Stereognosia is the ability to recognize objects based on mere tactile experience. The postcentral region of the parietal lobe is primarily involved in this ability, in addition to the individual components of the sense of touch. Lesions in these regions can disrupt this ability and cause what is known as astereognosia (stereoagnosia).
What is stereognosia?
Stereognosia is the ability to recognize objects based on mere tactile experience. The postcentral brain areas of the parietal lobe play a crucial role in the human sense of touch. These sections of the brain are sometimes relied upon for the ability to recognize shapes and consistencies through tactile processes, and to assign the object sensed to a specific object based on its specific properties. These abilities are summarized by the term stereognosia. Stereoagnosia depends on the one hand on the intact structures of the sense of touch and on the other hand on the interpretation ability of the individual. The absolute inability to recognize objects by means of tactile processes is called stereognosia in medicine. Stereoesthesia is to be distinguished from stereognosia. This ability is a basic prerequisite for stereognosia, but it should not be regarded as a synonym for it. Stereoesthesia is based on a combination of epicritic sensitivity and depth sensitivity and is one of the most complex qualities of tactile sensitivity. Failure of this ability is called stereoesthesia and automatically results in simultaneous stereoagnosia.
Function and task
Perception by active palpation of specific objects is called haptics. Together with tactile perception, it forms the totality of the sense of touch, whose biophysiological basis is the somatosensory system and the sensorimotor system. Stereognosy is a quality of haptic perception. Different receptors are involved in each haptic perception, so mainly the mechanoreceptors. They are sensitive to stretch, pressure, and vibration stimuli and are estimated to number up to 600 million within the skin layers. The most common mechanoreceptors are the Vater-Pacini corpuscles for vibration stimuli up to 300 Hz, the Meissner corpuscles for pressure changes, the Merkel cells for sustained pressure stimuli, and the Ruffini corpuscles for tissue stretch. Human body hair is also equipped with such touch sensors. These sensors are completed by the touch-sensitive nerve endings in the upper layer of the skin. Unlike other sensory perceptions, haptic perception relies on an integration of multiple information from different receptors. The receptor density at the fingertips is extremely high and is thus particularly crucial for stereognosy. Information from the individual receptors travels down afferent sensory pathways to the spinal cord and reaches the cerebral cortex via the thalamus. Within the thalamus, the information undergoes circuitry in the nucleus ventralis posterior. Resident neurons project to the secondary and primary somatosensory areas. Cortical processing continues via afferents to the parietal lobe. Its posterior regions in Brodmann areas 5 and 7 are particularly crucial for stereognosia. The somatosensory regions and temporal parietal areas 22, 37, 39, and 40 also play a role. The same is true for the insula and the temporal or frontal association cortices. Multisensory integration is primarily performed by neurons in the posterior parietal cortex. These areas decide all cognition that takes place based on percepts. Connections to the insula help assign shape information to an object and control affective components. Memory processes take place in the temporal lobe based on prior tactile experiences, which help with object recognition. Stereognosy depends, on the one hand, on the integrity of the described structures and, on the other hand, is influenced by chains of associations and stored tactile experiences in the respective brain areas.
Diseases and disorders
Stereoagnosia may arise on the basis of brain lesions or damage to afferent neural pathways. The same is true for stereoesthesia and accompanying stereoagnosia. Brain lesions in the areas described can be caused by stroke, for example.Inflammatory lesions are also conceivable causes. The same applies to tumors or traumatic injuries such as craniocerebral trauma. Stereoagnosia can manifest itself in various ways. For example, if afferent pathways are damaged, tactile information no longer reaches the brain at all and thus cannot serve object recognition. Even when tactile information does reach the brain, it does not necessarily lead to object recognition. If, for example, the memory for tactile information is affected by lesions, the patient can no longer classify the object despite the object properties perceived during palpation, because he lacks the frame of reference for this. In this case, although the transmission and processing of the information is intact, the ability to interpret it is lacking. Problems in multisensory integration can also promote stereoagnosia. According to current knowledge, such integration disorders may have a genetic component and thus be congenital. The neurological disease multiple sclerosis is also frequently associated with stereagnosia. The disease is an autoimmune disorder. The immune system identifies the body’s own nerve tissue of the central nervous system as a danger and attacks it. The antibodies cause inflammation in the brain or spinal cord and can thus also affect the pathways that conduct sensory information. Similarly, they can cause inflammation in brain areas involved in processing, such as the postcentral brain areas of the parietal lobe, damaging the basis for stereognosia. Depending on exactly where the inflammation is located, destruction of central nervous tissue caused in this way can manifest as stereognosia of various types. All types of stereognosia have one thing in common: objects can no longer be recognized with closed eyes on the basis of mere tactile experience.
