Bathroom Reading

Your brain probably suppresses your own poop smells.

Posted Jul 04, 2017

Source: Dreamstime

Do you like to read in the bathroom? Do you lose yourself there in books or magazines? Do you spend extended times doing crossword puzzles on the throne? You probably don’t declare to the world that you do, but if you’re like most people, the time you spend “en toilette” probably exceeds the time for basic functions.

Why don’t you get grossed out by the bathroom smells you make? It’s probably not that your poops are more fragrant than other people’s. You may be special in many ways, but generating balmy BMs is probably not one of them.

As far as I know, there has been no scientific research on why people find their own poop smells tolerable. Maybe that’s because most researchers don’t want to be associated with disgusting topics, though the understanding of disgust has been of longstanding interest to psychologists. For me as an experimental psychologist, the question of why we don’t mind our own toilet odors is as legitimate as any other. 

So why don’t your own poops smell? I’ll venture a hypothesis, focusing on the fact that to be indifferent to your own bathroom smells, you need to have recently generated those smells yourself. Merely knowing the smell is yours doesn’t explain your aromatic immunity, for if you return to “the scene of the crime” not too long after you’ve done your business, you’ll be as grossed out as anyone else. Similarly, if someone you live with happens to walk on in you in the midst of your toileting, that person, though he or she may adore you, will complain mightily about how disgusting your output can be.

The fact that you’re not bothered by your own poop smells if you just produced them is akin to a phenomenon of visual perception I share with my students when I lecture about perception and performance. I often ask a pair of students to come to the front of the classroom and gaze into each others’ eyes. Then I hand one student a mirror and invite that student to watch his or her own eyes in the mirror. The other student watches the mirror-holder’s eyes. “Can you see your own eyes move?” I ask the student who is looking into the mirror. “No,” he or she answers, echoing a classic finding in the literature. Then I turn to the other student. “Can you see the mirror-watcher’s eye movements?” I ask. “Yes,” the observer answers. The two students then reverse roles and, sure enough, the one looking in the mirror now discovers that he or she can’t see his or her own eye movements, but the other student – the first mirror-watcher – can. Therefore, neither student’s eye movements are too quick to be seen. Rather, each student’s eye movements, when generated, become invisible to the person generating them.

An analogous finding applies to touch. You can’t tickle yourself, or if you can, the experience is damped down. If someone else tickles you, you may laugh or shriek with delight, but if you tickle yourself, you just feel foolish. Self-tickling, it turns out, leads to predictable inputs, which your brain suppresses. Tickles from others yield less predictable inputs, and the experience becomes more rousing. (Sex with others versus oneself probably works the same way.)

Another example of perceptual suppression of self-generated outputs is related to blinking. You blink roughly once every 2 seconds, and when you blink, your lids cover your eyes for about .2 seconds. So your vision is interrupted by your blinks about 10% of the time, which means if you’re awake 16 hours a day, you’re in the dark for about 1.6 hours. Experiments by vision scientists have shown that being impervious to blink-related darkening is due to the brain’s suppression of the perceptual results of its own lid-closing commands.

The realization that the brain engages in self-suppression of perceptual input came from the great 19th century German scientist, Hermann von Helmholtz, whose inventiveness you’ve experienced when you’ve had your eyes checked. Helmholtz invented the ophthalmoscope, the device doctors use to peer into people’s pupils. Helmholtz realized that the brain makes adjustments to perceptual changes caused by the body movements it effects. His insight has been rediscovered and extended many times since he announced it in the 1800’s.

Given that auto-suppression characterizes vision and touch, it’s not surprising that it might arise in other domains. If your brain makes you blind to the predictable visual inputs you produce, and if your brain makes you tactually insensitive to the predictable touches you generate, then it probably also attenuates the smells you manufacture.

Is this idea worth testing, and if so how might it be tested? The idea deserves checking because if what you do prepares you for what you’ll smell, that could have important consequences. Is the mustard on your hot dog less tangy than you’d like, for example? Having someone else put the mustard on the dog should make it tangier according to the hypothesis offered here. What the effect of that experiment will be a few hours later is a private matter.

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