the thumb with a hammer, you would go, yeah, that hurts, that hurts quite a bit. It's on my thumb. Now it's throbbing. Boy, does that hurt. But you wouldn't have the typical emotional reaction - oh, heck. Oh, that hurts - ow, ow, ow. Oh, that hurts. That's terrible. That's awful. People who have pain asymbolia are not masochists. They don't enjoy pain. For masochists, pain has enormous positive emotional meaning. Pain asymbolics have no emotional component of pain whatsoever.

And then there are people who sustain damage to the opposite system, the one for the discriminative facts. And in that case, they hit themselves on the thumb with a hammer or you hit them on the thumb with a hammer and they go, oh, oh, that's terrible. And you say, oh, I'm so sorry. Where on your body is the pain? And they say, I have no idea. And you say, what is the quality of the pain? Is it stabbing or burning or aching? And they say, I have no idea.

GROSS: David Linden will talk about the link between pain and depression in the second half of the show. And he'll offer his suggestion for the best thing to do to protect and strengthen your cognitive function. Linden is a professor of neuroscience at Johns Hopkins University and the author of the new book "Touch." If you're curious about Bob Dylan's new album of songs that were recorded by Sinatra, stick around for Ken Tucker's review which is coming up. Why don't we hear a track from the album now? It's called "Shadows In The Night." I'm Terry Gross, and this is FRESH AIR.

(SOUNDBITE OF SONG, "WHERE ARE YOU?")

BOB DYLAN: (Singing) Where are you? Where have you gone without me? I thought you cared about me. Where are you? Where's my heart? Where is the dream we started? I can't believe we're parted. Where are you? When we said goodbye, love...

(SOUNDBITE OF MUSIC)

GROSS: This is FRESH AIR. I'm Terry Gross, back with neuroscientist David Linden, author of the new book "Touch" about the sense of touch and the circuitry that enables us to experience pleasure and pain. Linden is a professor of neuroscience at Johns Hopkins University and is a former chief editor of the Journal of Neurophysiology. When we left off, he was explaining that the physical and emotional aspects of pain are processed in different parts of the brain.

So if pain produces one response in the pain center of the brain and another response in the emotion center of the brain, that helps explain why chronic pain can be so emotionally upsetting and really lead to depression.

LINDEN: Well, yes, it's very clear. You're absolutely correct. So chronic pain is very much linked with depression, and conversely, depression is linked with the development of chronic pain, as well.

GROSS: So can you elaborate on how chronic pain can lead to depression, but also how depression can lead to chronic pain?

LINDEN: Yes, well, we don't entirely understand why depression leads to chronic pain. But it's part of a larger phenomenon in which pain perception is modulated by all kinds of situational factors having to do with mood and expectancy and surprise. So it turns out that the emotional pain centers are richly interconnected with regions of our brain having to do with cognition and anxiety and anticipation.

So this is why many people who suffer from chronic pain can get partial relief from anti-anxiety medication. It's not that the anxiety medication directly affects pain perception. Rather, what it does is it breaks this horrible positive feedback loop between anxiety and chronic pain. So if you have chronic pain, then you become anxious about when is it going to stop? When is it going to recur? And that anxiety seems to trigger more chronic pain. And if you can interrupt that, say, with a benzodiazepine drug, then oftentimes, that can bring at least partial pain relief.

GROSS: We often say, oh, what you said just really hurt my feelings. So when your feelings are hurt, are both part of the brain involved in that pain - the pain center and the emotion center?

LINDEN: When your feelings are hurt, the emotional pain center of your brain - the posterior insula - is very strongly activated, and the physical pain center - the somatosensory cortex - is activated just a whisper. But I think what this shows is that the phrase you hurt my feelings is not merely a metaphor. It's literally true.

And I think it points up a very interesting thing in our language. Why are feelings called feelings? Why are my tender emotions made analogous with our touch sense? Why is it that when someone is emotionally clumsy, we call them tactless - literally, they lack touch? We have this strong idea in the culture and in the language that emotion and touch are deeply linked. And I think the neuroanatomy bears this out.

GROSS: You write that chronic pain produces long-term changes in the brain's pleasure circuitry. So can you explain why chronic pain produces long-term changes in the brain's pleasure circuitry?

LINDEN: We don't really know.

GROSS: But would you know something about the significance of that, the implications of that?

LINDEN: Well, I think the implication is very important. And that is when you have chronic pain, then your ability to experience pleasure becomes reduced. And it's not just one kind of pleasure; it's all kinds of pleasure. And it's not just the pleasure from the so-called vices, like food and sex and alcohol and cannabis, but even the pleasure from our prosocial pleasure-seeking things, like learning and donating to charity and other things, meditation and prayer - the prosocial things that activate the brain's pleasure circuits.

GROSS: Let's talk a little bit about painkillers and how they work. Like, what does morphine do? And all the morphine derivatives - what do they do to alleviate or diminish pain?

LINDEN: The morphine derivatives activate a set of neurons in the base of the brain - in the brainstem. And cause them to send messages that interrupt pain signals in the spinal cord before they ever reach the brain. They also have secondary actions in the brain, as well. The interesting thing about morphine is that morphine attenuates both the discriminative sensory aspects of pain and also the emotional aspects of pain.

GROSS: Does it do other things to the emotions, too? Does it diminish all of your emotions?

LINDEN: No, it doesn't diminish all of your emotions. One of the remarkable things about morphine for pain is that when you're in pain, and you get morphine, you don't typically become euphoric in the way that you would if you used morphine or heroin or oxycodone recreationally. There's a great expression that says a lot of what drugs do to the mind is in the mind. In other words, drugs will have different effects based upon your expectation of what they will do to you.

GROSS: You're not going to get high if you're taking morphine or morphine derivative for pain?

LINDEN: Well, it's not that you won't get high at all or you won't get high if you take an enormous dose, but generally speaking, people who are properly dosed on morphine for pain aren't reporting high levels of euphoria. And that same dose, given to someone in a recreational context, would almost always produce very strong euphoria.

GROSS: Recent research shows that there really is a connection between pleasure and pain in the sense that dopamine, which has always been associated with pleasure, is also released with pain. Can you explain what that means?

LINDEN: Yeah, it's really kind of counterintuitive - isn't it? - because you think, well, pleasure and pain are opposites. And we don't entirely understand it. Part of what we know is that when pain ends, that is pleasurable - thinking of take off your ski boots after the end of a day of skiing. It feels really good. But there's something more complicated than that.

And what I think it is is that both pain and pleasure are emotionally salient. They mark experiences that are important for your life. And in terms of memory, they're the signal that says this is important. Write this down and underline it. Don't forget it.

The example that everyone uses is that everyone uses is that everyone remembers what innocuous thing they were doing when they got the news about 9/11, even if they were doing something really silly, like ironing clothes or feeding the dog. That memory that would otherwise be utterly discarded is now seared into one's consciousness forever because of the strong emotional nature of that moment.

So both pain and pleasure are - share salience. I think this is part of the explanation of why chili peppers are popular in food. They're a bit painful. Why should we want to put something painful in our food? I think it is because it is rewarding to eat something that's a little bit of a threat.

GROSS: Well, it certainly gives more sensation to the food.

LINDEN: Well, it plays with other aspects of sensation. And I think this is - brings up, I think, a really interesting point. Most of us are used to thinking that our food sensations come from a mixture of taste receptors on our tongue and smell receptors in our nose. But it turns out that many of the things that we think of in the experience of eating food actually come from touch receptors. Not only things like mouthfeel - is that apple crunchy and good, or is it mealy and soggy? Are these fries fresh and crispy, or they are they wilted and stale? But also the action of things like chili peppers to give a heating sensation and mint to give a cooling sensation and wasabi and horseradish to give a pungent sensation - those are actually all from the activation of touch receptors that happen to be in the oral cavity.

GROSS: The word placebo usually gets a bad rap. (Laughter) Like, it's like somebody's been tricked into thinking something's going to help them. And they may think it's helped them, but it really hasn't 'cause it's just a placebo. It's just, like, a sugar pill. But, you know, scientists are getting hip to the fact that now that - like, if a placebo can make you feel like it helps, maybe it's engaging your mind in such a way that it really is helping. It's helping your mind overcome the pain and, you know, heal itself.

So in the context of what we've been talking about - of, like, pain and pleasure and how the mind - how the brain handles that - what are some of your thoughts about placebo and the opposite of that, which is a nocebo, which is when you're given something that you're told will intensify the pain. You can definitely experience an intensification of that pain, even if it's just, like, a sugar pill.

LINDEN: Well, I think the general thing I take from this is that in the end, substrate is biology. When things work, whether they are drugs or the placebo effect or acupuncture or meditation or psychotherapy, they work because they're changing the functions of brain circuitry. And my feeling is that if it works, it works, and it should be used.

There's no reason to abandon something that works just because we don't understand all the biological steps in the way it works. In truth, many of the most popular drugs in the armamentarium for neuropsychiatric disorders we still don't understand how they work, anyway. We don't entirely understand how anti-depressants work. We don't understand how lithium works for bipolar disorder. So if the placebo effect works, then let's use it.

GROSS: If you're just joining us, my guest is David Linden. He's the author of the new book "Touch: The Science Of Hand, Heart, And Mind." He's a professor of neuroscience at Johns Hopkins University. Let's take a short break, then we'll talk some more. This is FRESH AIR.

(SOUNDBITE OF MUSIC)

GROSS: This is FRESH AIR. And if you're just joining us, my guest is David Linden. He's the author of the new book "Touch: The Science of Hand, Heart, And Mind." He's also the author of the previous book "The Compass Of Pleasure," which is about how the brain experiences pleasure. And he's a professor of neuroscience at Johns Hopkins University.

One of your areas of research is memory. And I would love it if you would tell us something exciting in the forefront of science now that relates to our knowledge of memory, how it works and how it diminishes (laughter).

LINDEN: Well, I think to my mind, the most important realization that's come in recent years about memory is that it is not like a computer's hard drive or a card catalog full of cards with information written on it that you pull out and look at and then put back and then come back to again and again. What we now know is that every act of recollecting a memory makes it a little bit labile, so that every time we recall a memory we can alter it a little bit. We render it susceptible to change. And you might think, well, why would you want to do that? Isn't that just a bug? Doesn't that just degrade the fidelity of memory? But the important thing is that it's actually a feature. Memory is only useful to us if we can link it with all our life experiences and our ongoing experiences that accrue after the memory was initially laid down in the brain. If we can't cross-reference it, then memory isn't very useful. And this process of making it labile during recall is what allows us to cross-reference it with subsequent experience.

Now, the dark side of that is that it also means that our memories for facts and events are often not very accurate, and eyewitnesses are not very accurate. And what's worse than that, it's not just that our memories aren't that good, it's that we don't even know when they're not that good. So if you ask people in situations where you can check up on them, how confident are you that your memory is accurate about this event, their indication of their confidence is no prediction at all about how accurate they really are.

GROSS: So first of all, I've never heard the word labile before. That means changeable? What does it mean?

LINDEN: Labile means changeable, yes.

GROSS: So when you say that memories are constantly changing and they need to be constantly reintegrated into who we are and what we're thinking, is one example of that how the memories we made as a child we keep kind of reinterpreting them as we get older and a little smarter and more knowledgeable?

LINDEN: Not only can we continually reinterpret them, but we are subject to bias from people around us, and we are the most subject to this bias when we were small. It turns out if you want to implant a false memory in a 5-year-old, it's an extraordinarily easy thing to do. You say, you remember that time the clown came to your kindergarten last week? No. The next week - you remember the time the clown came to the kindergarten? Yeah, I think I do. The third week - do you remember the time the clown came to your kindergarten? Yes, I do. I remember he had a funny, red nose and green hair. So not only can you implant false memories, but you can then cause someone to start elaborating very reasonable-sounding details based on those utterly false memories.

GROSS: So as a professor of neuroscience, do you do certain things to take care of your brain to either strengthen it or to protect it?

LINDEN: What I do to strengthen and protect my brain is physical exercise. The single best thing you can do for your cognitive function, particularly when you're in middle age like me, is to get out of your chair and move your body around. It is a much, much bigger effect than any of these brain-training games or puzzles or things that people want to sell you.

GROSS: Why is that true?

LINDEN: Well, it turns out that these puzzles only produce a very small improvement and that the improvement does not generalize very much behind the task of the puzzle itself. It turns out that when you exercise, you are dilating the blood vessels in your brain, you are changing the metabolic capacity of your brain. You're causing your brain to secrete chemicals called trophic factors that appear to keep neurons healthy and changeable. And we don't entirely understand the molecular basis of the beneficial aspects of exercise, but the aspects are enormous. It reduces anxiety; it prevents depression; it improves cognitive function. If there is a single thing to do for your brain health, it is to do 30 minutes of aerobic exercise a day.

GROSS: OK. Well, this has just been fascinating. I thank you so much for talking with us.

LINDEN: Well, thank you so much. It was lots of fun.

GROSS: David Linden is a professor of neuroscience at Johns Hopkins University and author of the new book "Touch." You can read an excerpt on our website, freshair.npr.org. Coming up, Ken Tucker reviews Bob Dylan's new album of songs that have been recorded by Sinatra. This is FRESH AIR. Transcript provided by NPR, Copyright NPR.