The tactile sense

The fine touch

The sense of touch is the first to develop in utero. From birth, it is an essential source of information for children to become aware of their body and to discover the reality of objects and living beings around them.

By touching objects (or beings) with their hands, but also with their lips and tongue, children gain access to certain properties that are inaccessible to the other senses. By combining tactile information with proprioceptive information from the muscles and tendons in action, they can also judge whether an object is hard or soft, heavy or light.

Active tactile exploration allows, by grasping an object and moving the fingers over it, to access a representation of the shape, curves and contours of objects in three dimensions [1]. Stereognosia is the ability to recognise the shape of an object solely by tactile information from the hands, in the absence of visual information (e.g. by wearing a mask over the eyes). The tactile component is then combined with proprioceptive information that allows precise knowledge of finger movements in space. Determining the shape of an object by touch, which is done sequentially on the object, requires a lot of attention and working memory. It is necessary to link specific information into a global mental construction. Stereognosy is an essential complement to what vision indicates for the three-dimensional mental representation of objects.

Pacini’s corpuscles

How does the hand detect tactile information ?

The hairless skin of the palm (the part that is invisible when the fist is closed) contains a variety of skin corpuscles that act as mechanoreceptors to signal deformations of the skin’s surface. A recent paper [2], from which the following images are taken, describes the anatomical and functional differences of these different corpuscles.

For example, Meissner’s corpuscles consist of a nerve fibre, the end of which surrounds a lamellar cluster of glial cells, the whole surrounded by a capsule. The end of the axon contains proteins that are sensitive to mechanical deformations, allowing the detection of low-frequency vibrations and fine movements of the skin. They appear very early (36 weeks of gestation). They are found particularly densely on the fingertips, on the palm, but also on the lips, the arch of the foot and the genitals. Their density decreases with age.

Pacini’s corpuscles are shaped like ovoid onions. They consist of the end of a nerve fibre surrounded by several layers of cell lamellae, all enclosed in a capsule. In the skin of the fingers, where they are particularly numerous, they are fully developed 4 months after birth. They are sensitive to pressure and vibrations between 20 and 1500 Hz, with maximum sensitivity between 200 and 400 Hz.

There are also many other corpuscles in the skin of the palms: Ruffini’s corpuscles (but at a low density) which are activated when the skin is stretched or when it comes into contact with a rough surface, and Golgi-Mazzoni’s corpuscles, present only in the tips of the fingers, but about which little is known. Merkel’s discs signal continuous pressure on the skin

When the skin is stimulated, proteins sensitive to deformation open up, allowing ions to enter the nerve fibre, which then emits nerve impulses that travel rapidly to the brain. The precise functioning of all these sensing entities is still not well known. Some signal continuous stimulation, others only signal the beginning of stimulation for a longer or shorter period of time. Meissner’s and Pacini’s corpuscles respond to spatially or temporally moving stimuli on the skin surface. Merkel’s discs and Ruffini’s receptors signal constant mechanical stimuli. Merkel’s discs and Meissner’s corpuscles have a small receptive field (the area of the skin to which they are sensitive is relatively small), whereas Pacini’s and Ruffini’s corpuscles have a large receptive field.

All these mechanoreceptive structures thus play different but complementary roles. They are not activated for the same mechanical causes and therefore transmit complementary information to the brain. The brain works on all this nervous information.

In addition to the corpuscles, the skin has other nerve fibres that indicate temperature (some fibres are sensitive to cold, others to heat) and pain (e.g. in case of very strong pressure or tissue damage).

All these mechanoreceptive structures therefore play different but complementary roles. They are not activated for the same mechanical causes and therefore transmit complementary information to the brain. The brain works on all this nervous information.

In addition to the corpuscles, the skin has other nerve fibres that indicate temperature (some fibres are sensitive to cold, others to heat) and pain (in the event of very strong pressure or tissue damage, for example).

How is this tactile information processed by the brain ?

Location of the somatosensory cortex (in red)

The nerve information generated by the skin’s deformation detectors ends up in a large cluster of neurons in the primary somatosensory cortex. This cortex runs along the back of Rolando’s central sulcus in each hemisphere, which separates the frontal and parietal lobes.

Each point of this cortex corresponds to a particular region of the body’s skin, in a very orderly fashion: feet up, head down. The right somatosensory cortex receives information from the left side of the body, and vice versa. The somatosensory cortex works in close collaboration with the motor cortex located on the other side of Rolando’s sulcus: the simple act of bending a finger activates both cortices simultaneously, since there is a motor component and a sensory component.

Within each somato-sensory cortex there is a precise representation of the whole body according to the density of the sensors rather than the surface. (A similar representation can be found in the motor cortex on the other side of Rolando’s central sulcus). ) Thus, the fingers and hands are managed by a very large number of cortical neurons, as are the lips and tongue. This has been known since 1937 and the work of Wilder Penfield and Edwin Boldrey. The hyperdevelopment of the brain area managing the tactile information of the hand is a characteristic of primates. A recent article [3], from which the following image is taken, gives more details on this representation of the body in these cortices.

Recently, extremely precise data on the representation of the body in the somatosensory cortex of 50 patients were obtained by means of electrostimulation during brain surgery [4]. In this work, which took 8 years to complete, the patients indicated during electrical cortical stimulation in which region of the body they felt a tingling sensation. The general pattern is identical to that described above. But this work also shows that, in the region of this cortex devoted to the hand, the information coming from each finger concerns distinct sets of neurons with a very localised and precise topography (each part of the fingers is even managed by close but distinct regions).

The discriminating tactile sensitivity of the hand is therefore very high: two points of contact can be distinguished 3.5 mm apart on the index finger. This distance rises to 8.8 mm on the cheek and 16.4 mm on the foot (hallux) [4]. The palm of the hand is therefore a region of the body that allows very precise analysis of tactile information, due to the high density of cutaneous sensors with special properties, but also to a vast cortical territory that is perfectly defined to manage all this information.
Illustration of homunculus: Penfield and Boldrey

What about children ?

Children are well equipped, from birth, to discover the world with their hands. However, full tactile functionality, particularly for shape and texture recognition, requires mastery of finger muscle dexterity to manipulate and navigate the object efficiently. However, the development of manual dexterity is a slow process, up to adolescence for some aspects, which requires the maturation of fine motor skills [5].

We can easily consider helping him to become aware of the potential of this irreplaceable sense. For example, we can measure his tactile sensitivity by using metal filaments of different diameters. We start with a 1 mm filament applied to the skin for 1.5 seconds and ask him if he has perceived a tactile sensation (he is blindfolded). Different parts of the body can be tested (index finger, cheek, thumb, back of the hand etc.). Then smaller diameter filaments (up to 0.1 mm) are used. This is a method used to measure the threshold of tactile point sensitivity.

We can also work on his stereognosia. While blindfolded, a child is asked to recognise common objects (coin, pen, pencil, cup, glass, comb, sponge, towel, etc.) by touch, using each hand independently, then both simultaneously. It is also possible, on the basis of tactile information alone, to ask a child to associate the shape of an object (triangle, round, square, rhombus, etc.) with its imprint on a wooden support.

Many other possibilities exist to develop children’s sense of touch..

Editor: Didier Trotier