Astereognosia And Tactile Agnosia: Symptoms And Causes
Astereognosia, also called tactile agnosia, is a little-known disorder because it does not usually affect the lives of people who suffer from it in a very negative way. This is a type of agnosia (that is, a disorder in the identification of objects that is not due to sensory alterations) that specifically impairs recognition through touch.
In this article we will describe the most significant clinical features and the most common causes of astereognosia or tactile agnosia Before proceeding, we will briefly stop at the concept of agnosia, since it is important to properly contextualize astereognosia and compare it with other disorders of the same class.
Agnosias are a set of disorders that are characterized by the lack of recognition of stimuli that are presented in a certain sensory modality, such as touch or hearing. In these cases the deficits are not a consequence of alterations in the sense organs but at higher levels of the perceptual pathways.
This type of symptom generally appears as a consequence of lesions that damage the cerebral cortex, interfering with the transmission of sensory impulses to pathways related to conscious recognition. Some of the most common causes of agnosia include ischemic strokes and neurodegenerative diseases.
In general, agnosias occur in a single modality, and they are frequently classified depending on the direction in which the alteration occurs. Thus, we can find visual, auditory, tactile or somatosensory, motor and body agnosias which consist of difficulties in identifying one’s own body or a part of it, often one of the halves.
An example of this type of disorder would be the inability to recognize that the object in front of the person is a towel through sight, although they could identify it through touch; In this case we would talk about visual agnosia. Sometimes, if the brain damage that causes the alteration is very severe, several sensory modalities may be affected.
Defining astereognosia and tactile agnosia
“Astereognosia” is a term commonly used to refer to tactile agnosia, that is, the inability to identify objects through touch in the absence of anomalies in perception itself. On the contrary, stereognosia would be the basic ability that allows us to perceive and recognize this type of stimuli in a normal way.
In this type of agnosia, the person is unable to recover from memory the information necessary to identify stimulus cues related to touch, such as temperature, texture, size or weight. However, he is able to do this when using other senses (usually sight), unless other types of agnosia are present.
Some authors use the name “tactile agnosia” only in cases where the involvement is limited to one of the hands or at most both, while if the problem involves tactile perception in a more general way they prefer to talk about astereognosia. In any case, there does not seem to be a consensus around these nomenclatures.
In many cases, astereognosia and tactile agnosia are not diagnosed because they do not usually interfere significantly with the functioning of those who suffer from them. This has led to an underestimation of the number of cases of astereognosia, as well as the scarcity of research on the matter that is detected when reviewing the scientific literature.
The available evidence reveals that astereognosia appears as a consequence of lesions in two specific regions of any of the cerebral hemispheres: the parietal lobe and the association cortex (composed of parts of the parietal, temporal and occipital lobes). It is also associated with damage to the dorsal or posterior column of the spinal cord
The specific location of the lesions determines the peculiarities of the symptoms. In this way, when the ventral part of the cortex is damaged, tactile perception of three-dimensional objects is especially affected, while if the same thing happens in the dorsal cortex, it is more common for recognition problems to have a cognitive nature.
One of the disorders that is most directly related to astereognosia is Alzheimer’s disease, characterized by progressive cognitive deterioration that affects memory especially intensely. This association supports the approaches that defend that agnosias are primarily a memory disorder and not of perception.
Tactile agnosia, or more specifically digital agnosia (affecting the fingers), is also a characteristic sign of Gerstmann syndrome. In this disorder, astereognosia presents with other peculiar symptoms such as difficulties in orienting oneself between left and right, in calculating or in making graphic representations, especially in writing.
