As haptic perception ( ancient Greek ἁπτός haptόs , German , tangible ' , ἁπτικός haptikόs , German , fitting to touch' ) refers to the tentative "understanding" in the literal sense, that is the perception by actively exploring in contrast to the passive tactile perception . The term haptics goes back to the German psychologist Max Dessoir , who in 1892 recommended naming scientific teaching on the tactile system based on "acoustics" and "optics", namely analogous to acoustic or optical haptic perception.
The generic term haptics includes both interoception and exteroception , with a distinction being made between tactile and haptic perception. The basis of the biophysiological tactile and haptic perception by the somatosensory system ( somatosensory , sensory information), and the sensorimotor system ( sensorimotor formed sensory and motor information).
Haptic perception includes the following perception aspects:
- haptic sensitivity (component of surface sensitivity , perception of mechanical stimuli in the form of pressure, vibration and tissue expansion)
- Proprioception ( deep sensitivity , ability to perceive the position and movement of the limbs and one's own body in space; movement perception is sometimes also referred to as kinesthesia),
- Visceroception (perception of information about organ activities)
- Pain perception ( nociception ),
- Temperature perception ( thermal reception )
Stimulus absorption by mechanoreceptors
A large number of different receptors are involved in haptic perception processes. In addition to the information from the mechanoreceptors of the skin , the information from the stretch, pressure and vibration receptors of the joints , tendons and muscles are also integrated into a haptic perception.
The number of receptors in the various layers of the skin alone is estimated to be between 300 and 600 million. The most common receptors include Vater-Pacini bodies (highest sensitivity to vibration stimuli in the range between 40 and 300 Hz), Meissner bodies (register pressure changes of up to 1 µm), Merkel cells (register sustained, vertical pressure stimuli) and Ruffini bodies (stretching of tissue). Pacini and Ruffini bodies in particular are not only found in the skin, but also in many cases in the connective tissue , joints, muscles and internal organs . There are also Golgi tendon organs and muscle spindles .
In addition, each of the approximately 5 million human body hairs is equipped with around 50 touch sensors that register every slightest deformation of the respective hair. In addition, there are the touch-sensitive free nerve endings in the epidermis , which, in addition to mechanical stimuli, primarily register temperature and pain stimuli. With a rough estimate of one free nerve ending per µm² skin area, the total number is around 2 × 10 12 in adults. In contrast to other sensory perceptions, haptic perception requires the integration of multiple information from different body regions and different receptor systems.
All information from the mechano- and proprioreceptors of the muscles, joints and skin is projected via the long afferent sensory pathways of the spinal cord via the thalamus to the cerebral cortex ( cortex cerebri , cortex) . In the thalamus, the haptic information is connected primarily in the nucleus ventralis posterior . Neurons there project directly into part of the secondary (SII) and all primary somatosensory (SI; postcentral gyrus ) parts of the contralateral half of the cerebral cortex. For further cortical processing, afferents go from the SI to the parietal cortex (especially posterior regions; Brodmann areas BA 5 and 7) as well as to secondary somatosensory regions (SII) and from there to more temporally located parietal areas (BA 22, 37, 39, 40) , to the insula , the frontal and temporal association cortices (for more information, see :).
The neurons of the posterior parietal cortex are involved in the multisensory integration (including visual with somatospatial and somatosensory with proprioceptive information), the necessary short-term storage and attention as well as motor control. They are crucial for body perception in space and form the basis for cognitive processes that are based on perceptions.
So far, relatively little is known about the complex higher functions of the compounds of SII. Connections to the insula likely play a role in the processing of form information and affective components. The frontal lobe is likely involved in haptic decision-making processes. The connections to the temporal lobe serve the relevant memory processes.
Differences in neural excitation in tactile and haptic stimulation
In haptic perception, the motor cortex is always active; it is in contrast to the perception of passive stimuli, such as being touched. Neurons were found in the primary and secondary somatosensory cortex of monkeys that
- fire when the monkey touches something, but not when the same monkey is touched in the same place with the same object;
- fire when the monkey is attentive to touch, but not when distracted;
- fire when the monkey grabs something square, but not when it grabs something round.
Exploration strategies for object recognition
The following exploratory procedures were identified:
- Sweeping over the surface ( lateral motion )
- Press ( pressure )
- Include ( enclosure )
- Follow contours ( contour following )
- Static contact ( static contact)
- Unsupported Hold ( unsopported holding )
Recorded object properties include size, weight, contour, surface and material properties, strength and temperature of an object or subject.
Limits of haptic perception
The sense of touch , in the form of (unspecific) responses to touch, is the first sense that develops in the fetus. Reactions triggered by skin contact are visible in extrauterine (surviving) fetuses as early as 6 weeks after fertilization.
The haptic perception threshold ( stimulus threshold ) in actively exploring, healthy adults is around 1 µm (1 mm = 1000 µm). Experimental investigations have shown that a single elevation of this size can still be perceived on an extremely smooth surface. This means that the resolution capacity of haptic perception is many times greater than that of tactile perception . The limits of perception for the immobile subject (tactile perception) are around one millimeter at the most sensitive parts of the body (fingertips and lips).
Disorders of the haptic sensitivity
A multitude of diseases and disorders can impair haptic perception. In addition to injuries to the skin (cuts, burns, etc.), nerve lesions (e.g. due to injuries or circulatory disorders) are the main causes. In addition, metabolic , toxic and immunological causes can cause neuropathies that can lead to sensitive failures. Some diseases that lead to sensitive neuropathies are: diabetes mellitus , chronic kidney failure , thyroid disorders (hyper- and hypothyroidism ) as well as hepatitis , liver cirrhosis and alcohol dependence .
- Force feedback
- Haptic design
- Haptic rendering
- Body therapy
- haptic brands
- Art therapy
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- Martin Grunwald (Ed.): Human Haptic Perception. Basics and Applications. Birkhäuser, Basel a. a. 2008, ISBN 978-3-7643-7611-6 (English).
- Haptics research laboratory at the University of Leipzig
- Virtual reality and the role of haptics, University of Regensburg
- Project page for a dissertation on the topic of operating haptics: Betriebshaptik.de
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