The 6 types of visual agnosia and their symptoms

Visual agnosia is an acquired neurological condition that is characterized by a difficulty in visually recognize and process objects. It has been described since the end of the 19th century and different types and manifestations are currently 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.

• Related article: "The 5 types of agnosia (visual, auditory, tactile, motor and body)"

What is visual agnosia?

Visual agnosia is an acquired difficulty in identifying objects through vision. It is presented without damage to the ocular system, without visual disturbances 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 the ability to recognize their characteristics and therefore to integrate them is lacking as an operative mental representation.

Visual agnosia occurs when the visual process takes place irregularly. This process involves the participation of the receptors of the retina, which is an extension of the system central nervous system, with nerve cells and circuits, as well as photoreceptor cells called rods and cones. The latter react to light, and transmit the message to other cells that carry 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, neurons are distributed in different areas according to the stimuli they process, and in broad strokes they are in charge of analyzing the attributes of visual images. All of the above helps form an initial representation of objects and their characteristics, which is translated into a specific perception of the observer, and then in a recognition stage focused on the object and its semantic information (the nomination proceeds).

It is in these last stages that some difficulties that cause visual agnosia have been identified.

Background and first definitions

In the year 1890, the German neurologist Heinrich Lissauer defined this difficulty for 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 needed to perform visual analysis and attribute meaning to them.

It was in the year 1891 when Sigmund Freud, who besides being a psychoanalyst was a neurologist, baptized this condition as “agnosia”. The word agnosia comes from the Greek "gnosis" which means knowledge, and the prefix "a" which means “Absence of”, as it refers to a condition characterized by an “absence or lack of knowledge".

6 types of visual agnosia

Since its first definitions, various types of visual agnosia have been identified. For example, we speak of a pure visual agnosia when it manifests only through the sensory channel of vision, not However, on many occasions it is also linked to the tactile or auditory channels (tactile agnosia, and agnosia auditory).

In any case, some of the main subtypes of visual agnosia are apperceptive agnosia, associative agnosia, prosopagnosia, achromatopsia, alexia and acinetopsia.

1. Aperceptive visual agnosia

Aperceptive visual agnosia is characterized by a difficulty in connecting the parts of an image into an understandable whole. This results in a difficulty in understanding the relationships that exist between objects.

In other words, there is no structuring of the visual stimuli received, with which it is a question of a condition that affects the discriminative stage of visual identification, which ultimately affects the inability to represent these 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 a difficulty in evoking information associated with the names, uses, origins, or specific characteristics of objects.

Both apperceptive agnosia and associative agnosia are often evaluated, for example, based on the person's ability to copy pictures. In this case, the person can perform tasks such as drawing or picture matching, but has difficulty naming them. Similarly, the person can use the objects shown, but has difficulty telling what object it is.

3. Prosopagnosia

The prosopagnosia consists of the difficulty to recognize the faces. It is caused by the specific functioning of the fusiform area, which is a region of the brain precisely associated with facial recognition. Prosopagnosia can occur for example in people who have Alzheimer's and other neurodegenerative conditions.

• You may be interested: "Prosopagnosia, the inability to recognize human faces"

4. Achromatopsia

Achromatopsia is characterized by difficulties in recognizing the colors of objects. In some cases there is recognition of the 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.

5. Alexia

Alexia is the difficulty in visually recognizing words. Sometimes people can speak and write without much difficulty, but they keep trouble saying what word it is once they see it written.

• Related article: "Alexia and agrafia: alterations in written language due to brain injury"

6. Acinetopsia

Acinetopsia is characterized by a difficulty in recognizing motor activity. This means that the person presents some problems to perceive the movement of objects as a whole. In other words, movements are perceived as sequences of instantaneous actions without continuity. The latter can occur to varying degrees. When the condition is severe, the person may lose the ability to recognize any type of movement.

Bibliographic references:

• Healthline (2018). What causes agnosia?. Retrieved June 22, 2018. Available in https://www.healthline.com/symptom/agnosia.
• Maritza, J. (2010). Visual agnosia Science and Technology for Visual and Ocular Health. 8(1): 115-128.