I would like to express my upmost gratitude for the vast amount of receptors you possess. Among my favorites – the practical ones that allow me to find keys at the bottom of my large jet-black bag, the emergency ones that warn me the fajita plate is searing, and those truly special ones that enable me to feel my lover’s caress and help me achieve orgasm…
Such would begin a letter of appreciation to a physical sense of great importance, exquisitely described in the book Touch: The Science of Hand, Heart, and Mind. In his revealing yet jargon-free narrative, John Hopkins neuroscientist David J. Linden describes how touch works and, more importantly, how intricately it is tied to our emotions. The pages are packed with astonishing facts and pictures, as Linden successfully weaves together the history of sensory research with current scientific breakthroughs for a thoroughly entertaining read.
Touch is social
From the common vampire bat (Desmodus rotundus) soliciting regurgitated blood by grooming other bats to gelada baboons’ (Theropithecus gelada) social grooming behavior, all the way to kangaroo care for human pre-term infants, touch is integral to the communication of a variety of emotional signals, “including support, compliance, appreciation, dominance, attention-seeking, sexual interest, play and inclusion.”
Linden lucidly explains how specialized touch receptors create our entire tactile experience and color our emotional world. He takes the menial task of finding a quarter in a deep pocket and explains our reliance on the main types of touch sensors and their associated nerve fibers embedded in the glabrous (non-hairy) skin of the fingertips. The mundane, almost mindless task, which would baffle even the most sophisticated modern-day robots, requires four types of sensors for mechanical stimuli: Merkel disks, Meissner’s corpuscles, Pacinian corpuscles, and Ruffini endings.
Distinguishing the texture and size of a quarter from other objects in the pockets calls upon all the main types of touch sensors, yet the main one activated is the Merkel or “touch” cell, which is made up of specialized epidermal cells clustered in disks. These cells send the tactile information to the spinal cord and finally to the touch-sensing region of the brain. It is still a mystery how mechanical deformation of the skin is translated into the language of the nerve fibers and brain: electrical signaling.
While finding a quarter is a useful perfunctory task, the mechanoreceptor’s crowning glory is replacing sight with touch to aid communication. Linden explains the origins of the global standard for blind reading: the Braille system. Louis Braille, who lost his sight as a child due to an accident, later adapted French Army Officer Captain Jacques Barbier’s “night writing” method which consisted of raised dots and dashes. Although Braille had no knowledge that Pacinian corpuscles or Merkel cells in the skin activate visual areas in the brain, he created an extraordinarily fast mechanism for processing of tactile information still in use today. In other words, Braille allowed the blind to veritably “see” words.
Touch is pleasurable
Linden delves deep into the anatomy of a touch, highlighting two distinct systems that transmit touch information to the spinal cord and brain: A-beta fibers and C-fibers. A-beta fibers are all about facts — where and when did a touch occur? C-fibers produce a positive sensation globally on the skin and are used to integrate tactile information over a large area of skin to discern emotional tone. This is the caress system.
The emotional tone, he explains, is crucial. It is this social information that is encoded at an early age and is necessary for the development of trust and cooperation. The two touch systems work together in order to interpret what touch means. For example, a light brush from a stranger: danger or flirtation?
In the same vein, mapping caress and sexual touch in the brain results in different patterns of brain activation depending on which genital region was stimulated (external clitoris, vagina and cervix). However, the patches are adjacent to each other and, in some cases, overlap. In women, cortical regions activated by nipple stimulation significantly overlapped with those of the genital regions. This suggests that nipple stimulation is sexually exciting, although the same has not been proven in men. The question of whether more sex (or for that matter, nipple stimulation) results in expanded cortical representations of vagina and external clitoris, in the same way daily piano practice expands the sensory map of the hands, has yet to be investigated.
Further, the zenith of sexual pleasure, the orgasm, is an accumulation of touch signals passed through sensory nerves to the spinal cord and the brain. Surprisingly, what feels like fireworks in the body makes the cerebral cortex go dark in a brain scanner. Linden describes how the lateral orbitofrontal cortex and anterior temporal pole, regions responsible for “thoughtful decision making, self-control, moral choice and social evaluation” are temporarily offline during orgasm. Clearly, not the time to make important decisions. Why orgasm deactivates these regions and how they come back online is still a complete mystery.
Touch is necessary for survival
Finally, Linden explores the concept of negative touch, or pain.
Imagine not knowing the meaning of the word “pain.” Children born with a mutation on gene SCN9A experience no pain. SCN9A directs production of a voltage-sensitive sodium channel essential for the propagation of electrical signals in pain neurons. The result of the mutation is flat-lined electrical current. No pain, no problem? Not so. The mutation silences neurons that convey pain signals to the spinal cord and brain, despite having normal numbers of A-alpha and C-fibers. These children have no obvious signs of dysfunction, they speak and behave normally, experience pain for others and still feel emotional pain. Unfortunately, they are constantly injuring themselves. They bite their own tongues or rub dirt in their eyes and damage their corneas. Many of these children do not survive past their teenage years. Its counterpart — a gain-of-function mutation on the same gene results in:
“the pain sensing neurons [that] are now like machine guns with a hair-trigger: Stimuli that might make normal neurons fire one or two electrical spikes now make them fire a sustained burst. [This activity] can trigger a bout of extreme pain.”
Pain, however, does not have to be physical to hurt. Social rejection is coded in the brain in the same manner as a needle prick, activating overlapping regions of the brain’s emotional circuit: the limbic system. Even a mild form of social rejection produces activation in the dorsal anterior cingulate cortex, anterior insula and somatosensory cortex, which are all discriminative pain centers.
Linden’s Touch is a quick and captivating read in its entirety, filled with illuminating insights of why onions make you cry, why some bats can detect infrared radiation and some cannot, and why heroin and oxycodone use can trigger intense bouts of itching. Linden teaches us that touch is primal, necessary and universal. Without it, we would be shells of our human-selves.
Mostly, I am grateful for how you have influenced my social cognition in inexplicable ways.
Human living inside you.