How does the somatosensory system work?

Tactile information is processed by the somatosensory system. Somatosensory information is derived from different receptors in the skin, muscles and joints that transmit information about different basic sensory modalities. Such as gnostic touch (pressure, vibrations), proprioception (contains information about the position and movement of the body), pain and sensitivity to hot and cold, and affective touch. There are two systems responsible for converting somatosensory input to the brain: 

1. The medial lemniscal system. This system is concerned with gnostic touch and proprioception.
2. The spinothalamic channel. This channel mediates pain, thermal and affective tactile information.

Input is then forwarded to the primary somatosensory cortex (S-I), also known as somatosensitive bark. This is located in the parietal lobe. The somatosensory cortex contains somatotopic maps of the contralateral side of the body. When the somatosensory cortex is damaged, limitations can be seen in processing the physical and elemental characteristics of tactile stimuli. Higher order somatosensory processes involve more widely distributed networks, including the secondary somatosensory cortex (S-II). These processes include deriving the characteristics of an object, recognising an object and body perception-related processes.

What are primary somatosensory disorders?

Primary tactile disorders consist of an inability to detect basic somatosensory aspects, including limited sensitivity to pressure on the skin, reduced spatial acuity, loss of vibration, and limitations in proprioception. These disorders often result from damage to the contralateral primary somatosensory cortex, thalamus or subcortical ascending to the somatosensory pathways. The deficits may relate to only a somatosensory submodality, while the others remain entirely intact. Primary tactile disorders can lead to higher order problems, such as the inability to recognise objects based on touch. However, higher order tactile disorders can also be present in the absence of primary elemental deficits. 

Which higher order tactile disorders can we distinguish?

• Discrimination against haptic features. Haptic features include texture, substance, size, shape, weight and hardness of a stimulus. There are two categories related to the micro and macrogeometric properties of an object. Texture, density and thermal properties are considered to be microgeometric aspects. Size and shape are considered as macro geometric aspects. It is still unclear whether the two functions are also segregated at the neuroanatomical level.

• Tactile apraxia. This is a deviation that occurs with actions related to the active sense of touch. This is not specific to the use of objects, but has to do with any action using the hand as a sensing means. These are problems in matching the hand movements with the characteristics of an object under certain circumstances. Tactile apraxia is often related to damage to the superior posterior parietal areas.

• Tactile agnosia. This concerns the inability to recognise an object by touch.

• Tactile aphasia. Tactile aphasia occurs when a person is unable to name an object based on touch (but can name the object when perceived through another modality). The patient is also able to visually depict and categorise the object based on meaning, indicating that semantic knowledge about the object is available to the patient. 

What are body-related disorders?

Information about our body is based on the integration of visual, proprioceptive and tactile input. A distinction is made between body image and body scheme. The body image concerns a conscious perceptual identification of body characteristics. It is more visually based and is influenced by existing knowledge of body structure and semantics. The body scheme concerns the position of body parts in space for directing action. It is mainly based on tactile input and proprioceptive information and is continuously updated as our body moves and changes. Body representation disorders can include features of both types.

What are structural body representation disorders?

Structural body representation concerns the knowledge about the composition and shape of body parts. This is essential for forming body awareness. Here are some examples of disorders related to structural body representation and body awareness:

• In autotopagnosia, patients are unable to pinpoint their own body parts on a visual schedule. In heterotopagnosia, patients are unable to identify the body parts of another. These disorders are associated with mid-temporal and parietal leasies in the dominant hemisphere.

• Patients with asomatognosia experience that a body part is no longer there. In somatoparaphrenia, the patient experiences asomatognosia with extensive delusions, misidentifications and fabrications about the affected body part.

• The phantom limb phenomenon concerns the persistent experience of the postural and motor aspects of a limb after the physical loss of that limb.