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Touching the world: human skin receptor biomechanics



It is no secret that the largest organ of the human body is its skin. In addition to protecting the body from external stimuli, the skin also performs the function of a sensor that collects information, along with eyes, ears, tongue and nose. The information received by the skin allows a person to assess the environment, better understand the situation in which he is and act in accordance with it. Despite the great importance of tactile information, we don’t know much about how everything works. Therefore, scientists from the University of California (USA) decided to examine the human skin from a mathematical angle in order to understand the mechanism of occurrence and transmission of tactile sensations. What happens when we take something in our hands, how our skin processes the information received,and how to apply this study in practice? We will find answers to these questions in the report of scientists. Go.

Study basis


In an adult, the area of ​​his skin can reach 2.3 m2, which makes it the largest organ. However, the dimensions are nothing if there is no functionality behind them. The skin performs quite a lot of functions: protective, respiratory, excretory, thermoregulatory, immune, metabolic, etc. In other words, trying to evaluate different organs by their importance, putting the skin in last place would be a mistake.

The most mysterious function of the skin is the collection of information, i.e. the formation of touch - one of the types of human senses. Such is the temperature in the room, rough or smooth wallpaper, how soft a chair is - all this and many other data are collected precisely by the skin.

The incredible sensitivity of the skin lies in the presence of a huge number of nerve endings, i.e. receptors. They all differ from each other in shape and structure, because they perform different tasks (some collect information about the texture of the object, others - about the temperature, for example).

Skin receptors can be divided into two main types: free nerve endings and non-free nerve endings. The former consist solely of the final branches of the axial cylinder and are located in the epithelium. These receptors collect data on temperature (thermoreceptors), pressure (mechanoreceptors) and pain (nociceptors).



The categorization of non-free nerve endings is much more extensive:

  • Pacini bodies - pressure receptors in the subcutaneous fat;
  • Meissner bodies - pressure receptors in the dermis;
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This is only a short list, without a deep examination of the receptors, their functions and structure, but this is enough to understand the complexity of the skin as an organ of senses.

Researchers themselves interpret touch as the encoding of mechanical signals collected by the skin and subcutaneous tissues into neural signals. Neural responses to tactile stimuli are often associated with mechanical influences arising from small areas of the skin, however, there is evidence that dynamic touch causes mechanical waves in the tactile frequency range that propagate throughout the arm, with transient excitations decaying for 30 ms. Thus, dynamic tactile influences can stimulate widespread afferentation * .
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It was found that these waves, caused by touch, contribute to a subtle perception and can be used to determine the characteristics of the object that was touched, the contact area of ​​the object with the hand and further actions. There is also evidence that the receptive fields of neurons in the somatosensory regions of the cerebral cortex cover large areas of the hands and several fingers.

The large contact area in the early stages of signal processing causes the cortical neurons to respond to input signals that are delivered back to the contact area.

Thus, somatosensory processing may depend on information carried by mechanical waves, which propagate in tissues to distant sites remote from places of direct mechanical contact.

Scientists believe that if the transfer of mechanical waves in the hand contributes to the effective coding of somatosensory information, then it should be possible to describe tactile stimuli in small areas by means of informative parameters. In other words, convert the touch feeling to numbers.

In their work, scientists show how mechanical waves in the hand efficiently encode tactile input data. After conducting experiments using high-precision sensors, scientists were able to create a kind of dictionary of spatio-temporal signals, which together allow you to classify incoming information with an accuracy of more than 95%. That is, they managed to create a map showing where and which areas of the skin of the hand are activated when in contact with an object.

Research results


Scientists depicted tactile information modeling in the form of matrix decomposition. The coding was evaluated using a database of tactile stimuli collected during the experiments for the entire hand, including spatio-temporal changes in the skin a (x, t). Special sensors in 30 sections (x) were attached to the volunteer's arm. During the experiment, 13 gestures and 4600 interactions with various objects were performed.


Image No. 1

Each of the stimuli w i (x, t) entered into the data set had its own activation time h i (t), which was also taken into account in the model to obtain more accurate “tactile basic patterns” ( 2A ), which aggregates encode all emerging stimuli and transmitted signals.


Image No. 2

These basic patterns (hereinafter referred to as the bases) can also be interpreted as a set of analysis filters that extract information from external stimuli using various additional patterns of spatial and temporal integration of mechanical signals in the hand. According to scientists, these filters can be compared with the functions of spectral-temporal tuning in auditory processing or with filters of the spatio-temporal receptive field during the work of the retina.

Summing up, scientists created a mathematical model in which the signals felt throughout the arm were represented as a small number of simplified patterns. This technique allowed us to obtain the main wave patterns - skin vibrations throughout the hand, which are involved in the collection and transmission of tactile information.

Despite the fact that the analysis did not take into account the conditions for the appearance of signals, tactile bases resembled the sensory function of the hand ( 2A and 2B ). Most of them were initially localized at the distal ends of one of the fingers (the most densely innervated areas of the hand). The speed of the signals was about 1-10 m / s, and the signal attenuation was observed 10-30 ms after its occurrence. Other tactile bases evolved from the distal region of individual fingers to diffuse regions of the surface of the hand ( 2A) In the aspect of frequency, a pair of bases showed a similar spatial arrangement, but different frequency characteristics. For example, there is a pair of bases localized within one finger, but having different filtering properties (relative to the transmitted signals): the lower range from 20 to 80 Hz ( 2V , basis 2) or the upper range from 80 to 160 Hz ( 2B , basis 6 )


Image No. 3

Scientists believe that spatio-temporal tactile bases are associated with a specific finger, i.e. have their own working area, so to speak. For example, 45% of the 4,600 tactile stimuli analyzed were caused by gestures when only one finger was in contact with the object. After re-analysis, excluding tactile signals created with just one finger, the same tendency was found.

The space of possible tactile stimuli is limited by the mechanics and duration of contact ( 3A ).

Then the scientists decided to check how many bases should be used to determine the signal source. As it turned out, if you use at least 7, then the accuracy of the determination will be 90%, and if 12, then 95%. However, not all incentives require activation of such a large number of bases to increase accuracy. The logic is fairly straightforward: when several fingers are involved in a gesture, several bases are activated; if only one finger is involved in the gesture, then there will be one basis, maximum two. Moreover, the bases themselves also varied depending on the gestures. That is, different gestures, although the same fingers are involved in them, will activate different bases.

The model also showed that five bases are enough to maximize the accuracy (80%) with which the stimuli from one participant in the experiments could be classified using data from other participants (3C). These five bases were almost universal among all participants and corresponded to the five fingers of the hand ( 3B ).

The totality of the above observations suggests that the elasticity of the skin itself plays an important role in the collection and transmission of information, because it increases the area of ​​contact with the object. In addition, the waves of signals propagating in a specific pattern allow us to classify the information received, which also helps to accelerate its processing directly by the brain.

Similar signal processing mechanisms can be compared with the work of the middle ear, which, by distributing sounds with different frequency contents to different sensory receptors in the ear, helps the coding of sounds by the auditory system.

For a more detailed acquaintance with the nuances of the study, I recommend that you look into the report of scientists and additional materials to it.

Epilogue


This study showed us that the skin is a much more complex system than previously thought. If earlier the process of signal transmission could be described linearly (touch - the occurrence of a signal - signal transmission to the brain), now this process is more like wave activity. Signals received from objects interacting with the skin propagate in waves along the nerve endings of the skin, depending on the contact area, its duration and the nature of the surface. In other words, in the collection of information about the contact object, not only receptors in the direct contact place are involved, but also receptors around this zone.

Researchers believe that in this complex process, the elasticity of the skin plays an important role, which allows to increase the contact area from the point of view of signal propagation, and not from the point of view of the contact itself.

According to scientists, their work will not only better understand the functioning of the human brain and nervous system, but will also come in handy in the development of new prostheses and even robots that can tactilely collect environmental information more accurately.

Friday off-top:

LEGO .

Thank you for your attention, stay curious and have a great weekend everyone, guys! :)

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