Perception – Tactile

Perception - Tactile

From chemicals and waves, to tactile sensations. As the body's largest organ, the skin provides protection, thermoregulation and prevents excess water loss (Kolarsick et al., 2011). It's divided into three layers: the epidermis (outer layer), dermis (middle layer), and hypodermis (fat layer beneath).

The Epidermis

The epidermis, the outmost shield of the skin is mostly compromised of cells called keratinocytes, which can also be found within the inner most layers of the skin and provide protection through the synthesization of keratin (Kanitakis, 2002). Nonkeratinocyte cells include Melanocytes, responsible for UV light protection and the production of melanin; Merkel cells which are responsible for touch sensation and; Langerhans cells which act as guards for the immune system. The junction between the Epidermis and the inner layer of the skin, Dermis, is comprised of sweat glands that contribute to thermal regulation of the body. Additionally, the two layers are separated with membranes that allow fluid and cell exchange. Hair follicles have several functions, such as protection from the elements, distribution of sweat and a vital psychosocial role (Kolarsick et al., 2011).

Dermis

The Dermis is a connective tissue below the Epidermis. In interacts with the epidermis to provide development of cells, heal wounds and remodel the surface of the skin. Mainly, the Dermis provides strength, elasticity and mobility of the skin.

The connective tissue components of the dermis include collagen, elastic fibers, and glycosaminoglycans. Collagen is the main structural protein and provides tensile strength to the skin. Elastic fibers, composed of elastin and protein filaments, maintain skin elasticity. They are fine in the papillary dermis and coarser in the reticular dermis. Glycosaminoglycans, such as hyaluronic acid, are minor components in normal dermis but can accumulate in pathological conditions.

The muscles of the dermis include both smooth and striated types. Smooth muscles such as the arrector pili contract to produce gooseflesh, while specialized structures called glomus bodies assist in thermoregulation. Striated muscles include the platysma in the neck and the muscles of facial expression.

The nerves of the dermis contain specialized sensory structures. Meissner corpuscles, concentrated in the fingertips, palms, and soles, mediate touch, while Pacinian corpuscles, found in deeper dermis and regions such as the genitalia and nipples, detect pressure. Pain, itch, and temperature sensations are transmitted by unmyelinated nerve fibers. Autonomic nerves also regulate vasoconstriction, sweat secretion, and contraction of arrector pili muscles. Mast cells are found mainly in the papillary dermis near blood vessels. They contain histamine, heparin, proteases, and cytokines, and play a key role in allergy, inflammation, host defense, and conditions such as asthma, arthritis, and malignancy.

Subcutaneous fat

The subcutaneous fat (hypodermis) is composed of lobules of fat cells separated by collagen and blood vessels. This layer functions as a storehouse of energy, provides insulation and buoyancy, and has endocrine activity, including hormone conversion and the production of leptin, which regulates body weight.

Tactile Illusions: When touch tricks the mind

Just as other sensory illusions, tactile illusions mostly showcase the limitations of the human body (Patel et al., 2019). Some examples of tactile illusions may include:

Geometrical Based Illusions: such as the Weber's Illusion (or the Hollow-Face Illusions), where tactile size perception depends on the illusions that perceive it. For example, is found that individuals perceive materials to be larger when they perceive items with their fingertips where the receptors are more prominent, whereas when items are perceived with other body parts, the size of an item was perceived to be smaller. This phenomenon showcases that our brains do not perceive spatial information as they actually are.

Material Based Illusions: such as the social softness illusion, where we perceive other's skin as softer than ours, which is believed to act as a social mechanism to promote social bonding and physical interaction.

References

• Kanitakis, J. (2002). Anatomy, histology and immunohistochemistry of normal human skin. European journal of dermatology, 12(4), 390-401.
• Kolarsick, P. A., Kolarsick, M. A., & Goodwin, C. (2011). Anatomy and physiology of the skin. Journal of the Dermatology Nurses' Association, 3(4), 203-213.
• Patel, P., Ray, R. K., & Manivannan, M. (2019). Power law based "out of body" tactile funneling for mobile haptics. IEEE transactions on haptics, 12(3), 307-318.