As the human brain senses where we touch a held object, it can also pinpoint the exact location of where that object then touches another object, Science Alert reported Monday. Mapped neural activity shows our efforts to discern where an object that we’re holding makes physical contact with something else. We experience the sense of touch through a complex network of cells.
According to Science Alert, new evidence is showing that our perception of touch—the most direct of the five senses—treats certain objects as extensions of ourselves. If you hold a ruler at one end, for example, and someone else takes hold of the ruler without touching you and without removing it from your grasp, your brain can roughly detect where on the ruler the other person is touching it. When we physically feel something touching us, it’s a combined work of four groupings of cells under our skin.
Merkel Receptors and Ruffini Cylinders
The different types of cells under our skin respond to physical touch in different ways, and although we only have one conscious perception of touch, it comes to us through a combination of different cell types reacting to external stimuli.
“Merkel receptors live very close to the surface of the skin, just beneath the tough, outermost layer of skin called the epidermis,” said Dr. Peter M. Vishton, Associate Professor of Psychology at William & Mary. “They’re small, and in some areas of the skin, they’re very densely packed together. This gives them the ability to encode very fine details of a pattern of pressure when something presses against your skin.”
For example, Dr. Vishton said, Merkel receptors are the reason we can feel the difference between the bark of an oak tree versus a maple tree.
Ruffini cylinders, he said, reside a bit deeper than Merkel cells and they respond to the stretching of the skin, whether by our own movement or from external friction. “These cells seem to be especially important for kinesthetic perception; that is, our sense of our body’s posture.”
Meissner and Pacinian Corpuscles
At the same narrow depth under the skin as Merkel receptors are a cell group called Meissner’s corpuscles. “These cells are substantially larger than Merkel cells and, thus, can’t encode fine details very well,” Dr. Vishton said. “What they can encode is vibration. If you press on a Meissner’s corpuscle, it would respond but only for a few milliseconds, after which it would turn off. If you release that pressure, it responds again.”
In other words, he said, Meissner cells respond not to pressure but to changes in pressure, and since they respond so quickly, vibration is the best stimulus for them.
They pair with Pacinian corpuscles, a cell group far deeper in the skin that responds to vibrations at higher cycles per second than Meissner corpuscles.
“As with color vision, the relative responses of these two different types of cells enable us to infer just what the actual rate of vibration that’s out there in the world is,” Dr. Vishton said. “If the two cells are responding equally, then the vibration is probably in the middle of their preferred ranges; if only the Meissner’s cells are responding, then a slower rate of vibration is present; lots of Pacinian corpuscle response and no Meissner’s cell response, then probably a high rate of vibration is present.”
With so many cells reacting so differently to every external stimulus we touch, it’s little wonder that our brains can even sense secondary physical contact to an extent.
Dr. Peter M. Vishton contributed to this article. Dr. Vishton is Associate Professor of Psychology at William & Mary. He earned his Ph.D. in Psychology and Cognitive Science from Cornell University.