Honey and candy are shown together on a tabletop.

Stickiness and Slipperiness

Some sensations seem to come out of nowhere. We lack stickiness receptors, yet we know it when we feel it. Baseball players know that a smear of pine tar improves their grip on bats and balls, but maybe they don’t know why. It’s not as simple as seeing a flash of light or hearing a firecracker.

A similar puzzle arises with sensations of slipperiness and wetness. Sensory receptors are necessary to feel them, but touch receptors have never been specialized for them. We call them perceptions rather than sensations because they require some further analysis to make sense of what’s happening. For example, stickiness isn’t just a signal from touch receptors because we can judge stickiness by what our eyes and ears tell us as well as our skin.

Colors and depth perceptions are also perceptions that are created by top-down processing, though much more is known about them than of stickiness. Stickiness is said to be one of the important touch qualities, due to friction, sometimes studied as skin tribology. Stickiness on the skin is not judged instantaneously, and the duration of stimulation is important to the magnitude of stickiness.

(Stickiness of spaghetti can supposedly be judged visually as well, though the temperature (too  hot or too cold) and perhaps the variety of pasta that is used may be important.)

An ice skater is racing on slippery ice.

BIO: The stickiness we feel can often be reduced to chemical bonding. Adhesiveness is a matter of chemical interactions. Dry sugar is not very sticky, but wet sugar is quite sticky. This makes the success of peanut brittle a question of keeping sugar and water separate. And if you add too much water, the stickiness changes to slipperiness.

Since our bodies are mostly water, bioadhesion—our inner stickiness—varies all the time, allowing blood platelets to clump in clotting blood, fat to cling to the inside of blood vessels, earwax to be just sticky enough to trap dust and pollen in the ear, particularly of Europeans and Africans, and mucus to do the same in the nose. Snails may not sense the stickiness of their slime at all, but it works for them in two ways: lubricating their slow travel as a slippery substance and sealing their shells against invaders when it’s sticky. (Slime is a gastropod Ooblek. In yet another way it keeps snails, and someday humans, healthy.)

No doubt it’s clear by now that we can’t divide the world into sticky and slippery substances. Context and circumstances are important, as we saw in making sugar sticky. Something similar is responsible for making ice slippery: Again, it’s liquid water that makes the difference. Liquid water might help aging stage magicians with their dry hands, too, but we’d suspect a performer who dipped his or her hands too often in the fingerbowl. Papercreme to the rescue! In east Asia people have been moistening grains of rice to make a serviceable glue for time out of mind. I learned the trick from a Korean friend in Seoul who noticed a piece of fabric that had worked its way loose in my car. This does not require so-called “sticky rice” (chapssal).

PSYCHO: Chemistry isn’t all there is to stickiness. It’s a perception, not a sensation, remember: a matter of interpretation. Runners feel sticky roads when walkers don’t feel it at all. We expect anything that is glossy to feel more slippery than sticky.

As with slipperiness, so with the perception of wetness, which is not a question of hydrogen bonding and its quantum quirkiness. Chemistry lays a groundwork for the behavior of different materials, but our perceptions do not always correspond.

We lack sensory receptors for wetness, or hygroreceptors, just as we do for many perceptions. Hygroreceptors are possible, and are found in some insects. To some, a perception without specific sensory receptors is necessarily an illusion. Indeed, much of our perception could even be labeled hallucinatory, in the sense that it is constructed, top-down, from what our sensory receptors tell us. In common usage, of course, hallucinations are reserved for maladaptive phenomena, while perception, though similar to hallucinations in some ways, is adaptive and normal.

Though we explain water’s qualities by the bonds between hydrogen and oxygen, the bonds do not explain how we feel that water is wet. Rather, wetness and some other perceptions are emergent (see minute 1:22), developing through interactions too numerous and complex to permit psychological predictions. For this reason, mainly, seek the laws of sensation and perception in psychology rather than in physics and chemistry, though subject to the laws of evolution in biology.

Emergence of the sort we are discussing is not an illusion; it follows rules. It is not confined to psychology but is a useful concept also for analyzing health and disease and many other phenomena such as traffic. It’s interesting that researchers eight years ago, dismayed by the limits of reductionism, predicted greater success for emergent thinking in the 21st century.

SOCIAL: How, then, do we make the same identifications of our sensations if it’s not directed by genes and their instructions to our brains? The outcome depends not only on our sensory receptors but on our cognitive skills. What we hear as a letter of the alphabet depends on the language we learn, while the color we see varies with the names of colors.

It comes as a surprise to young students that sensory stimuli are usually ambiguous. Shapes and luminances can be duplicated by alternative stimuli, and boundaries between colors and speech sounds are drawn differently in different cultures. Consequently, every percept is an interpretation, a choice among alternatives

Usually we “disambiguate” what we see too fast to be aware of our analysis.  Sometimes a pattern may be so blatantly ambiguous that we alternate between interpretations, as with the Necker cube and the Young-girl/Old-woman illusion. When ambiguity defeats interpretation, we are alerted instantly to the perplexity. A few years ago a car company drew attacks for installing rows of nearly identical buttons on the dashboard, some with ambiguous symbols, that forced drivers to look away from the road. The airplane cockpit is even more challenging.

Our concepts of color and speech sounds (phonemes) are not driven by physical stimuli as directly as the raw sensations generated in the retina and the cochlea, yet we all agree with our friends in distinguishing the sounds of /b/ and /p/ or blue and green, almost as if the tasks were biologically constrained. More likely, it is emergence following biological rules in the brain to generate results that can’t be predicted from genetics and neuroanatomy. Our senses and our senses’ brain become tuned to special stimuli.

Colors are hard to learn, yet we learn the same distinctions within each culture. Learning to tell English speech sounds, or phonemes, apart can be hard, yet all English speakers end up making the same distinctions. To learn a new language, we have to learn to hear sounds differently. (An allophone, incidentally, is a variant pronunciation of a phoneme that doesn’t make you hear a different phoneme.)

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