Sense of Touch

"It could not have been ten seconds, and yet it seemed a long time that their hands were clasped together. He had time to learn every detail of her hand. He explored the long fingers, the shapely nails, the work-hardened palm with its row of callouses, the smooth flesh under the wrist. Merely from feeling it he would have known it by sight."

Nineteen Eighty-Four by George Orwell

Odd choice, I know. Originally, I had planned on something more like this: 

But as it happened I was reading 1984 while writing this piece and the quote in its context of lovers in impossible, desperate, and dehumanizing circumstances—of love as an act of rebellion, of love to remind them that they were still human, if only fleetingly so—seemed to capture the essence and intensity of the sense of touch: "Merely from feeling it he would have known it by sight." 

Could the imagery of any other sense capture so aptly what it is to be human? 

Ask anyone who has seen 'All Quiet on the Western Front' to recall the most poignant scene from the film, and it will no doubt be this scene:



Movieclips via YouTube

It is not enough to simply see things with our eyes, or to hear things with our ears; no, something deep inside us drives our desire to touch, to connect. 

Seeing is not believing. We do not believe something is real and true until the sense of touch sends the message to our brain telling us it is so, cue poor old doubting Saint Thomas:

What perhaps most differentiates the sense of touch from the other senses is that it is not only a source of commonality between humans as individuals (and as primates), but also between humans and the simplest organisms on earth. 

HOW THE SENSE OF TOUCH IS PROCESSED IN OUR BODIES

Human skin is comprised of millions of sensory receptors, including: [1][28]

• Thermoreceptors (temperature)
• Mechanoreceptors (movement/vibrations & pressure)
• Photoreceptors (light)
• Nociceptors (pain)  
• Chemoreceptors (chemicals)

The skin's four main mechanoreceptors  are: [37]

• Pacinian corpuscles
• Meissner's corpuscles
• Merkel's discs
• Ruffini endings

• Sensory receptors are each associated with nerve fibers (eg: A-beta, A-delta & C fibers). [30] [see Table 1] 

• When the sensory receptors in the skin are stimulated, they send electrical pulses to these nerve fibers, which then relay the pulses to the spinal cord. [28][30]

• From the spinal cord the pulses travel to the brain which translates the electrical signal. [28][30]

• From here the waters get a bit murky, but in 2011 researchers working with transgenic mice identified a group of neurons in the brain’s neocortex with a dedicated, “sub-network of connections that enable them to communicate with one another and amplify the information they are receiving from the stimulus.” [15]

Table 1: Touch Fibers [38-40]

Type of Nerve Fiber	Myelinated	Conduction Speed m/s	Information Carried by Fiber
A-beta (5 sub-types)	yes	35 - 90	Vibration Movement Indentation Stretch Movement of longer hairs
A-delta	yes	5 - 40	Movement of shorter hairs (unpleasant)
C	no	0.5 - 2	Movement of shorter hairs (pleasant)
			Temperature
			Deep Pain

Source: Pleasant to the Touch / The Scientist / September 1, 2012

A cortical sensory homunculus

THE CORTICAL HOMUNCULUS

Areas of the body with greater or more complex sensory or motor connections are represented as larger in the homunculus, those with fewer or less complex connections are represented as smaller. 

For instance, there is a large area of cortex devoted to sensation in the hands, as compared to the back. The resulting image is that of a distorted human body, with disproportionately huge hands, lips, and face. [16][24]

MORE ON HOW TOUCH IS PROCESSED

• Research published in 2012, found that the frequency of skin vibrations also has a role in tactile perception: [20]

When we run our fingers over a surface, the frequency of vibrations produced in the skin produce corresponding oscillations in nerves, which, in turn, convey specific messages about texture to the brain. (Think trying to find a light switch in the dark.) 

• Research published in the journal Cell (2011) using genetically engineered mice to investigate how touch is processed in the skin and nervous system found that each C-type low-threshold mechanosensory receptor branches, “to send projections to as many as 30 different hair follicles.”
• Three different hair types, thick and long (or guard hairs), short, and fine, are evenly spaced throughout the skin of mice and are associated with a different and specific set of nerve endings:

“The nerves connecting each patch of skin containing one guard hair and other associated smaller hairs line up in columns in the spinal cord—neighboring columns correspond to neighboring patches of skin." [25]

The researchers, "estimate that there are about 3,000 to 5,000 columns in the spinal cord, with each column accounting for 100 to 150 hair follicles." [25]

How Touch is Processed: Nerve cells stretch from the spinal cord to hair follicle cells, allowing detection of different levels of touch sensation. [25]

Image Credit: David Ginty lab

DIFFERENT TOUCH SYSTEMS: [23]

Discriminative Touch System: Involved in sensing the location, movement and strength of touch.

Emotional Touch System: Conveys information to the posterior insula, a part of the brain implicated in the process of social bonding. 

• Nerve fibers involved in social touch detect touch frequencies that are slow and gentle. [26] 

• These nerve fibers are abundant in the parts of the body we instinctively touch for comfort or to show affection. [26]

THE ACUITY OF OUR SENSE OF TOUCH LESSENS WITH AGE

• Studies have shown a decline in passive tactile spatial acuity (the ability to resolve the fine spatial details of an object pressed against the skin) with age. [34-36] 
• This decline is attributed to the gradual loss of touch, pain and temperature receptors that occurs as we age. [23]

CROSS-MODEL PLASTICITY

• Researchers have found that adult animals with hearing loss will re-route the sense of touch into the hearing parts of the brain. [17]

(I believe Lance Kerwin attempted similar research in the 70’s. … apologies, any excuse for a James at 15 reference.)

• Many studies have shown that tactile spatial acuity is enhanced among blind individuals compared to sighted individuals of the same age. [8–13][19]

• However, a McMaster University study found that blind individuals who could read Braille had greater sensitivity in their reading finger than non-Braille readers among blind study participants.
• This suggests that the heavier the reliance upon our fingertips, the greater our tactile acuity. [19]

TACTILE ILLUSIONS

Tactile illusions exploit the sense of touch. [4]

One example is the cutaneous rabbit illusion, in which a sequence of taps at two separated skin locations results in the perception that intervening skin regions were also tapped. [5]

Another is the Pinocchio illusion: [16]

“Researchers have found that if they vibrate the tendon of the biceps, many people report feeling that their forearm is getting longer, their hand drifting ever further from their elbow. And if they are told to touch the forefinger of the vibrated arm to the tip of their nose, they feel as though their nose was lengthening, too.” 

THE PLASTICITY OF BODY PERCEPTION

• Studies of monkeys learning to use a rake to obtain distant objects have found that multisensory brain cells began to respond to objects appearing anywhere along the length of the rake, suggesting the brain represented the rake as actually being part of the hand. [31]

• From the 2010 study, Rapid Assimilation of External Objects Into the Body Schema: [32]

“If you look towards your hand in a darkened room and see it illuminated by a bright flash, an afterimage of your hand remains after the flash. If you then move your hand, the afterimage changes, though no actual visual signal is present.”

EVER WONDER WHY WE CAN'T TICKLE OURSELVES? [33]

• Brain scientists at University College London have pinpointed the cerebellum as the part of the brain that prevents us from self-tickling. 
• The cerebellum can distinguish expected sensations from unexpected sensations. 
• Because our brain anticipates this contact from the hands and prepares itself for it, the feeling of unease and panic is removed, and thus the body no longer responds the same as it would if someone else were to tickle us. 
• For more on this, click on the link to view the video by HowStuffWorks Why Can't You Tickle Yourself? 

TOUCH & FIRST IMPRESSIONS

In the 2008 paper, Experiencing Physical Warmth Promotes Interpersonal Warmth, researchers found: [27]

• "Participants who briefly held a cup of hot (versus iced) coffee judged a target person as having a “warmer” personality (generous, caring)." 

• "Participants holding a hot (versus cold) therapeutic pad were more likely to choose a gift for a friend instead of for themselves." 

PROSTHETIC RESEARCH

• In the video below, the man is wearing a prosthetic hand that restores sensation via three small electrodes implanted on the peripheral nerves of the residual limb.
• He then uses the prosthetic hand to pick up and remove stems from cherries while blindfolded.
• With the sensation off, 9 out of 15 cherries were damaged. With the sensation on, 1 out of 15 cherries was damaged. [21]

DTSciVids

• Researchers in South Korea and the United States have created a stretchable artificial skin that can register heat, pressure and moisture. The artificial skin is also embedded with heated elements to maintain warmth. [22]

VOA News

6 MORE INTERESTING POINTS ABOUT THE SENSE OF TOUCH

• Somatosensation, a fancy name for the sense of touch, also plays an important role in proprioception, which is fancy name for the self-awareness of the relative positions of neighbouring body parts, and of the effort required for their movement. [15][41]  (Think trying to touch your nose with your eyes closed.) 

• Photoreceptors detect potentially damaging ultraviolet radiation, inducing increased production of melanin  by  melanocytes. [14]

• Astereognosis is the inability of an individual to identify an object by touch alone. [2][3]

• Patients in a deep vegetative coma will show skin responsiveness when touched. [16]

• Pain asymbolics (individuals who lack the emotional pain system mentioned above) are aware that pain is occurring, but are not sensitive to the pain. It is in effect, pain without unpleasantness. [26]

• Your sense of touch is the first sense to develop inside of the womb. [28]

***

FIN

REFERENCES

[1] https://en.wikipedia.org/wiki/Somatosensory_system

[2]https://en.wikipedia.org/wiki/Astereognosis

[3] O'Sullivan, S.B.; Schmitz, T.J. (2007). Physical Rehabilitation (5th Edition ed.). Philadelphia: F.A. Davis Company. pp. 1180–1181.

[4] https://en.wikipedia.org/wiki/Tactile_illusion

[5] Geldard, F. A.; Sherrick, C. E. (13 October 1972). "The Cutaneous "Rabbit": A Perceptual Illusion". Science 178 (4057): 178–179. doi:10.1126/science.178.4057.178. PMID 5076909.

[6] Peters, RM; Hackeman, E; Goldreich, D (2009). "Diminutive digits discern delicate details: fingertip size and the sex difference in tactile spatial acuity.". Journal of neuroscience 29 (50): 15756–61. doi:10.1523/JNEUROSCI.3684-09.2009. PMID 20016091.

[7] Dillon, YK; Haynes, J; Henneberg, M (2001). "The relationship of the number of Meissner's corpuscles to dermatoglyphic characters and finger size.". Journal of anatomy 199 (Pt 5): 577–84.doi:10.1046/j.1469-7580.2001.19950577.x. PMID 11760888.

[8] Goldreich, D; Kanics, IM (2003). "Tactile acuity is enhanced in blindness.". Journal of neuroscience 23 (8): 3439–45. PMID 12716952.

[9] Stevens, Joseph C.; Foulke, Emerson; Patterson, Matthew Q. (1996). "Tactile acuity, aging, and braille reading in long-term blindness.". Journal of Experimental Psychology: Applied 2 (2): 91–106. doi:10.1037/1076-898X.2.2.91.

[10] Van Boven, RW; Hamilton, RH; Kauffman, T; Keenan, JP; Pascual-Leone, A (2000). "Tactile spatial resolution in blind braille readers.". Neurology 54 (12): 2230–6.doi:10.1212/wnl.54.12.2230. PMID 10881245.

[11] Goldreich, D; Kanics, IM (2006). "Performance of blind and sighted humans on a tactile grating detection task.". Perception & psychophysics 68 (8): 1363–71. doi:10.3758/bf03193735.PMID 17378422.

[12] Wong, M; Gnanakumaran, V; Goldreich, D (2011). "Tactile spatial acuity enhancement in blindness: evidence for experience-dependent mechanisms.". Journal of neuroscience 31 (19): 7028–37. doi:10.1523/JNEUROSCI.6461-10.2011. PMID 21562264.

[13] Bhattacharjee, A; Ye, AJ; Lisak, JA; Vargas, MG; Goldreich, D (2010). "Vibrotactile masking experiments reveal accelerated somatosensory processing in congenitally blind braille readers.".Journal of neuroscience 30 (43): 14288–98. doi:10.1523/JNEUROSCI.1447-10.2010. PMID 20980584.

[14]  Zukerman, Wendy. "Skin 'sees' the light to protect against sunshine". newscientist.com. New Scientist.

[15] INTERNATIONAL RESEARCH EFFORT GIVES NEUROSCIENTISTS BETTER FEELING ABOUT SENSE OF TOUCH By Jocelyn Duffy / Carnegie Mellon University / January 5, 2015

[16] Primal, Acute and Easily Duped: Our Sense of Touch / www.nytimes.com / By NATALIE ANGIER / December 8, 2008

[17] Researchers studying hearing loss find auditory regions of the brain convert to the sense of touch / esciencenews.com /  March 24, 2009 - 10:39 in Biology & Nature

[18] Society for Neuroscience. "Women tend to have better sense of touch due to smaller finger size." ScienceDaily. ScienceDaily, 28 December 2009. <www.sciencedaily.com/releases/2009/12/091215173017.htm>.

[19] Practice, not loss of sight, improves sense of touch in the blind: study / esciencenews.com in Psychology & Sociology / May 11, 2011 / Source: McMaster University

[20] University of Chicago Medical Center. "How our sense of touch is a lot like the way we hear." ScienceDaily. ScienceDaily, 11 December 2012. <www.sciencedaily.com/releases/2012/12/121211154437.htm>.

Journal Reference:

E. L. Mackevicius, M. D. Best, H. P. Saal, S. J. Bensmaia. Millisecond Precision Spike Timing Shapes Tactile Perception. Journal of Neuroscience, 2012; 32 (44): 15309 DOI: 10.1523/JNEUROSCI.2161-12.2012

[21] A more natural sense of touch is brought to prosthetic hand / phys.org / December 6, 2013 / by Nancy Owano 

[22] http://www.voanews.com/content/researchers-artificial-skin-sense-of-touch/2583545.html

[23] 9 surprising facts about the sense of touch / www.vox.com / by Joseph Stromberg / January 28, 2015

[24] https://en.wikipedia.org/wiki/Cortical_homunculus

[25] Johns Hopkins Medicine. "Touching a nerve: How every hair in skin feels touch and how it all gets to the brain." ScienceDaily. ScienceDaily, 11 January 2012. <www.sciencedaily.com/releases/2012/01/120111103354.htm>.

Journal Reference:

Lishi Li, Michael Rutlin, Victoria E. Abraira, Colleen Cassidy, Laura Kus, Shiaoching Gong, Michael P. Jankowski, Wenqin Luo, Nathaniel Heintz, H. Richard Koerber, C. Jeffery Woodbury, David D. Ginty. The Functional Organization of Cutaneous Low-Threshold Mechanosensory Neurons. Cell, 2011; 147 (7): 1615 DOI:10.1016/j.cell.2011.11.027

[26] From Touch to Technology - Scientific American / www.scientificamerican.com / By Claudia Wallis / Jun 11, 2015

[27] Lawrence E. Williams et al. Experiencing Physical Warmth Promotes Interpersonal Warmth. Science. 2008 Oct 24; 322(5901): 606–607.

doi:  10.1126/science.1162548

[28] Cicero, Shannon.  "The Sense of Touch"  20 August 2009.  HowStuffWorks.com. <http://health.howstuffworks.com/skin-care/information/anatomy/touching.htm>  23 July 2015. {HowStuffWorks Source: Children's Mercy Hospitals].

[29] Lisa Skedung et al. Feeling Small: Exploring the Tactile Perception Limits Scientific Reports, September 12, 2013, Vol. 3 Article number: 2617 doi:10.1038/srep02617,

[30] Discover How Nerves Translate Different Types of Touch Sensations  / scientificamerican.com / By Andrea Alfano | Jun 11, 2015

[31] You Are What You Touch - Scientific American / www.scientificamerican.com

[32] Thomas A. Carlson et al. Rapid Assimilation of External Objects Into the Body Schema. Psychological Science July 2010 vol. 21 no. 7 1000-1005.

[33] "Why can\u0027t you tickle yourself?"  13 November 2000.  HowStuffWorks.com. <http://science.howstuffworks.com/life/inside-the-mind/human-brain/question511.htm>  23 July 2015.

[34]  Stevens, JC; Alvarez-Reeves, M; Dipietro, L; Mack, GW; Green, BG (2003). "Decline of tactile acuity in aging: a study of body site, blood flow, and lifetime habits of smoking and physical activity.". Somatosensory & motor research 20 (3-4): 271–9. doi:10.1080/08990220310001622997. PMID 14675966.

[35] Manning, Hélène; Tremblay, FranÇois (2006). "Age differences in tactile pattern recognition at the fingertip". Somatosensory & Motor Research 23 (3-4): 147–155.doi:10.1080/08990220601093460.

[36]  Goldreich, D; Kanics, IM (2003). "Tactile acuity is enhanced in blindness.". Journal of neuroscience 23 (8): 3439–45. PMID 12716952.

[37] https://en.wikipedia.org/wiki/Mechanoreceptor

[38] Pleasant to the Touch / The Scientist / September 1, 2012

[39] ASCENDING PAIN PATHWAYS / http://thebrain.mcgill.ca/ July 21, 2015

[40] The Social Power of Touch / Scientific American Mind / June 11, 2015

[41] https://en.wikipedia.org/wiki/Proprioception

1984 QUOTATION

Orwell, George (2014-05-06). The Collected Works of George Orwell: Nineteen Eighty-Four, Animal Farm, Down an (Kindle Locations 25692-25694). HarperCollins Canada. Kindle Edition.

ADDITIONAL RESOURCES

http://news.nationalgeographic.com/news/2013/13/130912-tactile-touch-perception-nanometers-psychology-science/

http://pss.sagepub.com/content/21/7/1000

https://www.ljmu.ac.uk/about-us/news/why-our-brains-need-touch

http://www.the-scientist.com/?articles.view/articleNo/32487/title/Pleasant-to-the-Touch/

IMAGE CREDITS

Still of John Hurt and Suzanna Hamilton in Nineteen Eighty-Four (1984) / December 22, 2010 © 1984 Metro-Goldwyn-Mayer Studios Inc. All Rights Reserved. Nineteen Eighty-Four at Amazon

"Creación de Adám" by Michelangelo - →This file has been extracted from another file: Creación de Adán.jpg.. Licensed under Public Domain via Wikimedia Commons - https://commons.wikimedia.org/wiki/File:Creaci%C3%B3n_de_Ad%C3%A1m.jpg#/media/File:Creaci%C3%B3n_de_Ad%C3%A1m.jpg

"Caravaggio - The Incredulity of Saint Thomas" by Original uploader was Dante Alighieri at en.wikipedia - Transferred from en.wikipedia; transferred to Commons by User:Tm using CommonsHelper.. Licensed under Public Domain via Wikimedia Commons - https://commons.wikimedia.org/wiki/File:Caravaggio_-_The_Incredulity_of_Saint_Thomas.jpg#/media/File:Caravaggio_-_The_Incredulity_of_Saint_Thomas.jpg

"Mikrofoto.de-volvox-4" by Frank Fox - http://www.mikro-foto.de. Licensed under CC BY-SA 3.0 de via Wikimedia Commons - https://commons.wikimedia.org/wiki/File:Mikrofoto.de-volvox-4.jpg#/media/File:Mikrofoto.de-volvox-4.jpg

"Creation poster" by Source. Licensed under Fair use via Wikipedia - https://en.wikipedia.org/wiki/File:Creation_poster.jpg#/media/File:Creation_poster.jpg

"Blausen 0809 Skin TactileReceptors" by BruceBlaus. When using this image in external sources it can be cited as:Blausen.com staff. "Blausen gallery 2014". Wikiversity Journal of Medicine. DOI:10.15347/wjm/2014.010. ISSN 20018762. - Own work. Licensed under CC BY 3.0 via Wikimedia Commons - https://commons.wikimedia.org/wiki/File:Blausen_0809_Skin_TactileReceptors.png#/media/File:Blausen_0809_Skin_TactileReceptors.png

“Interpreting Touch” Image from the article Pleasant to the Touch / The Scientist / September 1, 2012

"1421 Sensory Homunculus" by OpenStax College - Anatomy & Physiology, Connexions Web site. http://cnx.org/content/col11496/1.6/, Jun 19, 2013.. Licensed under CC BY 3.0 via Wikimedia Commons - https://commons.wikimedia.org/wiki/File:1421_Sensory_Homunculus.jpg#/media/File:1421_Sensory_Homunculus.jpg

‘How Touch is Processed’ Image by David Ginty lab, via Touching a nerve: How every hair in skin feels touch and how it all gets to the brain / www.sciencedaily.com