Visual agnosia is an acquired neurological condition that is characterized by difficulty in recognize and visually process objects It has been described since the end of the 19th century and currently different types and manifestations are recognized.
In this article we will see what are the types of visual agnosia what were its first definitions and what are its main manifestations.
Visual agnosia is an acquired difficulty identifying objects through vision. It presents without damage to the ocular system, without visual alterations and without significant intellectual modifications. It mainly affects the ability to perceive and process elements such as colors, shapes and movements.
In other words, it is a condition in which the ocular ability to perceive objects persists, but lacks the ability to recognize its characteristics and, therefore, to integrate them as an operational mental representation.
Visual agnosia occurs when the visual process is carried out irregularly. These processes involve the participation of retinal receptors, which is an extension of the central nervous system, with circuits and nerve cells, as well as photoreceptor cells called rods and cones. The latter react to light, and transmit the message to other cells that take it to the brain.
After a complex process involving different types of cells and microsystems, the message specifically reaches the primary visual cortex of the brain, located in the occipital lobe, near the calcarine fissure. The specific region associated with the visual system, and therefore with agnosia, is the bilateral occipito-temporal junction.
In the latter, the neurons are distributed in different areas according to the stimuli they process, and roughly speaking, they are responsible for analyzing the attributes of visual images. All of the above helps form an initial representation of the objects and their characteristics which translates into a specific perception of the observer, and then into a recognition stage focused on the object and its semantic information (nomination proceeds).
It is in these last stages where some difficulties that cause visual agnosia have been identified.
Background and first definitions
In 1890, the German neurologist Heinrich Lissauer defined this difficulty in visual recognition as “blindness of the mind” or “blindness of the soul,” and divided it into two main types: apperceptive and associative. In his theory, strongly based on recognition systems, agnosia is a consequence of disorganization of the processes necessary to perform visual analysis and attribute meaning to them
It was in 1891 when Sigmund Freud, who in addition to being a psychoanalyst was a neurologist, baptized this condition “agnosia.” The word agnosia comes from the Greek “gnosis” meaning knowledge, and the prefix “a” meaning “absence of”, so it refers to a condition characterized by an “absence or lack of knowledge”.
6 types of visual agnosia
Since its first definitions, several types of visual agnosia have been identified. For example, we speak of pure visual agnosia when it manifests only through the sensory channel of vision, however, on many occasions it is also linked to the tactile or auditory channels (tactile agnosia and auditory agnosia).
In any case, some of the main subtypes of visual agnosia are apperceptive agnosia, associative agnosia, prosopagnosia, achromatopsia, alexia and acinetopsia.
Apperceptive visual agnosia is characterized by a difficulty connecting the parts of an image into a whole that is understandable. This translates into difficulty understanding the relationships that exist between objects.
In other words, there is no structuring of the visual stimuli received, which is a condition that affects the discriminative stage of visual identification, which ultimately affects the inability to represent such stimuli For example, the person may have serious difficulties representing or matching objects through drawings and images.
It is generally caused by lesions in the temporal lobe or the parietal lobe, in both cerebral hemispheres.
2. Associative visual agnosia
Associative visual agnosia is characterized by difficulty evoking information associated with the names, uses, origins, or specific characteristics of objects.
Both apperceptive agnosia and associative agnosia are often assessed, for example, based on the person’s ability to copy drawings. In this case, the person can perform tasks such as drawing or matching images, but has difficulty naming them. In the same way, the person can use the objects shown to him, but has difficulty telling what object it is
3. Prosopagnosia
Prosopagnosia consists of difficulty recognizing faces. It is caused by the specific functioning of the fusiform area, which is a region of the brain associated precisely with facial recognition. Prosopagnosia can occur, for example, in people who have Alzheimer’s and other neurodegenerative conditions.
4. Achromatopsia
Achromatopsia is characterized by difficulties recognizing the colors of objects. In some cases there is recognition of colors but there is no possibility of naming them It is associated with lesions in the V4 region of the brain, and is related to the regions responsible for regulating linguistic activity.
Alexia is the difficulty in visually recognizing words. Sometimes people can speak and write without much difficulty, but they maintain problems saying what the word is once they see it written
6. Acinetopsia
Akinetopsia is characterized by a difficulty recognizing motor activity. This means that the person has some problems perceiving the movement of objects as a whole. In other words, movements are perceived as sequences of instantaneous actions without continuity The latter can occur in different degrees. When the condition is severe, the person may lose the ability to recognize any type of movement.
