Astereognosia and tactile agnosia: symptoms and causes
Asthereognosia, 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. It is a type of agnosia (i.e., a disorder of object identification not due to sensory disturbance) that specifically impairs recognition through touch.
In this article we will describe the most significant clinical features and the most common causes of tactile agnosia or asthereognosia . Before proceeding we will briefly discuss the concept of agnosia, since it is important to properly contextualize asthereognosia and compare it to other disorders of the same class.
What are agnosias?
Agnosias are a group of disorders characterized by a lack of recognition of stimuli that occur in a certain sensory mode, such as touch or hearing. In these cases the deficits are not the result of alterations in the sense organs , but rather in higher levels of the perceptive pathways.
This type of symptom usually appears as a consequence of injuries that damage the cerebral cortex, interfering with the transmission of sensory impulses to the pathways related to conscious recognition. Some of the more common causes of agnosia include ischemic strokes and neurodegenerative diseases.
In general, agnosias occur in only one mode, and are often classified according to 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 the body itself 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 he could identify it through touch; in this case we would speak of a visual agnosia. Sometimes, if the brain damage that causes the disorder is very serious, several sensory modalities can be affected.
Defining tactile agnosia and asthereognosia
“Astereognosia” is a term commonly used for tactile agnosia, that is, the inability to identify objects through touch in the absence of anomalies in perception itself. In contrast, stereognosia would be the basic capacity 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 retrieve from memory the information needed to identify stimulating cues related to touch, such as temperature, texture, size or weight. However, they are able to do so when they use other senses (usually sight), unless other types of agnosia are present.
Some authors use the name “tactile agnosia” only in cases where the affectation is limited to one of the hands or at most to both, while if the problem involves tactile perception in a more general way they prefer to speak of asthereognosia. In any case, there does not seem to be a consensus on these nomenclatures.
On many occasions, asthereognosia and tactile agnosia are not diagnosed because they do not usually interfere significantly with the functioning of those who suffer them. This has led to an underestimation of the number of cases of asthereognosia, as well as to the scarcity of research on the subject detected when reviewing the scientific literature.
Causes of this disorder
The available evidence reveals that asthereognosia 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 spinal cord .
The specific location of the lesions determines the peculiarities of the symptoms. Thus, when the ventral part of the cortex is damaged, the tactile perception of three-dimensional objects is particularly affected, while if the same happens in the dorsal cortex, it is more common for recognition problems to be of a cognitive nature.
One of the disorders most directly related to asthereognosia is Alzheimer’s disease, characterized by progressive cognitive impairment that particularly affects memory. This association supports the view that agnosias are mainly a memory disorder , and not a perception disorder.
Tactile agnosia, or more specifically digital agnosia (affecting the fingers), is also a characteristic sign of Gerstmann’s syndrome. In this disorder, asthereognosia is accompanied by other peculiar symptoms such as difficulties in orientation between left and right, in calculating or in making graphic representations, especially in writing.