Senses in the dark

Sense of touch

The importance of the sense of touch

The sense of touch is the first one that an individual develops in life. It continues to be the primary means of experiencing the world through infancy and well into childhood.

The sense of touch has emerged as an important factor for the facilitation of growth and development. Positive effects of supplemental mechanosensory stimulation have been demonstrated in a wide range of organisms, from worm larvae to rat pups to human infants.

Children that were deprived of normal sensory stimulation (like premature born children in incubators or institutionalized children) very often suffer from delay in growth and cognitive development and also have a higher incidence of serious infections.

The sense of touch and the size of your fingers

Tactile perception improves with decreasing finger size, and this correlation fully explains the better tactile perception of women, who on average have smaller fingers than men.

To find out whether receptors (Merkel cells) are more densely packed in smaller fingers, scientists measured the distance between sweat pores (Merkel cells cluster around the bases of sweat pores). People with smaller fingers had greater sweat pore density, which means their receptors are probably more closely spaced.

Blindness and the sense of touch

The sense of touch becomes an important additional source of information when sight is absent.

According to a number of studies, tactile spatial acuity is enhanced in blindness.  Already in 1964 scientists demonstrated that seven days of visual deprivation result in tactile acuity enhancement.  Still, short-term visual deprivation (periods ranging from under 10 to over 110 minutes) does not result in that kind of enhancement.

There are two competing hypotheses on how the sense of touch improves in blind people.  According to the tactile experience hypothesis, reliance on the sense of touch drives tactile-acuity enhancement.  The visual deprivation hypothesis, on the other hand, states that the absence of vision itself drives tactile-acuity enhancement.

A study trying to figure out which hypothesis is true, demonstrated that proficient Braille readers—those who spend hours a day reading with their fingertips, have much more sensitive  fingers than sighted people. But when the lips of the same participants were tested for sensitivity, the blind and sighted performed equally well. This study confirmed the tactile experience hypothesis, for if the visual deprivation hypothesis were true, blind participants would outperform the sighted on all body areas, even those that blind and sighted people use equally often, such as the lips.

Reading Braille code

Reading Braille code

The Braille code

Touch is an important means of written communication for blind people.

The Braille code uses variations of combinations of six dots to represent all the letters of the alphabet, numbers, punctuation marks and commonly-occurring groups of letters. There are additional Braille codes for different purpose e.g.  Braille for science and math notation, for music or foreign languages (naturally the Braille code differs for different alphabets).

Braille and the brain

In an interesting experiment, scientists measured the neural activity in eight people who had been blind since birth while they read Braille words or nonsense Braille.  They discovered that, when a word is read through touch, a specific part of the brain, known as the visual word form area, would peak in activity.  Interestingly, exactly the same area lit up when the same word was read by sighted readers.

Just like experiments with hearing and the sense of smell in blind people, this demonstrates that the different areas of the brain are not specialized to process information coming from defined senses.  It seems that the metamodal theory of brain function is closer to the truth.  According to this theory, the brain regions are defined by the type of information (e.g. spatial, linguistic etc.) to be processed, rather than the channel (hearing, touch) that the information comes from.

Braille music

Braille music notation was invented along with the literary code (in the 1920s). It also uses a system of six raised dots, here used to represent the pitch and rhythm of each note. You can see an example of Braille music code below. The top two rows represent the pitch and the bottom row is used for rhythm. As the code is read by hands, the Braille reader normally has to read, memorize and then play a piece.

 Braille music; From: Krolick B. 1998. How to Read Braille Music: An Introduction, 2nd Edition.

Braille music

Can all visually impaired people learn Braille?

Some blind and partially sighted people cannot  learn Braille. One of the reasons could be that their fingers are not sensitive enough to feel the relatively small Braille dots. For example many diabetic blind persons experience difficulty reading Braille. This is unexpected, because there is no clinical evidence that diabetic neuropathy affects the hands. Yet a study in 1969, comparing seeing, nondiabetic blind and diabetic blind subjects, demonstrated that the mean threshold for two-point discrimination was 1.6 mm in non-diabetic participants and 2.5 mm for diabetic blind participants (there was no difference in the result between the seeing and the nondiabetic blind subjects). The fact that the distance between the points in English Braille is 2.2 mm, explains the difficulty of these diabetic subjects in reading this code.

Other writing systems for the blind

Except the Braille there are also other writing systems for blind people – the Moon, Fishburne etc.  Note that the standard print alphabet cannot be used just by raising letters because of their complexity.

The Moon system is system of relief shapes similar to Latin, print alphabet.  Moon has been found particularly suitable for those who lose their sight later in life or those who do not have a keen sense of touch.

The Moon Alphabet

The Moon Alphabet

The Fishburne code is made up of four symbols (a dot, a vertical dash, a horizontal dash and a diagonal dash), which are used in a repeating pattern to make up the alphabet.

Tactile maps

Reading by touch also includes reading tactile maps and reliefs. They can be extremely useful for people with visual impairment and can improve their ability to manage the challenges of everyday life. They can be used as a tool that gives information about space and are ideal for making mental maps of the reality which are important for navigation. The first tactile maps were created in France in the 18th century.

To facilitate reading of tactile map for people with some (albeit little) remaining visual abilities, they are often coloured and inscribed with black conventional letters.

Tactile art

Croatian art historian and critic Nataša Jovičić brings works of art closer to a blind people through a tactile gallery, exhibiting sculptures and reliefs of various pieces of art. The author has developed a new concept in which the work of art is transferred to tactile diagrams. Important details are presented in separate tactile diagrams. Each colour in the picture is also associated with a specific texture and it is shown in a separate diagram. Everything is accompanied by audio navigation which helps the visitors through professional analysis of the work of art

Tactile sensitivity is heritable, and linked to hearing sensitivity

In study that involved identical and fraternal twins researchers revealed that touch sensitivity was highly heritable and connected closely with hearing ability. The better the twins could sense touch, the better they could hear, and vice versa.




Source and additional links

Lior Reich, Marcin Szwed, Laurent Cohen, and Amir Amedi. A Ventral Visual Stream Reading Center Independent of Visual Experience. Current Biology, 2011; DOI: 10.1016/j.cub.2011.01.040

Wong M, Hackeman E, Hurd C, Goldreich D (2011) Short-Term Visual Deprivation Does Not Enhance Passive Tactile Spatial Acuity. PLoS ONE 6(9): e25277. doi:10.1371/journal.pone.0025277

Zubek J.P., Flye J., Aftanas M. 1964. Cutaneous sensitivity after prolonged visual deprivation. Science 144: 1591–1593.

Wong M., Gnanakumaran V., Goldreich D. (2011) Tactile Spatial Acuity Enhancement in Blindness: Evidence for Experience-Dependent Mechanisms. The Journal of Neuroscience, 31:7028–7037.

Frenzel H, Bohlender J, Pinsker K, Wohlleben B, Tank J, et al. (2012) A Genetic Basis for Mechanosensory Traits in Humans. PLoS Biol 10(5): e1001318. doi:10.1371/journal.pbio.1001318

Ardiel E. L. and Rankin C. H. (2010) The importance of touch in development. Paediatr Child Health. 15: 153–156.

Peters R.M., Hackeman E., Goldreich D. (2009) Diminutive Digits Discern Delicate Details: Fingertip Size and the Sex Difference in Tactile Spatial Acuity. The Journal of Neuroscience 29: 15756-15761

Heinrichs R. W.  and Moorhouse J. A. (1969) Touch-Perception Thresholds in Blind Diabetic Subjects in Relation to the Reading of Braille Type. N Engl J Med ; 280:72-75.