Human understanding of their own bodies has been highly misleading and biased since ancient times. For instance, Aristotle believed that the dominant sensory ability of a living being determined intelligence, yet eagles have a much longer visual range than humans, dogs have a better sense of smell, and cats have a sharper sense of hearing. The reason humans are smarter than other animals is due to our more delicate sense of touch [Aristotle, On the Soul, Book 2].
Modern science would obviously not directly agree with such a conclusion, but among the five senses of humans, touch is the largest and most widely distributed sense. Although ordinary people can obtain the most direct tactile experience anytime and anywhere, its mechanism has been a problem that researchers have found difficult to fully conquer for decades. The 2021 Nobel Prize in Physiology or Medicine was awarded to David Julius and Ardem Patapoutian for their discovery of the molecular mechanisms of human touch. The Nobel Prize has made more people realize the importance of touch and has led to more research into human and machine touch. Pineal gland robots have been developing tactile sensors and products since their establishment in 2020, and in the past two years, with the continuous progress and application of products, everyone has found that touch is so interesting.
Have you ever thought about such questions in life:
- Why can fingertips easily distinguish the texture of objects, but the back of the hand finds it difficult?
- Blow on the palm and back of your hand, why is the back of the hand more sensitive to the wind this time?
- When tasting a steak, are you mainly relying on taste?
- The old saying goes, "Hold it in your hand and fear it will fall, hold it in your mouth and fear it will melt." Why is it the hand and mouth?
- How does the brain locate where the body itches?
- What is the phenomenon of phantom limbs after amputation, and if the phantom limb is "itchy," how should you "scratch it"?
- Raise your hand and draw a straight line along the distant wall, why can't you draw it straight?
- Close your eyes and gently touch the edge of the table to draw a straight line, why is it so easy this time?
- Why do humans express love through touch?
- How is the aesthetic sense of touch formed?
- What happens to vision, touch, the brain, and muscles when you easily pick up a cup of coffee?
- How do the blind rely on touch for massage?
This text preliminarily establishes a knowledge system about touch, such as the perception and conduction of touch, and the processing of touch by the central nervous system. I hope readers will start with ten thousand whys and end with a hundred thousand whys. If you have other interesting little questions about touch, you can also leave a message on the public account, and we will try to decipher together. The materials in this article can only represent the current stage of researchers' very limited understanding of human touch, so there will definitely be many details that cannot be traced to the root, or even fundamental errors. Therefore, I try to express the work of predecessors rigorously, and some of my own understanding and work results, please give readers more advice.
Human Tactile Receptors
The human skin is composed of three main tissue layers: the outer epidermis, the middle dermis, and the inner subcutaneous tissue. Whether it is hairy or hairless skin, the overall structure is the same. The main difference is that hairless skin is thicker and undulates, while hairy skin is thin and flat, with fine vellus hairs and slightly longer guard hairs. Humans only have hairless skin on the inner sides of fingers, palms, soles, lips, nipples, and parts of the genitals, and the undulating grooves of the epidermis form unique fingerprints, palm prints, sole prints, lip prints, etc.
In addition to being filled with blood vessels and sweat glands, these skin tissue layers also have specialized tactile receptor cells that detect touch and transmit signals to the central nervous system. Different types of tactile receptors are distributed in different locations, giving different parts of the body a specific combination of touch. Humans can easily perform complex finger operations, experience delicate caresses, and even feel pain, all thanks to these tactile receptors.
Tactile receptors convert the mechanical signal of touch into electrical signals at the nerve endings, and the mechanism of conversion is the ion channel discovered by the Nobel Prize winners mentioned earlier. In different receptors, the switching mechanism of ion channels is also different, providing different touch signal encoding mechanisms.
In hairless skin, the following types of tactile receptors are dominant:
- Merkel Disk: It grabs the concave part on the inner side of the epidermis like an octopus with a sucker, so it can feel the most delicate curvature changes of the skin. In addition, each Merkel disk is connected to the central nervous system by an independent nerve fiber, and it has the strongest spatial positioning ability among tactile receptors, so it is particularly sensitive to the shape and texture touched. Merkel disks can feel indentations from 0.05mm to 1.5mm, and the signal it transmits to the nerve fiber is continuous when an indentation is produced, and a deeper indentation triggers a stronger signal. Merkel disks are particularly dense on fingertips, lips, and tongues.
- Meissner Corpuscle: Its shape is a small sac, distributed at the edge of the dermis and epidermis, and the protruding part on the outside. This small sac deforms due to skin compression and releases an instantaneous signal, not continuous. Therefore, repeated low-frequency vibrations will make it release more excited signals, so vibrations produced by sliding when holding something will activate the signal of Meissner's corpuscles. It is more densely distributed on fingertips than Merkel disks, but unfortunately, all Meissner corpuscles in an area of 10 square millimeters share a nerve fiber, so its spatial positioning is very vague and cannot distinguish the subtle features of objects. What's even stranger is that these nerve fibers do not enter the brain; it is a spinal cord circuit, that is, a conditioned reflex. The advantage of this is that the transmission path is short, the processing time is short, and the reaction is very fast, so people can subconsciously hold the water glass that is about to slip from their hand.
- Pacinian Corpuscle: Its shape is a layered sac, like an onion, larger than Meissner's corpuscle, and is distributed deeper in the dermis. Pacinian corpuscles are only sensitive to weak, high-frequency vibrations from 200Hz to 300Hz, and other characteristics are very similar to Meissner's corpuscles, with signals being instantaneous and insensitive to spatial position. Its function is to judge the state at the other end of the tool in use through vibrations, such as judging what material the pen is writing on.
- Ruffini Nerve Ending: It is a strip-like sac, generally parallel to the deep dermis, and is only sensitive to horizontal stretching, with a very low density, so the spatial position is not sensitive. Ruffini nerve endings can judge the posture of the joint through the pulling of the skin.
- Free Nerve Endings: It connects the epidermis, is scattered, and is sensitive to pain, temperature, and itching.
Most of the human body's skin is hairy. In hairy skin, the distribution density of the above tactile receptors is much lower, mainly relying on the bending of the hair and the pulling of the surrounding tissue to produce touch, and in addition to Merkel disks distributed around the hair follicles, there is another receptor:
- Longitudinal Pinching Receptors: Compared to the continuous signals brought by Merkel disks, longitudinal pinching receptors produce instantaneous signals due to the deformation of the hair, and have better sensitivity. Because its sensation is brought by the movement of the hair, it does not necessarily directly contact the skin, so the function of this type of tactile receptor can be attributed to the sense of proximity, and humans often use this type of touch to express emotions.
So far, we have seen six types of tactile receptors, but in the human brain cortex responsible for touch, most of it is processing the signals sent by Merkel disks. Why is the status of Merkel disks so important? Because the most delicate work of humans is done by Merkel disks. A fact is that the blind can not only read Braille with their fingertips but also use their lips and tongues, and other parts, such as the soles of the feet, although more sensitive to slight touches than fingers, cannot distinguish the shape of Braille. The reason is that in addition to the fingertips, only the lips and tongue have a dense distribution of Merkel disks. Another interesting phenomenon is that regardless of the shape of the fingers, the number of Merkel disks each person has is roughly the same. Researchers speculate that this may explain why girls' small hands are more sensitive and flexible, because the distribution of Merkel disks on small hands is more dense, giving these hands the ability to distinguish more subtle structures [Ryan M. Peters, etc., Diminutive Digits Discern Delicate Details: Fingertip Size and the Sex Difference in Tactile Spatial Acuity].
Tactile Neural Conduction
The human nervous system is responsible for "sensing and analyzing changes in the internal and external environment of the body, and regulating the entire body to respond through changes in its output information." In simpler terms, it is the body's sensor and processor.
The central nervous system and the peripheral nervous system work together to transmit and process sensory information and coordinate body functions. The central nervous system includes the brain and spinal cord, which act as the control center. They receive data and feedback from sensory organs and the entire body, process information, and send back commands. The peripheral nervous system's nerve pathways carry afferent and efferent signals. 12 pairs of cranial nerves connect the brain with the eyes, ears, and other sensory organs, as well as the muscles of the head and neck. 31 pairs of spinal nerves extend from the spinal cord to the tissues of the chest, abdomen, and limbs, and are divided into sensory and motor nerves according to function. Sensory nerves transmit nerve impulses from receptors to the central nervous system; motor nerves transmit nerve impulses from the central nervous system to the surrounding motor units [visiblebody.com, Brain and Nerves: Five Keys to Unlock the Nervous System].
The human nervous system is highly developed, responsible not only for basic body regulation and visceral function regulation but also for complex behaviors such as language, science, and emotions. Here, we mainly focus on the parts related to whole-body tactile sensation, that is, sensory and motor nerves.
Nerves are wrapped with several nerve bundles, and nerve bundles are wrapped with several nerve fibers, which are the unit structure of nerve conduction.
As mentioned earlier, the central nervous system needs to locate each specific Merkel disk, for example, it needs to know what the local shape of the object touched by the finger is, from a systemic point of view, there are only two methods: one is that each receptor uses a separate nerve fiber to conduct the signal, which can be directly positioned; the other is that multiple receptors share a nerve fiber to conduct the signal, but the signal content emitted by each receptor needs to include an encoding of its own identity.
We already know that the electrical signal emitted by human tactile receptors is a simple one, and the information quantity is represented by the height and duration of the potential, and there is no evidence to support that it uses "Morse code" or "binary" to express its own identity or position. So I can only agree with such a conclusion, that is, under ideal circumstances, each Merkel disk independently occupies a nerve fiber.
Multiple Meissner corpuscles or Pacinian corpuscles connect to a nerve fiber through dendrites, so they are insensitive to position, so even though the finger can feel the particularly subtle vibrations through them, it can only obtain a rough position, and this signal is only processed through the spine and does not enter the brain. On the contrary, the finger can feel the very fine curvature of each touched position through Merkel disks and synthesize the precise shape of the touched object through the brain because Merkel disks independently occupy a nerve fiber and have a very high density.
So, how many nerve fibers does the human hand have?
I have not found a specific number, but I infer from the number of tactile receptors, plus the corresponding motor nerves, that the order of magnitude of the number of nerve fibers in the human hand is ten thousand.
The diameter of sensory and motor nerve fibers ranges from 0.3 microns to 22 microns [People's Medical Publishing House, Systematic Anatomy, 9th Edition], among which the nerve fibers corresponding to Merkel disks are about 10 microns. Assuming that ten thousand nerve fibers are bundled together to form a nerve, plus the outer membrane of the nerve bundle and the outer membrane of the nerve, the result should be at the millimeter level.
What a magical conclusion, after years of robotics, I can't believe that any robot can have a nerve composed of ten thousand wires on its arm, let alone that the diameter of this nerve is only one millimeter. So robots can only use time to exchange space, using time-sharing sampling and encoding methods to reduce the number of lines.
So, how fast is the conduction speed of nerve fibers?
A basic conclusion is that the larger the diameter of the nerve fiber, the faster the conduction speed. We assume that the length of the nerve fiber from the finger to the spine is 1 meter, then through the conduction speed calculation, the delay of proprioception and motor nerves is 1/120 to 1/70 seconds, the delay of Merkel disks, Meissner corpuscles, Pacinian corpuscles, and Ruffini endings is about 1/70 to 1/30 seconds, the delay of pain and temperature is 1/30 to 1/12 seconds, and the delay of the hair follicle touch sensation on hairy skin responsible for emotion is the slowest, at 1/2 to 1/0.6 seconds [People's Medical Publishing House, Systematic Anatomy, 9th Edition].
That is to say, if a robot needs to interact normally in a human environment, in terms of tactile sensing system delay, it needs to reach about 1/60 seconds; and the delay of the motion control system needs to reach 1/120 seconds. This requirement is not high and also provides space for time-sharing sampling of robot sensors.
Understanding of Tactile Sensation by the Central Nervous System
The signals from the skin's tactile receptors are transmitted through nerve fibers to the brainstem, then to the thalamus and cortex. In fact, the cerebral cortex has corresponding positions for the touch of the entire skin, and this part of the cortex is called the primary somatosensory cortex, and generally, adjacent positions of the skin are also adjacent on the primary somatosensory cortex. The reason we consciously feel that a certain position of the skin is touched is because the corresponding position of the cerebral cortex is activated. In particular, the regions of the primary cortex responsible for the skin with a high concentration of Merkel disks are enlarged.
Each nerve fiber's information is transmitted to different parts of the somatosensory cortex, but some abnormalities can occur, such as amputation, which leads to the disappearance of the input from the receptors of a specific body part. At this time, the somatosensory cortex area corresponding to that body part is usually "invaded" by adjacent areas. For example, it has been observed that when the face of an amputated person is touched, this person feels a touch sensation in his non-existent hand. This may be due to "invasion" or "cortical reorganization," because the facial and hand areas are adjacent on the somatosensory cortex map [Azadeharjmandi, Chasing a Ghost: Unravelling the Secrets of Phantom Limb Pain].
Some experiments have tried to stimulate the primary somatosensory cortex with external electrodes to make people gain simulated touch. However, so far, the touch experience obtained from these purely brain stimulations is very rough, lacks rich details, and is completely different from the natural touch feeling, and cannot give people a sense of immersion.
The primary somatosensory cortex is responsible for extracting the underlying features of touch, and the further advanced somatosensory cortex is responsible for summarizing and identifying these features and transmitting the results to the areas responsible for emotions, movement, and body balance.
For example, human touch behaviors such as hugging, holding hands, or caressing are called pleasant touch, which is very important for emotional health and development. A study has identified the mechanism of encoding and transmission of pleasant touch sensation and the spinal nerve circuit mechanism [Benlong Liu, etc., Molecular and neural basis of pleasant touch sensation].
Another example is human proprioception. Proprioception includes three aspects: joint position sense, which is the perception and sensing of the spatial position of joints; movement sense, which refers to the perception and sensing of the direction and speed of joint movement; and resistance sensation, which is the perception and sensing of the force acting on the joint or the force generated within the joint. Our muscles, joints, and fascia are all equipped with proprioceptive-related tactile receptors, and the previously mentioned Ruffini corpuscles are one of them. Every time we make a movement, the tactile receptors on the muscles, joints, and fascia receive movement information, which is then transmitted to the brain through afferent nerves. The brain then integrates and processes various movement information in a timely manner to make a response, and then our body parts can operate in coordination, and the movement is smooth.
Another function of the advanced somatosensory cortex is to process our expectations of tactile stimuli. This expectation is formed in life experiences and already exists in the brain before the event, and it even affects our perception of real stimuli. For example, when touching an object with your eyes closed, if you are told that this is a clean towel, you will feel pleasant; if you are told that this is a towel used by others, you will feel disgusted.
In addition, the brain's expectation of tactile stimuli can also correct our movements during movement.
How do the blind massage through touch?
At the beginning of the article, readers were asked several small questions, and I believe you have basically found the answers by reading here, so I only try to answer one of them in a framework. The problem of blind massage is a typical case of the application of the tactile system. The blind rely more on touch than normal people in daily life and are particularly suitable for massage. In fact, a considerable proportion of the blind work as massage therapists. It can be seen that massage does not particularly need visual participation and can be completed using touch. How do the blind massage through touch?
- Blind people who are blind from birth or in early childhood have a more sensitive tactile system. Because the blind do not need to process visual signals, and the nervous system has strong plasticity, so the cortex responsible for vision has undergone functional transfer, and some have strengthened touch processing, and some have strengthened the processing of hearing and language [Merabet, L., Pascual-Leone, A. Neural reorganization following sensory loss: the opportunity of change. Nat Rev Neurosci 11, 44–52 (2010)].
- The enhanced tactile system of the blind, after long-term life experience and basic massage training, first has a structured human body model in his brain. Here, I will not elaborate on the structured human body model, and there will be a chapter dedicated to it later. You can first simply understand it from the literal meaning, which is the prior knowledge naturally formed by the masseur through long-term training; followed by training each basic massage movement execution, which we call it the meta-action; during the execution of meta-actions, it is also necessary to train the expected touch, what kind of touch is in place, effective.
- With the results of the basic training, the next step is the upper-level understanding training, which is to associate specific diseases with basic massage movements. Most blind massage therapists are based on the theory and practice of traditional Chinese medicine massage for this level of training, and the result of this training is to learn to plan a massage plan according to specific situations.
- When the massage begins, the blind massage therapist will try to touch each feature of the guest's body with his hands to determine the specific parameters of the guest's structured human body model. In other words, the blind person synthesizes the guest's own human body model and initially identifies abnormal conditions in the body, such as a tense and stiff muscle in the shoulder, through touch.
- Based on the specific instance of the structured human body model obtained in the previous step and the customer's requirements, the blind massage therapist quickly plans a complete set of massage plans in his brain. This set of massage plans is first functionally arranged, such as: relaxing the shoulders and neck -> relaxing the waist -> kneading the tense muscles in the shoulders -> pushing the bladder meridian..., followed by estimating the parameters of each function according to the specific indications of the human body model, such as the target position of the action, the amplitude, and the expected contact state between the masseur's hand and the guest's body.
- Now the formal massage begins, and each functional arrangement is composed of a sequence of meta-actions, which are the results of normal training and have formed muscle memory. The blind massage therapist can naturally complete them without much involvement of the brain. At the same time, the tactile system is also working, and the four main tactile receptors of the human hand obtain real-time contact status, and quickly compare it with the expected contact status trained in ordinary training in the brain, and adjust the execution of meta-actions to obtain better expected results.
- During the massage process, the guest's posture will often change, and the guest's requirements for massage position, technique, and strength may change. The blind massage therapist will promptly adjust the structured human body model in his brain and the control parameters for implementing meta-actions so that he can successfully complete the entire set of massages.
The blind massage therapist has completed a very complex process. When humans implement a process like massage, it is often more troublesome during the training process because it requires long-term learning to establish a human body model, massage theory model, and proficient massage techniques in the brain, as well as to enhance and massage action-related muscles through hard practice. But in practice, it is more natural and smooth, the enhanced tactile system and muscle control system, together with the trained brain, appear to be very comfortable. Speaking of which, now that young people are less willing to work as massage therapists, can we only hope that machines will give us a massage in the future? Thinking about those massage chairs on the market that only know how to hard against, I think everyone is looking forward to a massage machine that can use flexible techniques like a person to appear as soon as possible.
---
Bionics is the best teacher for human scientific progress.
I have been complained by the little friends in the company that I always have too many new ideas, and these ideas are often a bit difficult to handle, even affecting the principles of technology. Everyone gets excited about the future impact when they listen to me promote the implementation of these new ideas, but they also complain that they can only keep up with the progress every day by working hard. We can develop several influential products from scratch in just over a year, thanks to these new ideas.
The continuous emergence of these ideas is actually inseparable from our observation and thinking about the operating mechanism of human beings themselves, which is bionics. This is also why I explain my understanding of human touch in the first chapter.
So, how should a machine obtain touch that is comparable to or even surpasses that of humans? The technical details are the content of the next chapter, but through this chapter's understanding of human touch, you may have your own inference about the following questions related to machine touch in advance:
- What performance should an ideal machine touch have?
- In fact, which aspects of machine touch can surpass humans? Which aspects may have limitations?
- Tesla's upcoming general-purpose robot "Optimus," if you were to design it, what performance should its touch at different locations have? Guess if "Optimus" can achieve your design.
---
Weiran YUAN
In July 2022, Shenzhen
Leave a comment
This site is protected by hCaptcha and the hCaptcha Privacy Policy and Terms of Service apply.