Tactile discrimination

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Tactile discrimination is the ability to differentiate information through the sense of touch. The somatosensory system is the nervous system pathway that is responsible for this essential survival ability used in adaptation. [1] There are various types of tactile discrimination. One of the most well known and most researched is two-point discrimination, the ability to differentiate between two different tactile stimuli which are relatively close together. [2] Other types of discrimination like graphesthesia and spatial discrimination also exist but are not as extensively researched. [3] Tactile discrimination is something that can be stronger or weaker in different people and two major conditions, chronic pain and blindness, can affect it greatly. Blindness increases tactile discrimination abilities which is extremely helpful for tasks like reading braille. [4] In contrast, chronic pain conditions, like arthritis, decrease a person's tactile discrimination. [5] One other major application of tactile discrimination is in new prosthetics and robotics which attempt to mimic the abilities of the human hand. In this case tactile sensors function similarly to mechanoreceptors in a human hand to differentiate tactile stimuli. [6]

Contents

Pathways

Somatosensory system

The somatosensory system includes multiple types of sensations from the body. This includes light, touch, pain, pressure, temperature, and joint /muscle sense. [7] Each of these are categorized in three different areas: discriminative touch, pain and temperature, and proprioception. Discriminative touch includes touch, pressure, being able to recognize vibrations, etc. Pain and temperature includes the perception of pain/ amounts of pain and the severity of temperatures. The pain and temperature category of sensations also includes itching and tickling. Proprioception includes receptors for everything that occurs below the surface of the skin. This includes sensations on various muscles, joints, and tendons. Each of these three categories have their own types of pathways and receptors. These pathways target the cerebellum in the brain. This section of the brain tracks what the muscles are doing at all times so any potential damage to this area can greatly affect one's senses. [8]

Pseudounipolar bipolar neurons Pseudounipolar bipolar neurons.svg
Pseudounipolar bipolar neurons

Within each Somatosensory pathway there are three types of neurons: the pseudounipolar neuron, secondary afferentme neurons, and tertiary afferent neurons. There are also slowly adapting receptors that signify the receptors that sense the indents made on the skin. Rapidly adapting receptors are also present in this system. An example of a slowly adapting receptor in use is when a person breaks his/her arm, the arm is immobilized until it is healed. He/she does not want to forget that it is broken and do something that could potentially worsen the damage in the arm. An example of a rapid adapting receptor in use is putting on clothes. [9] Initially you will feel the clothes being worn, but after a while you forget you are wearing clothes. It is not at the forefront of the brain to focus on the feeling of the clothes on your body; however, if you were to concentrate on that feeling, you could instantly feel the contact between your skin and the clothing being worn. [10]

Discriminative touch system

The discriminative touch system deals with everything from the toes to the neck through the spinal cord. The sensation experienced enters the periphery by axons. More specifically, the sensory axons. This signal passes through axon to axon from the distal to proximal process. The proximal end of the specific axon leads into the spinal cord on the dorsal half. This then moves towards the brain. These axons that are leading the signal towards the spinal cord to the brain are classified as primary afferents. This makes sense as afferent is defined conducting toward something. These neurons are sending signals towards the brain. [11] Those that receive the neuron synapses are classified as secondary afferents. These neurons go to the thalamus and then synapsed towards another set of neurons that move towards the cerebral cortex. [7]

Types of receptors

There are many types of receptors in the somatosensory pathway including:

WVSOM Meissner's corpuscle WVSOM Meissner's corpuslce.JPG
WVSOM Meissner's corpuscle

Types of tactile discrimination

Stereognosis

Stereognosis (Tactile Gnosis) is defined as the ability to tell the difference and identify objects via touch in the absence of visual or auditory contact. The subject will need to be able to recognize temperature, spatial properties, texture, and size to achieve reach an accurate conclusion to what the object is. [13] This type of tactile discrimination will give an indication of the status of the Parietal lobe of the brain. [14] When conducting this test, common objects that the subject is familiar with are used in order to ensure an accurate reading, and consistency amongst multiple tests with multiple different subjects. By utilizing this form of tactile discrimination, practitioners will be able to detect and track the presence or effects of Neurodegenerative Diseases such as Alzheimer's disease due to Astereognosis, which is the failure to recognize objects via touch without visual recognition. [15]

Graphesthesia

Graphesthesia is the ability in which a person is able to recognize a number or letter that is written on the person's skin. [3] Like other tactile discrimination tests, the test for this is a measurement of the patient's sense of touch, and requires that the patient perform the test voluntarily and without visual contact. The purpose of this form of tactile discrimination is to detect any defects in the Central nervous system such as lesions in the Brainstem, Spinal cord, Thalamus, or Sensory cortex. In order for this test to be carried out successfully, it is imperative that the subject's primary sensations be fully functional. A severe lesion in the Central Nervous System would suggest a loss in primary sensation. It is also important that the practitioner and the patient communicate ahead of time about the orientation of the characters, as well as where on the body the figures are to be drawn (usually on the palms of the hand). [3] In order to make this tactile discrimination more flexible, the patient may select the correct answer from a series of images in lieu of communicating verbally if the patient suffers from a speech or language impairment. The Graphesthesia test is also more versatile than the Stereognosis test since it doesn't require the patient to be able to grasp an object. [16]

Two-point discrimination

Two-point discrimination (2PD) is a neurological examination in which two sharp points are applied to the surface of a part of the body in order to see if the patient recognizes them as two discrete sensations. [2] The two-point threshold is the smallest distance between the two points that the patient can recognize. [17] By conducting this form of tactile discrimination, it is believed that practitioners will be able to discern the relative amount of nerves in the tested location. When conducting the procedure on the desired part of the body, the practitioner may apply both points simultaneously or with just one point. The practitioner may switch between the two at random. In order for the examination to be conducted in the most proper fashion, it is imperative that there be clear and open communication between the subject and the practitioner with the subject being fully conscious and not under any sort of influence while at the same time not making visual contact with the device. [18] The efficacy of Two-point discrimination has come under scrutiny from many researchers despite being commonly used to this day in a clinical setting. Research studies have shown that the 2PD test does a poor job of determining the degree to which the nerves regain their function after damage, as well as determining the sensory failures in the first place, owing to this form of tactile discrimination's simplicity, crudeness, and dependence on anecdotal evidence. The research studies have also shown that there is a discrepancy between the data obtained from 2PD tests and data obtained from other tests used to measure tactile spatial acuity. [19]

2-point orientation discrimination (2POD) compared to traditional two-point discrimination (2PD) 2-point orientation discrimination (2POD) compared to traditional two-point discrimination (2PD).jpg
2-point orientation discrimination (2POD) compared to traditional two-point discrimination (2PD)

Spatial discrimination

Spatial discrimination is another form of Two-point discrimination in which the practitioner tests for innervation of the skin with two blunt points of a compass (drawing tool). Just with like 2PD, the patient must be able to discriminate between the two applied points. All other parameters, methods, and objectives of the Spatial discrimination tactile discrimination and the 2PD tactile discrimination remain the same. [20]

Applications

Blindness

Common Keypad with Braille ATM keypad with braille.jpg
Common Keypad with Braille

When a person has become blind, in order to “see” the world, their other senses become heightened. An important sense for the blind is their sense of touch, which becomes more frequently used to help them perceive the world. People that are blind have displayed that their visual cortices become more responsive to auditory and tactile stimulation. Braille allows the blind to be able to use their sense of touch to feel the roughness, and distance of various patterns to be used as a form of language. Within the brain, the activation of the occipital cortex is functionally relevant for tactile braille reading, as well as the somatosensory cortex. [4] These various parts of the brain function in their own way, in which they each contribute to the effectiveness of how braille is read by the blind. People that are blind also rely heavily on Tactile Gnosis, Spatial discrimination, Graphesthesia, and Two-point discrimination. Essentially, the occipital cortex allows one to effectively make judgements on the distance of braille patterns, which is related to spatial discrimination. [4] Meanwhile, the somatosensory cortex allows one to effectively make judgements on the roughness of braille patterns, which is related to two-point discrimination. [4] The various visual areas in the brain are very essential for a blind person to read braille, just as much as it is for a person that has sight. Essentially, whether one is blind or not, the perception of objects that involves tactile discrimination is not impaired if one cannot see. When comparing people that are blind to people that have sight, the amount of activity within their somatosensory and visual areas of the brain do differ. The activity in the somatosensory and visual areas are not as high in tactile gnosis for people that are not blind, and are more-so active for more visual related stimuli that does not involve touch. Nonetheless, there is a difference in these various areas within the brain when comparing the blind to the sighted, which is that shape discrimination causes a difference in brain activity, as well as tactile gnosis. The visual cortices of blind individuals are active during various vision related tasks including tactile discrimination, and the function of the cortices resemble the activity of adults with sight. [21]

Chronic pain

Some non-neuropathic chronic pain conditions have been shown to decrease tactile acuity, the ability to precisely detect touch. [5] There is a difference between different chronic pain conditions and how they affect tactile acuity deficits. One of the conditions with the most profound deficits in tactile acuity is arthritis. This condition affects the tactile acuity both at the site of the pain and at remote locations away from the pain. [5] This suggests that the deficit may be a result of a cortical reorganization, or cortical remapping in the patient's brain. Other conditions, like complex regional pain syndrome and chronic lower back pain, show deficits only at the site of pain. Still other conditions like burning mouth syndrome shows no deficit in tactile acuity at all. [5] Although there is evidence that some chronic pain conditions cause a decrease in tactile acuity there is no evidence to suggest when this deficit becomes clinically meaningful and affects the function of the patient. [5]

Robotic tactile discrimination

Early robotic prosthetic hand, made in 1963. On open public display at the main shopping mall in Belgrade. Belgrade hand.JPG
Early robotic prosthetic hand, made in 1963. On open public display at the main shopping mall in Belgrade.

As robots and prosthetic limbs become more complex the need for sensors capable of detecting touch with high tactile acuity becomes more and more necessary. There are many types of tactile sensors used for different tasks. [6] There are three types of tactile sensors. The first, single point sensors, can be compared to a single cell, or whiskers, and can detect very local stimuli. The second type of sensor is a high spatial resolution sensor which can be compared to a human fingertip and is essential for the tactile acuity in robotic hands. The third and final tactile sensor type is a low spatial resolution sensor which has similar tactile acuity as the skin on one's back or arm. [6] These sensors can be placed meaningfully throughout the surface of a prosthetic or a robot to give it the ability to sense touch in similar, if not better, ways than the human counterpart. [6]

Related Research Articles

<span class="mw-page-title-main">Sensory nervous system</span> Part of the nervous system responsible for processing sensory information

The sensory nervous system is a part of the nervous system responsible for processing sensory information. A sensory system consists of sensory neurons, neural pathways, and parts of the brain involved in sensory perception and interoception. Commonly recognized sensory systems are those for vision, hearing, touch, taste, smell, balance and visceral sensation. Sense organs are transducers that convert data from the outer physical world to the realm of the mind where people interpret the information, creating their perception of the world around them.

<span class="mw-page-title-main">Afferent nerve fiber</span> Axonal projections that arrive at a particular brain region

Afferent nerve fibers are axons of sensory neurons that carry sensory information from sensory receptors to the central nervous system. Many afferent projections arrive at a particular brain region.

A mechanoreceptor, also called mechanoceptor, is a sensory receptor that responds to mechanical pressure or distortion. Mechanoreceptors are innervated by sensory neurons that convert mechanical pressure into electrical signals that, in animals, are sent to the central nervous system.

<span class="mw-page-title-main">Spinothalamic tract</span> Sensory pathway from the skin to the thalamus

The spinothalamic tract is a part of the anterolateral system or the ventrolateral system, a sensory pathway to the thalamus. From the ventral posterolateral nucleus in the thalamus, sensory information is relayed upward to the somatosensory cortex of the postcentral gyrus.

<span class="mw-page-title-main">Dorsal column–medial lemniscus pathway</span> Sensory spinal pathway

The dorsal column–medial lemniscus pathway (DCML) is a sensory pathway of the central nervous system that conveys sensations of fine touch, vibration, two-point discrimination, and proprioception (position) from the skin and joints. It transmits information from the body to the primary somatosensory cortex in the postcentral gyrus of the parietal lobe of the brain. The pathway receives information from sensory receptors throughout the body, and carries this in nerve tracts in the white matter of the dorsal column of the spinal cord to the medulla, where it is continued in the medial lemniscus, on to the thalamus and relayed from there through the internal capsule and transmitted to the somatosensory cortex. The name dorsal-column medial lemniscus comes from the two structures that carry the sensory information: the dorsal columns of the spinal cord, and the medial lemniscus in the brainstem.

Stereognosis is the ability to perceive and recognize the form of an object in the absence of visual and auditory information, by using tactile information to provide cues from texture, size, spatial properties, and temperature, etc. In humans, this sense, along with tactile spatial acuity, vibration perception, texture discrimination and proprioception, is mediated by the dorsal column-medial lemniscus pathway of the central nervous system. Stereognosis tests determine whether or not the parietal lobe of the brain is intact. Typically, these tests involved having the patient identify common objects placed in their hand without any visual cues. Stereognosis is a higher cerebral associative cortical function.

Sensory substitution is a change of the characteristics of one sensory modality into stimuli of another sensory modality.

A topographic map is the ordered projection of a sensory surface, like the retina or the skin, or an effector system, like the musculature, to one or more structures of the central nervous system. Topographic maps can be found in all sensory systems and in many motor systems.

Cutaneous innervation refers to the area of the skin which is supplied by a specific cutaneous nerve.

<span class="mw-page-title-main">Group C nerve fiber</span> One of three classes of nerve fiber in the central nervous system and peripheral nervous system

Group C nerve fibers are one of three classes of nerve fiber in the central nervous system (CNS) and peripheral nervous system (PNS). The C group fibers are unmyelinated and have a small diameter and low conduction velocity, whereas Groups A and B are myelinated. Group C fibers include postganglionic fibers in the autonomic nervous system (ANS), and nerve fibers at the dorsal roots. These fibers carry sensory information.

Pallesthesia, or vibratory sensation, is the ability to perceive vibration. This sensation, often conducted through skin and bone, is usually generated by mechanoreceptors such as Pacinian corpuscles, Merkel disk receptors, and tactile corpuscles. All of these receptors stimulate an action potential in afferent nerves found in various layers of the skin and body. The afferent neuron travels to the spinal column and then to the brain where the information is processed. Damage to the peripheral nervous system or central nervous system can result in a decline or loss of pallesthesia.

<span class="mw-page-title-main">Two-point discrimination</span> Ability to discern between single and pairs of points on ones skin

Two-point discrimination (2PD) is the ability to discern that two nearby objects touching the skin are truly two distinct points, not one. It is often tested with two sharp points during a neurological examination and is assumed to reflect how finely innervated an area of skin is.

<span class="mw-page-title-main">Somatosensory system</span> Nerve system for sensing touch, temperature, body position, and pain

In physiology, the somatosensory system is the network of neural structures in the brain and body that produce the perception of touch, as well as temperature (thermoception), body position (proprioception), and pain. It is a subset of the sensory nervous system, which also represents visual, auditory, olfactory, and gustatory stimuli.

<span class="mw-page-title-main">Proprioception</span> Sense of self-movement, force, and body position

Proprioception, also referred to as kinaesthesia, is the sense of self-movement, force, and body position. It is sometimes described as the "sixth sense".

A somatosensory disorder is an impairment of the somatosensory system.

Amorphosynthesis, also called a hemi-sensory deficit, is a neuropsychological condition in which a patient experiences unilateral inattention to sensory input. This phenomenon is frequently associated with damage to the right cerebral hemisphere resulting in severe sensory deficits that are observed on the contralesional (left) side of the body. A right-sided deficit is less commonly observed and the effects are reported to be temporary and minor. Evidence suggests that the right cerebral hemisphere has a dominant role in attention and awareness to somatic sensations through ipsilateral and contralateral stimulation. In contrast, the left cerebral hemisphere is activated only by contralateral stimuli. Thus, the left and right cerebral hemispheres exhibit redundant processing to the right-side of the body and a lesion to the left cerebral hemisphere can be compensated by the ipsiversive processes of the right cerebral hemisphere. For this reason, right-sided amorphosynthesis is less often observed and is generally associated with bilateral lesions.

<span class="mw-page-title-main">Cross modal plasticity</span> Reorganization of neurons in the brain to integrate the function of two or more sensory systems

Cross modal plasticity is the adaptive reorganization of neurons to integrate the function of two or more sensory systems. Cross modal plasticity is a type of neuroplasticity and often occurs after sensory deprivation due to disease or brain damage. The reorganization of the neural network is greatest following long-term sensory deprivation, such as congenital blindness or pre-lingual deafness. In these instances, cross modal plasticity can strengthen other sensory systems to compensate for the lack of vision or hearing. This strengthening is due to new connections that are formed to brain cortices that no longer receive sensory input.

Many types of sense loss occur due to a dysfunctional sensation process, whether it be ineffective receptors, nerve damage, or cerebral impairment. Unlike agnosia, these impairments are due to damages prior to the perception process.

<span class="mw-page-title-main">Tactile hallucination</span>

Tactile hallucination is the false perception of tactile sensory input that creates a hallucinatory sensation of physical contact with an imaginary object. It is caused by the faulty integration of the tactile sensory neural signals generated in the spinal cord and the thalamus and sent to the primary somatosensory cortex (SI) and secondary somatosensory cortex (SII). Tactile hallucinations are recurrent symptoms of neurological diseases such as schizophrenia, Parkinson's disease, Ekbom's syndrome and delerium tremens. Patients who experience phantom limb pains also experience a type of tactile hallucination. Tactile hallucinations are also caused by drugs such as cocaine and alcohol.

Tactile induced analgesia is the phenomenon where concurrent touch and pain on the skin reduces the intensity of pain that is felt.

References

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  9. ↑ Costanzo. "Tactile Discrimination and Proprioception" (PDF). ecurriculum. Retrieved 8 April 2018.
  10. ↑ Costanzo. "Tactile Discrimination and Proprioception" (PDF). ecurriculum. Retrieved 8 April 2018.
  11. 1 2 Dougherty, Patrick. "Somatosensory Pathways". Neuroscience. Retrieved 2 April 2018.
  12. ↑ Costanzo. "Tactile Discrimination and Proprioception" (PDF). ecurriculum. Retrieved 8 April 2018.
  13. ↑ "Stereognosis Medical Dictionary". Merriam-Webster.com. Merriam-Webster. Retrieved 26 April 2018.
  14. ↑ Silvestri: Saunders Comprehensive Review for the NCLEX-RN Exam
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  16. ↑ O'Sullivan, S. B., & Schmitz, T. J. (2007). 'Physical Rehabilitation' (5th Edition ed.). Philadelphia: F.A. Davis Company.
  17. ↑ Physical rehabilitation. O'Sullivan, Susan B., Schmitz, Thomas J. (5th ed.). Philadelphia: F.A. Davis. 2007. ISBN   9780803612471. OCLC   70119705.{{cite book}}: CS1 maint: others (link)
  18. ↑ Hal., Blumenfeld (2010). Neuroanatomy through clinical cases (2nd ed.). Sunderland, Mass.: Sinauer Associates. ISBN   9780878930586. OCLC   473478856.
  19. ↑ Lundborg, G.; Rosen, B. (October 2004). "The two-point discrimination test--time for a re-appraisal?". Journal of Hand Surgery (Edinburgh, Scotland). 29 (5): 418–22. doi:10.1016/j.jhsb.2004.02.008. PMID   15336741. S2CID   34717685.
  20. ↑ spatial discrimination. (n.d.) Medical Dictionary. (2009). Retrieved March 17, 2018 from https://medical-dictionary.thefreedictionary.com/spatial+discrimination.
  21. ↑ Marina, Bedny (2017). "Evidence from Blindness for a Cognitively Pluripotent Cortex" (PDF).{{cite journal}}: Cite journal requires |journal= (help)
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