The skin is the largest organ of the human body and is responsible for tactile sensory input, but even though it covers almost the entire body, why is our sensitivity not the same in each of its parts? Why can we recognize a stimulus accurately through the touch of our fingers, but we do not have the same ability with the skin that covers our backs or legs? On the other hand, why do stimuli in these regions lead to such pleasurable sensations despite this low discriminatory capacity?
A very common method used by doctors to assess an individual's tactile sensitivity is the two-point discrimination test. In this test, two needles are lightly pressed against the person's skin, and they report whether they can perceive two points of stimulation or just one. At the fingertips, a person can distinguish two distinct points when the needles are only 1 to 2 millimeters apart. In contrast, on the back, the distance between the needles must be 30 to 70 millimeters for both points to be perceived. This difference is related to our body map in the somatosensory cortex.
Although the senses begin in specialized organs, such as the skin (in the case of touch), their understanding occurs in the central nervous system. Sensory signals from all modalities end in the cerebral cortex, each in a specific region. Tactile sensations reach the somatosensory cortex, where different parts of the body are represented with a high degree of localization, forming a map. Some areas occupy large regions in the somatic cortex – the lips have the largest representation, followed by the face and thumb – while others are represented by relatively small areas, with the dimensions of these brain regions being directly proportional to the number of receptors present in each respective part of the body. The figure below illustrates our brain's sensory map.
In evaluating the quality of a lipstick, several factors are taken into consideration, including: the absence of an unpleasant taste, ease of application (smoothness, greasiness, glide), thickness and homogeneity of the film deposited on the lips, and the comfort generated. In fact, the size of the area occupied by the representation of the lips in the cortex may explain, at least in part, the large number of criteria involved in the appreciation of this cosmetic.
Many studies provide detailed mapping of these cortical areas, but the most interesting aspect is that they also show that these maps are dynamic, changing according to learning and other environmental conditions. A frequently cited example is the so-called "phantom limb" sensation, which causes amputees to continue feeling the lost limb. This occurs because the cortical area previously occupied, for example, by the arm, instead of becoming inactive after amputation, starts to represent neighboring regions, such as the shoulder or neck, causing stimuli in these areas to be interpreted as coming from the arm.
In addition to the representation in the brain map, tactile sensitivity allows for a pleasurable form of stimulation through touch, massage, which has been used as a technique to increase well-being and reduce anxiety, stress, and pain. A recent study used functional magnetic resonance imaging to test the hypothesis that the combination of human touch with movement leads to the stimulation of brain areas involved in pleasure. It compared human touch with or without movement and touch through a rubber glove, also with or without movement. The results showed that human touch with movement was rated as more pleasurable and led to greater activation of the anterior cingulate cortex, a brain area involved in pleasure.
This work clearly shows the hedonic effect of touch and makes me think about the beneficial effects that may exist in simple gestures. For example, we know that creams are made with ingredients for skin nutrition and protection, but does the act of applying it to the skin, through touch and massage, not bring other positive effects? In the next post , we will return to this topic and see that caressing is, in fact, essential!
Maria Cristina Valzachi is a pharmaceutical biochemist graduated from the Faculty of Pharmaceutical Sciences at USP and holds a master's degree in Pharmacology from the Institute of Biomedical Sciences at USP. Currently pursuing a doctorate at the same institution, she is dedicated to research and studies in the areas of Neurochemistry and Behavioral Pharmacology, with an emphasis on adolescence. She has a special interest in all fields related to education and the dissemination of ideas.
Contact: cris.valzachi@gmail.com
References
Hall, JE. Somatic sensations: I. General organization, sensations of touch and body position. In: Medical Physiology Treatise (Guyton, AC; Hall, JE). 2011; p. 603-614, Elsevier;
Lent R. The senses of the body. In: One Hundred Billion Neurons – Fundamental Concepts of Neuroscience (Lent, R). 2004; p. 211-39, Atheneu Publishing;
Lindgren L, Westling G, Brulin C, Lehtipalo S, Anderssonc M, Nyberg L. Pleasant human touch is represented in pregenual anterior cingulate cortex. NeuroImage. 2012;59:3427–32;
Tagliari MP, Stulzer HK. General aspects of lipstick technology. Cosmetics & Toiletries. 2007;19:72-5.
Figure 1: http://commons.wikimedia.org/wiki/File:Massage_Frankfurt.jpg?uselang=pt-br
Figure 2: Sensory homunculus. http://commons.wikimedia.org/wiki/File:Sensory_Homunculus.png
