Haptic perception is a sensory quality that allows humans to feel objects or subjects based on active exploration. Haptic perception is distinct from tactile perception, which corresponds to passive skin perception. Multisensory integration disorders, neurological diseases, and receptor diseases interfere with haptic perception.
What is haptic perception?
Haptic perception is a sensory quality that allows humans to feel objects or subjects based on active exploration. The human skin sense has different qualities. The passive qualities are summarized under the term tactile perception to the sense of touch. Tactile perception is composed of protopathic and epicritic perception and thus consists of the passive sensation of temperature, the passive sensation of pain and the likewise passive sensation of touch. However, the human skin also has the ability to perceive the qualities of objects and living beings through active exploration. This active exploration is subsumed under the term haptics. The term goes back to Max Desoir, who coined the term in the 19th century. Haptics encompasses both interoception and exteroception, i.e. the active perception of stimuli on the surface of the body as well as the active perception of stimuli from within the body. Biophysiologically, the basis of tactile and haptic perception is composed of the somatosensory and sensorimotor systems. Haptics encompasses pain perception in the sense of nociception, temperature perception in the sense of thermoreception, and haptic surface sensitivity in the sense of perception of mechanical stimuli as pressure, vibration, and tissue stretch. Also included in haptics is proprioception as depth sensitivity or the ability to perceive one’s own body position in space. In addition, kinesthesia and visceroception are often counted as part of haptics.
Function and task
Haptics allows humans to perceive object properties such as the size, weight, contour, material properties, strength, and temperature of a subject or object. Different receptors or sensory cells are involved in haptic perception. The mechanoreceptors of the skin are among them, as are the stretch, pressure, and vibration receptors in the tendons, joints, and muscles. The haptic system integrates this information into a common perception. Up to 600 million receptors are located in the individual skin layers, for example the Vater-Pacini corpuscles for vibration stimuli, the Meissner corpuscles for pressure changes, the Merkel cells for sustained pressure stimuli and the Ruffini corpuscles for tissue stretching or the Golgi tendon organs and the muscle spindles. The body hairs are also equipped with about 50 touch receptors for registering deformations. The free nerve endings in the epidermis perceive temperature and pain stimuli in addition to mechanical stimuli. Unlike other sensory perceptions, the integration of multiple receptors thus plays a major role for haptic perception. The information from the mechano- and proprioceptors travels via afferent sensory pathways of the spinal cord through the thalamus into the cerebral cortex. In the thalamus, circuitry occurs via the ventral posterior nucleus. Resident neurons project directly to the secondary and primary somatosensory portions of the two contralateral cerebral hemispheres. Cortical processing has afferents from there to the parietal lobe and to secondary somatosensory regions. Projection continues at this point to temporal parietal areas, frontal temporal association cortices, and insular cortex. Neurons in the posterior parietal cortex are tasked with the multisensory integration of haptic information. They form the basis for cognition. Touch memory is provided by connections to the temporal lobe. Efferent signals travel to the parietal lobe via neuronal connections with subcortical and cortical regions. Differences emerge for tactile and haptic sensory stimulation. For haptic perception, unlike tactile perception, there is always activity in the motor cortex.
Diseases and complaints
Because haptics depends in large part on the integration of multisensory information, disruption of these integration processes may be related to generally impaired haptic perception.Sensory integration disorders impair the interpretation of and response to certain stimuli. As a result, those affected seem to behave inappropriately and may, for example, exert inappropriate amounts or little pressure when touching objects or people. Haptic overactivity has a strong heritability and can be treated thanks to modern therapies in the field of multisensory integration. An inability to integrate haptically may also be present after lesions in the posterior parietal cortex. Such lesions can be caused, for example, by ischemia, by stroke, or by neurological diseases such as multiple sclerosis. However, haptics may also be impaired independently of multisensory integration processes. This can be the case, for example, with damage to the afferent nerve pathways in the spinal cord. Damage to all other haptically relevant regions of the central nervous system can also be the cause of impaired haptics. Depending on the localization of the lesion, haptic memory may be impaired, for example. Equally conceivable are lesion-related erroneous tactile information, such as might result from impaired surface sensitivity. Receptor-related diseases are rather rare in this context, but can cause the impaired surface sensitivity as well as neuronal diseases. Receptor disorders are often associated with intoxication. Far more often, however, haptic insensitivity is associated with peripheral or central nervous damage. Peripheral nerve damage can occur, for example, in the context of polyneuropathy and in this case is associated with vitamin deficiency, alcohol abuse, diabetes, toxins, or cancer and infectious diseases, among others. Accordingly, there are many possible reasons for haptic perception disorders. The diagnosis of a specific disease therefore turns out to be extremely challenging in this context.
