Development of touch
Among the four somesthetic qualities of touch, warmth, coolness, and pain described by Mountcastle (2005), touch is the most difficult to define due to its multimodality. The sense of touch is enabled by “afferents sensitive to mechanical stimulation of the skin; they provide signals to the brain concerning the form, texture, location, intensity, movement, direction, and temporal cadence of mechanical stimuli, forms of somesthesis highly developed in the hand” (Mountcastle, 2005, p 72). The sensations included in the sense of touch are also categorized as epicritic. In the glabrous skin, the sense of touch is mediated by four types of classically described cutaneous receptors (Merkel, Ruffini, Pacini, Meissner). Tactile information from the body travels through large myelinated axons in the peripheral nerves to the dorsal root ganglia. From there, the information ascends to the medulla via the ipsilateral dorsal columns (gracilis and cuneatus tracts). In the dorsal column nuclei, the second-order neurons send projections that cross the mid-line, where they form the medial lemniscus, which further ascends in the pons and mid-brain to terminate in the ventral posterior lateral nucleus of the thalamus. From there, third-order neurons send their axons to the primary somatosensory cortex in the post-central gyrus (Kandel, 2000). The “development of touch” sensation is dependent upon maturational processes affecting mechanoreceptor populations, cortical neurons and myelinated fibers.
Four types of receptors have been identified in the human glabrous skin (Vallbo and Johansson 1984).
- Merkel discs: slow adapting type I (SAI) receptors that are dynamically sensitive and exhibit a response linked to the strength of maintained skin deformation. They have small and well-defined cutaneous receptive fields.
- Meissner corpuscles: fast adapting type I (FAI) receptors that have small and well-defined cutaneous receptive fields. They only respond to changes in skin deformation.
- Ruffini receptors: slow adapting type II (SAII) receptors that are dynamically sensitive and exhibit a response linked to the strength of maintained skin deformation. They have receptive fields that are larger and less well defined than those of type I receptors.However, the role of Ruffini receptors has been put into question due to the few number of such receptors in the glabrous skin of the human hand (Paré et al., 2003).
- Pacini corpuscles: fast adapting type II (FAII) receptors that respond to changes in skin deformation. They have receptive fields that are larger and less well defined than those of type I receptors.
The cortical areas dedicated to touch and the encoding of sensory information are the primary somatosensory cortex (SI, including Brodmann’s areas 3a, 3b, 1 and 2); the secondary somatosensory cortex (SII) and the insular (retro and posterior) cortex. The insular cortex, which receives projections from SII, is thought to be important for tactile learning and tactile memory (Kandel, 2000). In addition, associative functions of the posterior parietal cortex play an important role in the sense of touch (Kandel, 2000). These cortical areas, along with the entire cortex, develop the first synapses from the 23rd week of gestation. The cortex continues to develop until birth. Afterward, changes persist; cortical thickness and size continue to increase until 4 years of age. Dendritic connections, synaptic stabilization, myelination and maturation of associative pathways are developed post-natally following a predetermined sequence. The primary areas develop early, followed by the secondary association areas and finally by the terminal zones, i.e. the long association fibers that may only become operative in the second decade of life (Connolly and Forssberg, 1997).
The myelination of both central and peripheral pathways may also play a role in the development of the sense of touch. The myelination follows a defined order: peripheral nerves are myelinated first, followed by the spinal cord, the brainstem, the cerebellum, the basal ganglia and the thalamus. Cortical myelination begins last (Yakovlev and Lecours, 1967). Whereas peripheral nerves, such as the ischiatic nerve, are already myelinated at 12 weeks of gestation, short association fibers in the cortex are not fully myelinated before the age of 16 years (Connolly and Forssberg, 1997).
The description of how the sense of touch evolves throughout life is a complex challenge due to both the multimodal aspect of tactile perception and the lack of systematic investigations in the field. Tactile sensations can be roughly categorized (Jones and Lederman, 2006) into simple stimuli, such as touch detection or vibration, or complex stimuli (texture, spatial acuity/orientation, size/shape/form, manual exploration).
Touch detection, sensing pressure and vibration, is provided primarily by the Meissner and the Pacinian receptors, respectively. No publication has systematically investigated the changes in sensitivity to pressure and vibration throughout life; however, a decline in both has been observed in aging adults (Thornbury and Mistretta, 1981; Bruce, 1980; Kenshalo, 1986; Gescheider et al., 1994; Goble et al., 1996; Verrillo et al., 2002).The decline of the vibrotactile sensitivity has been observed by studying detection thresholds of vibrotactile signals (Gescheider et et al. 1994 I and II), as well as by measuring absolute difference limens (Gescheider et al., 1996). The investigation of the subjective intensity of vibration provided also higher thresholds in older subjects (Verrillo et al., 2002). Touch detection measured with Semmes-Weinstein aesthesiometer filaments also showed increased thresholds with age (Thornbury and Mistretta 1981; Bruce 1980).
Texture discrimination, spatial acuity and orientation, and size/shape/form and manual exploration have been described as complex stimuli (Jones and Lederman, 2006). The Merkel-SA1 afferents are selectively sensitive to particular components of local stress-strain fields, which makes them sensitive to edges, points and curvatures (Johnson, 2001). These receptors are believed to be determinant for size/shape/form perception, spatial acuity and orientation. Texture discrimination, or a simplified conception thereof, is also commonly attributed to the Merkel-SA1 receptors.
- Texture
- Orientation and spatial acuity
- Size / shape / form
- Manual exploration
Whereas many of the multi-modal capabilities related to tactile perception can be observed in infants aged less than one year, the refinement of touch requires at least the first decade of development to reach young adult values. These observations could have implications for children’s manual abilities.
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Dynamic touch, Encyclopedia of touch,Texture from touch,Touch-vision integration