Wednesday, January 31, 2007

The Physics Guide to Hooking Up

Another Saturday night is just around the corner, and you're looking forward to cruising for action at the local hot spots.

How'd it work out for you last time? Did you hook up with your dream guy or gal, or did you strike out? Either way, it's possible that you could improve your odds by applying the physics published in the journal Physical Review Letters a few years ago.

Listen to the podcast with Text-to-Speech roboreaders Kate and Paul.

Or, if you're in a rush, Skip to the Tip in this week's post.

In 2002, a group of Spanish and Brazilian physicists looked at two types of search strategy that might be employed by such things as predators in search of prey, bees in search of flowers, or other creatures (like you) in search of mates. They found that the searchers could dramatically improve their odds by tailoring their strategies depending on the distribution and motion of their targets. Theoretically, you should be able to improve your odds of finding that special someone as well.

When you go out on the town looking for love, you have at least two options. For one thing, you could pick a bar and settle in for the night, while doing your best to mingle as you work to attract or seduce someone.

This type of search strategy is called a Brownian random walk. You just bounce around to search randomly for a love connection in some small area, such as the dancefloor of your favorite bar. Eventually, you might drift to another nearby establishment. But in any case, you don't cover a lot of ground over the course of the night.

Alternatively, you could bar hop - drop in on a bar, work the room, and then if there's nothing promising, dash to another bar to do it again.

This second type of strategy is called a Lévy flight search. Lévy flights involve poking around in one location, and then zipping off to poke around somewhere else. Lots of creatures use Lévy flights for searching large areas, when there are sparse distributions of what ever it is that they're after. Bees often hunt for pollen rich flowers this way, and there's a good chance that you look for your lost keys with a similar search pattern. (You might check the dresser, skip down to look through the desk, pop over to the closet to check your coat, etc.)

As the research team ran their simulations, they found that when the targets were relatively stationary and far apart, searchers increased their odds of success by performing Lévy flights from place to place. Picture, for example, groups of eligible singles nestled at bars around town, with the bars far enough apart that you have to drive or walk a long way to get from one place to the next.

If instead, the targets moved around a lot or there were many of them packed in a large area, then searchers were more successful when they avoided Lévy flights and just flitted around randomly in a small area.

The first scenario sounds a lot like the club scene in most major cities, and the second scenario is more like the flowing crowds at Carnival in Rio or Mardi Gras in New Orleans.

At first glance it seems like the best bet is simply to zip from bar to bar with a series of Lévy flights, so long as you're cruising in town. But if you're at a big event with lots of available singles around, you should stay in one place.

Unfortunately, things aren't always so easy. If you're making lots of Lévy flights to search the clubs, and your targets are making frequent Lévy flights as well, then the chances are you're going to miss many of your potential love connections while in transit. When targets are highly mobile the physics model suggests that a searcher, like you, should pick one place and stay there to wait for your potential mate to make a Lévy flight right into your lap. In fact, the faster your targets are moving the less you should stray from your barstool.

In some cases it makes sense to evaluate the traffic flow and adjust your strategy throughout the course of the night. That would've worked best for me when I was in college and I used to head with my buddies into town to meet girls. Early in the evenings, clusters of women would seem to be roaming everywhere. And we were roaming too, occasionally flirting as we were going along. In retrospect we could probably have met more women if we'd settled somewhere and waited for them to come to us.

But as the night progressed and the alcohol kicked in the girls tended to travel less, and many of them eventually took up residence at various bars. That would've been the time to zip from bar to bar looking to hook up. Of course, by then we were usually pretty tipsy too and probably in no shape to walk or drive very far.

If I had it to do over again, I would spend the early part of the evening sipping mild drinks and sodas while attempting to charm the girls who were cruising through one of my favorite hangouts. Then I could've turned to Lévy flight searches from bar to bar, once the women slowed down.

The physicists who ran the search simulation weren't specifically thinking of the singles scene, they also considered things like the relative sizes of searchers and targets. For large creatures in search of small targets - like foxes hunting for rabbits - Lévy flights usually work best. For small searchers on the prowl for large targets - like parasites hoping to latch onto passing horses - it's best to sit tight and wait.

For humans, who are all roughly the same size (from a physics point of view) relative size isn't an issue. We only have to worry about the relative motion and the distribution of the people we'd like to meet.

So the next time you head out to the club scene to hook up, stop to take stock of the situation. If the kind of people you're after are bar hopping, you should stop in one establishment and mingle. If there's not much traffic in and out of the bars, then consider Lévy flight searches. And if you're going to Mardi Gras, pick a location, sit tight, and wait for the prospective mates to come to you.

Read the rest of the post . . .

Monday, January 29, 2007

Book Review: The Science of Orgasm

The Science of Orgasm by Barry R. Komisaruk, Carlos Beyer-Flores, and Beverly Whipple has got to be one of the best science books on sex that you can buy, if you can deal with wading hip deep through medical and biological research jargon.

I learned something new on just about every page, and each fascinating factoid and phenomenon - from the horrific sexual behavior that can result from certain types of brain damage to the question of whether or not orgasms are good for your health - is backed up with citations from top research journals and institutes. The authors themselves are responsible for a significant amount of the original research in the book. It seems pretty clear that that they know what they're taking about.

Still, you have to be awfully determined to plow through passages like this, "The participation of the adrenal cortex as a source of steroids capable of maintaining sexual response in women after bilateral oophorectomy has often been suggested" (page 179).

It can't hurt to have browsers open to Gray's Anatomy (the medical text, not the TV show) and Wikipedia as you make your way through the book, just to keep up with the lingo.

It's interesting that the publisher would choose cover art that resembles a plain brown wrapper, as if you're going to buy a copy to read under the covers while your mom thinks you're sleeping in. No matter how sexy the topic, doctor-speak is hardly a turn on. (Although, I'm sure it works for someone.)

If you read nothing else in this book, I highly recommend the brief section addressing the biological function of the female orgasm (pages 10-15). I have never seen a more coherent and compelling argument that orgasms in women serve some vital, if only partially understood, purpose. It's an excellent counterpoint to arguments claiming that orgasms are critical rewards to induce men to mate, but in women are only evolutionary accidents.

I'm not going to go into details here, but basically the authors point out that women seem to have at least some specialized anatomy that lets them experience types of orgasms that have no male equivalent. (The authors even invented a device to give women orgasms by stimulating only the cervix, which is an exclusively female body part.) The female-only orgasms can't be something left over from male anatomy, they conclude, if they can't exist in male bodies.

I plan to explain things more fully in a future post, but if you can't wait and you think reading the Journal of the American Medical Association is a good way to pass the day, pick up a copy of The Science of Orgasm. If nothing else, it's a good addition to your sexual science reference shelf.

Read the rest of the post . . .

Thursday, January 18, 2007

Is the Select Comfort air mattress good for sex? A PoS experiment

If you're eagerly anticipating part 2 of Sex and Sensibility, we'll have that for you next week.

In the meantime, we decided to send two of our writers, Buzz Skyline and Martica, into the field to do an experiment inspired by a portion of the Physics of Sex post Sexual Rhythms.

Specifically, we wanted them to see what physics could tell us about the Select Comfort brand adjustable bed. We hoped they would learn enough to help you determine if it's the best bed for your love life.

You can listen to roboreader Heather interviewing Martica and Buzz in our latest podcast, or read the transcript below, to find out what (if anything) the Sleep Number bed has to offer for sex.

Skip to the tip in this week's post, if you're in a rush.

In any case, please take part in The Great Physics of Sex Bed Test. We want you to test out your bed and send us the data so that we can figure out, once and for all, what type of bed is best for sex. But don't do it for us. Do it for yourself. Do for the world. Do it for science.


Heather: Welcome to the Physics of Sex podcast. My name is Heather.

If you've been listening to our past episodes, you know we usually give a little lecture about a fascinating aspect of physics, as it applies to your love life. But this week, we decided to try something different.

We sent two of our writers into the field to do a few experiments on an unusual kind of bed, in order to find out how it might affect your sex life. Here to report on what they found out about the Select Comfort sleep number bed, are Martica. . .

Martica: Hi Heather.

Heather: . . . and Buzz Skyline.

Buzz: Hi Heather.

Heather: So guys, you ventured out to test a bed. Did you do what I think you must have done? How'd you keep from being arrested?

Martica: Well, we didn’t actually do anything that's not appropriate in a mattress store. We actually just went and jumped on the bed.

Buzz: Well, we sat and bounced on the bed.

Martica: Exactly.

Buzz: We didn’t stand on the bed.

Heather: OK, well tell us about the bed.

Martica: It’s a Select Comfort Sleep Number bed, which [allows you to] change the firmness of the mattress.

Buzz: You can change the amount of air inside the mattress.

Martica: Right.

Buzz: And they call it the firmness.

Martica: It’s a big air mattress with a pump and a little remote control which you can use to pump the air in or let the air out and that changes the firmness, what they call the firmness, of the mattress.

Buzz: And it basically is just inflating this bladder that’s inside the bed instead of springs.

Heather: So this is the bed Lindsay Wagner promotes on TV. Great. How did you do the experiment?

Martica: We sat on the bed and bounced up and down.

Buzz: At different numbers. We set it for different numbers and we took turns bouncing on it. Martica bounced on it a few times at different settings and I bounced on it at a few different settings, a few different sleep number settings, and the last time . . .

Martica: We sat next to each other and bounced up and down at the same time, which is actually really hard – but fun. (laughs)

Heather: What sort of results did you expect?

Buzz: So, what we thought would happen was we assumed the sleep number really was what you call firmness, which on a spring would basically be the spring constant. It would tell how strong the springs are. And that means that as you turned it up it should increase your resonance frequency- the frequency that you bounce on the bed. And as you turn it down, it should decrease your resonance frequency. And so we started out with Martica on the bed, and what we found was no matter what sleep number we chose, she bounced at about the same rate.

Martica: I was trying! (laughs)

Buzz: It wasn’t her fault. It was obviously the physics. And so we thought there must be something wrong. So I sat on the bed and tried it for several numbers and I also bounced at almost exactly the same rate every time.

Martica: But a little bit more because you’re a little bit heavier than I am.

Buzz: Yeah, I bounced slower than Martica did because I weigh about 50 pounds more. So you would expect it to be a little slower, and it was a little slower, but by the same amount every time than Martica’s bounce was.

Martica: No matter whether the bed was really, really, really firm and full of air or really, really, really soft and the balloon was almost flat.

Buzz: So basically that means that whatever the sleep number tells you, it doesn’t really tell you the firmness in the same way that a bed is firm, it doesn’t tell you how firm the springs are. It’s changing something else.

Heather: All right then, what's going on?

Martica: So what it actually is that’s changing is the damping of the bed. It’s like having a giant pillow that you can compress and make it firmer or make it softer.

Buzz: Yeah. So if you were to open the bed and look at the bag, you would either see that it was completely full at a hundred percent, or a hundred sleep number, but usually it was kind of floppy.

Heather: So the sleep number doesn't matter for sex? You just set it wherever you want and things don't change?

Buzz: Well we did find that there was a change as we turned the sleep number up or down.

Martica: Right, it got harder to bounce on.

Buzz: It was more . . .

Martica: It took more energy because you’re bouncing on a less inflated balloon, basically.

Buzz: You are tuning something. You’re not tuning the spring constant, but we were tuning the damping. It made it harder to maintain a bounce because it was . . . it wasn’t changing the spring constant, but it was sucking energy out of the bed, it was damping the energy. It meant that we had to work harder to bounce when you turned the sleep number down, which corresponds to turning up the damping but not changing the spring constant.

Heather: Do people who actually buy the bed think about this sort of thing?

Buzz: In fact, we had the same question. So I went back when the store was closing and there weren’t any customers to distract the salesman, and I asked him that very question. And this is what he said.

(Mall noise)

Buzz: And so the feature we’re doing is a question of whether or not some beds are better for a love life than other beds.

Salesman: Uh huh.

Buzz: And we’ve gone around and we’ve measured basically the resonance frequency of various beds and . . .

Salesman: Uh huh.

Buzz: . . . things like that to try to understand how they’re different.

Salesman: Huh.

Buzz: So I was wondering – do people ever take that into consideration? When they talk to you when they are about to buy a bed, is that something that ever comes up?

Salesman: Nah, it never comes up. Maybe, maybe they think about it, or maybe they talk to each other about it.

Buzz: So it’s not like a waterbed store where you know what they’re there for . . .

Salesman: Right.

Buzz: This is more for . . .

Salesman: I’ve never, never heard anybody say anything like that.

Buzz: How long have you been sellin’ the beds?

Salesman: Since November.

Buzz: Oh OK, so you haven’t been doing it too long. So some people may think that . . . But if they did there’s not . . .

Salesman: I’ve slept on one. I’ve slept on one for about six years. But my wife and I, we don’t talk about it.

Buzz: You don’t adjust the pressure . . .

Salesman: No.

Buzz: . . . or anything?

Salesman: No no. Nope.

Buzz: Oh OK. (laughs) Alright. Well I know they’re strange questions.

Salesman: No no no. It’s all part of research.

Buzz: Exactly.

Salesman: I’m doing my part for science.

Buzz: OK. Alright.

(Mall sounds fade.)

Heather: So, having put the Sleep Number bed through it's paces, what are your recommendations?

Martica: Well if you already have a sleep number bed then you can turn it up for sex, if you want to make sure you’re getting the most bounce for the energy you put in.

Buzz: With a regular mattress, you can add damping. You can add something like comforters or pillows or something. You have to add additional material to make it softer to make it more comfortable or to change the rhythms of sex. Whereas with the Sleep Number bed you could potentially, with just this one adjustment, quickly go from what’s comfortable for sleeping to what’s comfortable for sex and back again.

Heather: Do you have plans for follow up experiments?

Martica: We plan to go to a mattress discounter store where there will likely be more mattresses with springs inside of them rather than air inside of them, and jump up and down on the beds there as much as they’ll let us. And ask some of the same questions. If maybe people who buy spring beds are looking to see how their sex lives will be changed by these beds. And if we go to a waterbed store, maybe people are even more interested in sex when they come into a waterbed store looking to buy a waterbed.

Buzz: It would be a completely different experience. And we could also consider alternative sleeping surfaces, like futons.

Heather: That's great. You've certainly given us something to think about. Thanks for stopping by.

Martica: Thanks Heather. It was great to talk to you.

Buzz: Bye.

Heather: After the select comfort adventure that Martica and Buzz went on, it occurred to us to ask our listeners and readers, to tell us about your beds. What type of bed do you sleep on? Is yours good for sex? Is it fast or slow? Do you prefer lots of damping or just a little?

We'll post instructions on our website,, to let you know how to send us some data. We'll analyze it and hopefully report back with the results in a few weeks.

Thanks for listening to the Physics of Sex podcast. I'm Heather.

Download us next time for the second segment of last week's show, part two of Sex and Sensibility, the physics of the nervous system.

Bye for now.


The Great Physics of Sex Bed Test

It's this easy.

Send us an email or write in the comments section the information you collect by following these instructions.

1. Tell us what kind of bed you have (spring mattress, waterbed, space-age foam, futon, air mattress, etc.)

2. Estimate the damping on your bed - is it plush (like a pillow top bed), moderately padded, or a plain mattress?

3. Measure your bed's resonance frequency by

a. getting a watch with a second hand

b. sitting on the bed and bouncing at the rate that feels most natural

c. timing how long it takes to bounce 25 times

d. It couldn't hurt to do the experiment a few times, or even get your partner or friends to try it. Be sure to note both the time it takes to bounce 25 times and the weight of the person bouncing. We'll need both numbers to calculate your bed's spring constant.

4. Tell us whether your bed is good for sex on a scale of 1 to 5, where five is Nirvana and one is like doing your taxes.

5. Send the information to us in an email to, or paste it into the comments section for this blog entry.

Read the rest of the post . . .

Tuesday, January 16, 2007

The Physics of Sex Cited Among the Best of Science Blogging

The Physics of Sex post Sexual Rhythms is included in a brand new anthology of the 50 best science blog posts of 2006.

The Open Laboratory: The Best Writing on Science Blogs 2006 was edited by Bora Zivkovic (aka Coturnix of Seed Media's, who is probably one of the hardest working folks in the blogosphere.

Bora chose the entries with the help of a select group of the top science bloggers around. The whole process is detailed on Bora's web page.

Buy the book and find out what wild and wonderful things are out there under the heading of science blogging. (No one at The Physics of Sex is getting a penny for this endorsement, BTW. We just like to encourage initiative like Bora's.)
Read the rest of the post . . .

Wednesday, January 10, 2007

Skip to the Tips

One of the great things about pondering the Physics of Sex is that it naturally leads to tips to enhance your sex life. Several of the posts that you can read here have a physics inspired suggestion or two in them.

Some of the suggestions - such as how to choose a good bed for sex - include information that you may not be able to find anywhere else.

For those of you who don't have time to read the full posts or listen to the podcasts, we have made a list of the tips that lets you jump straight to the good stuff.

You'll see this little icon next to italicized text to highlight the Physics of Sex Tips in each post.

Check out the tips below.

The Physics of Sex Tips (so far . . .)

-> Find the best bed for your sexual style. (from Sexual Rhythms)

-> Take control of your vibe with damping. (from Sexual Rhythms)

-> Choose the right lube. (from Slip, Slide or Stick)

-> Find out how the pinch technique can enhance your erection . . . or how it can be adapted to women. (from Pumped Up and Ready for Love, Part 1)

-> How you should adjust your smoking habits to reduce the negative impact on your sexual function. (from Pumped Up and Ready for Love, Part 2)

-> How losing weight can improve your circulation and make you better in bed. (from Pumped Up and Ready for Love, Part 2)

-> A safer alternative to the dangerous practice of erotic asphyxiation. (from Pumped Up and Ready for Love, Part 2)

-> How minding the negative feedback in your sensory receptors can help you make sex more intense. (from Sex and Sensibility, Part 1)

-> How the "start-stop method" helps you manipulate your sensory receptors to get past premature ejaculation. (from Sex and Sensibility, Part 1)

-> Two ways the physics of sensory receptors can add an extra tingle to sex. (from Sex and Sensibility, Part 1)
Read the rest of the post . . .

Tuesday, January 09, 2007

Part 1 of Sex and Sensibility: the Physics of the Nervous System

Sensation – it’s not all that sex is about, but it’s a lot. Sure, intimacy is important, and so are trust and communication. But when it comes down to it, one of the best things about sex is that it feels good.

(Listen to the podcast with Text-to-Speech roboreaders Heather and Graham.)

You might wonder why the touch of a fingertip, lips or tongue that is barely detectable on one region of your body causes a surge of pleasure somewhere else. And what is it that makes a caress, pinch, slap, or tickle feel just right at one moment and completely wrong at another? There’s more to it than simple physics, but a look at the nervous system through the eyes of a physicist can help you get a handle on the sources of your sexual pleasure. . .

In order to enjoy sexual sensations your body needs to do at least three things. First it must detect the sensations. Next it has to send information about them to your brain. And finally, it must interpret those sensations as pleasurable. The three components in your body that accomplish these tasks - sensory receptors, nerves, and specialized regions of your brain - are portions of your overall nervous system.

These parts of your body are responsible for much more than making sex pleasurable, of course. The nervous system controls the movement of muscles, both voluntary ones such as your arms and legs and involuntary ones including your intestine and heart. It also monitors the status of your organs and generates thoughts and emotions. In a way, the nervous system is the part of the body that makes us who we are. After all, a person who loses a limb or has a heart, eye, or even face transplant is still the same person.

In effect all the parts of someone’s body could be replaced, but as long as the lump at the end of your spinal cord - which we call the brain - remains intact most of us would feel the essential person that is you is still here. Death, in fact, is defined as the cessation of activity in the brain, regardless of the condition of the rest of the body.

When you have sex or fall in love, revel in a moment of ecstasy or sink into the depths of despair it’s your nervous system that is experiencing all these events. The rest of you - including the bones, muscle, fat, and organs - is only a compilation of components in the vehicle that carries your nervous system around and lets it enjoy the world.

In this week’s edition of The Physics of Sex, we’ll start at the beginning - by looking at the structures in your body that first respond to events around it.

All sensations, sexual or otherwise, start with a sensory receptor. Rods and cones in the retina of your eye react to light, tiny hairs deep inside your ear detect sound, chemical receptors in your nose and on your tongue reveal smells and flavors, and receptors in your skin alert you to cold, heat, pressure and pain. In each case a receptor converts a stimulus into a signal that nerves transmit to other parts of your body.

To a physicist, a sensory receptor is a kind of transducer. Transducers take incoming signals of one type and change them to a convenient form that is easier to transmit or interpret.

Manmade transducers often convert information into electrical signals. A microphone, for example, changes sound into electrical waves before it sends it over metal wires. A speaker, in turn, is a transducer that converts the electrical signal back into something you can hear.

Long before humans thought to build transducers, nature had already discovered essentially the same trick. For example, when cold receptors in skin are exposed to low temperatures they move chemicals around to produce a small electrical voltage.

Heat receptors do the same thing when exposed to elevated temperatures, and receptors on the tongue and in your nose produce voltages in response to chemicals. Mechanical receptors, located primarily in your skin and muscles, produce electrical signals when they are squeezed, stretched, bumped or shaken.

Regardless of their particular sensitivity, all sensory receptor cells are really just small bags of electrically charged fluid. The bags are made of membranes of fatty cells, (much like the soap micelles described in the Physics of Sex chapter on lubrication).

Tiny chemical motors called ion pumps are embedded in the membrane. The pumps move electrically charged ions in and out of the cell. Ions that have excess electrons are negatively charged and ions with missing electrons are positive. The pumps push more of the positive ions out of the cell than into it, so the fluid inside becomes charged from the excess of negative ions that stay behind. The charge is small, roughly 70 thousandths of a volt, which is about twenty times smaller than the charge on a fresh flashlight battery.

When a sensory cell is triggered, pores open up in the cell membrane. This lets positively charged ions flow back into the cell, which makes the electrical voltage inside surge upward. If the stimulus is too mild, the voltage only goes up a tiny bit and the ion channels shut down again to let the pumps restore the cell to its negative resting voltage.

For a larger stimulus, above what is called the triggering threshold, the flow of charge is large enough that it causes more and more of the cell's other ion channels to open, leading to a dramatic voltage change and the cell fires an electrical pulse.

Triggering a sensory cell to fire is a bit like setting off an avalanche on a snow bank. A whisper may not get the snow to break loose, but a shout or a handclap can sometimes be a large enough trigger to send a wall of snow careening down a mountain. Similarly, when some ion channels open up in a sensory cell membrane they induce others to open as well. If too few open up to begin with, they all shut down in a fraction of a second. But if enough open up initially, they trigger an avalanche of ion channels to open.

Mathematically, a sensory cell’s response to stimulation is known as positive feedback. It occurs whenever a stimulus causes an effect that in turn increases the stimulus. Positive feedback typically leads to dramatic events, such as stock market crashes and orgasms, in addition to sensory cell bursts.

Once the flow of positive ions into a sensory cell raises the voltage to 10 millivolts or so, then no more positive ions can squeeze in and the ion channels clamp down. In the same way, an avalanche ceases after all the snow has slid down a valley wall. At this point the sensory cell and the snow filled valley are in their refractory states, which means that they cannot respond to stimulation, at least for a while. In a sensory cell the ion pumps charge the fluid back to a negative voltage and the cell is ready to fire again. Valleys prone to avalanches, however, must wait for snow fall to build up before they can go off once more.

The whole process takes a few thousands of a second in a sensory cell. If the stimulus remains after the cell has completed its cycle, it will fire another pulse.

The positive feedback in a sensory cell ensures that any stimulation, regardless of its strength, will lead to the same electrical burst, provided that the stimulation is above the triggering threshold. (Anything below the threshold - too gentle a touch, too quiet a sound, etc. - will simply go unnoticed.) Nevertheless, it's clear that we can tell the difference between a light touch and a heavy touch. Sensory cells reflect the intensity of a given stimulation by firing a train of electrical pulses. The more intense the stimulus, the more rapid the pulse train. In cases of extreme stimulation, the sensory cells fire immediately upon recovering from their refractory period.

Over time, a receptor exposed to an unchanging stimulus will gradually cease to respond. If you were to record the pulses coming out of a touch receptor, you would see that when it is first stimulated by a firm touch it emits a rapid string of pulses. After a few moments the pulses slow. At this point, the receptor has adapted to the stimulation. Removing the stimulus would then lead to another string of rapid pulses, which again taper off over time.

It is this sort of desensitization that allows you to ignore the touch of clothing against your skin, forget that you have your sunglasses resting on top of your head, and gradually come to tolerate the temperature of a hot shower.

Adaptation to an unchanging sensation can be thought of as a kind of short-term cellular memory. Once a sensory receptor adapts to a stimulus, it acts as though it can’t remember a time when the stimulus wasn’t there. Taking it away is a shock, but the cell soon gets used to the absence of stimulation as the new status quo, and loses all memory of the earlier stimulus. In contrast to the positive feedback avalanche of a firing sensory cell, sensory adaptation is a form of negative feedback.

Despite the term’s pessimistic-sounding name, negative feedback is a good thing most of the time. If a radio is too loud, you turn it down. If it is too quiet, you turn it up. Eventually you'll find a comfortable volume. This is negative feedback. Anything that must remain stable over time needs negative feedback, whether it's your weight, body temperature, emotional state, or even your bank balance.

There must be some flexibility in negative feedback systems to adjust for new situations. For example, there was likely a time when you were young that your appropriate weight was fifty pounds and negative feedback helped you stay close to that number, at least for a while. In order to grow, your body had to allow your weight to increase over the course of months and years. Most negative feedback systems operate this way - working to reduce sudden changes while adapting to gradual shifts. It is as if negative feedback systems have short-term memory and long-term amnesia. In sensory cells, the long-term amnesia results in raising or lowering the cell’s triggering threshold.

Negative feedback helps explain why it’s important to mix things up in bed. Pinching a nipple may be pleasurable for a few moments, but the sensation will fade if it's not varied and the sensory cells have a chance to adapt to the pinch. On the other hand, simply releasing the pressure on a nipple can be pleasurable as the sensory cells react to the change in status.

Varying the type and location of stimulation on a penis or clittoris during oral sex, and switching sexual positions from time to time during lovemaking, take advantage of the short-term memory of sensory receptors.

By focusing on one portion of an erogenous zone, you allow the sensory receptors in other places to adapt to the lack of stimulation, which makes them particularly sensitive when it’s their turn.

A commonly repeated myth claims that women who use vibrators will eventually lose sensation in their genitals. While it’s true that extended vibrator play in a session can lead to numbness, the effect is only a temporary resetting of the triggering threshold of the mechanical sensory receptors. It is certainly possible that intense vibration can rupture cells, but we all have plenty of pain sensors in our genitals that produce clear signals to warn of tissue damage. The intense agony of misusing a vibrator would make most women stop well shy of doing themselves any permanent injury. Thanks to the dense packing of sensory receptors in our erogenous zones, the risks that come with using vibrators are low and the orgasmic benefits are very high. In fact, many women find that using a vibrator helps them learn to orgasm more easily, even when they are not using powered toys.

Sensory adaptation also provides ways to alleviate problems with premature ejaculation in men. One method for increasing a man’s staying power involves repeatedly stimulating him to bring him to the brink of orgasm. Stopping just short of ejaculation and waiting a few moments, and then repeating the procedure several times, raises the triggering threshold of the sensory receptors in the penis to the point where a hair-trigger man can maintain self-control for longer and longer periods. Sex therapists often call this the stop-start method.

It may seem paradoxical that sensory adaptation can enhance sexual sensations under one set of circumstances, and desensitize a man’s penis under another. The key to the different benefits lies in the timing. Consider a hot shower, for example. As you step under the showerhead the steaming water might be barely tolerable. After a minute or so, the pain subsides as your temperature sensors set their pain threshold higher. Provided you remain under the shower, or move away for only brief periods, the water will still feel hot but won’t hurt. If you step out of the shower for several minutes or neglect to run water on your back for a while, the water will sting again when it hits the regions of your skin that have readapted to cooler temperatures.

Temperature sensors in your skin adapt more slowly than touch sensors, so you only have to wait a few seconds for a nipple, penis or clitoris to become re-sensitized after removing its stimulation. Similarly, in helping a man to stave off premature ejaculation it’s important not to entirely cease stimulating him for too long as he nears orgasm. Just ramp it back briefly. Otherwise you may end up speeding things along rather than slowing them down.

There are other ways to manipulate sensory receptors. You can use an ice cube to set temperature thresholds in your skin very low, say by cooling down a nipple, and then sucking on it to give your lover a sensory roller coaster ride. You can achieve a similar effect chemically with oils and lubricants that make your skin feel hot or icy cold (sometimes in rapid succession). Cooling and heating liquids and lubes contain compounds such as camphor and menthol that cause skin sensors to overreact to small temperature changes. If your partner breathes gently onto skin exposed to such a chemical, the warm breath will feel oddly hot, and if your lover blows vigorously on your skin, the rapidly moving air will feel surprisingly cold. In each case their breath is actually very close to body temperature, but the slight difference in skin temperature is amplified by the effect of camphor or menthol on the sensory receptors.

Once a sensory cell has fired in response to a stimulus, the signal is passed to nerve cells. But that is a topic that will have to wait for now. To learn about the physics of the nerves that connect to your sensory cells, be sure to come back next week to read part 2 of Sex and Sensibility: the Physics of the Nervous System.

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