Visual agnosia is an acquired neurological condition that is characterized by a difficulty in recognizing and visually processing objects . It has been described since the end of the 19th century and currently different types and manifestations are recognized.
In this article we’ll see what are the types of visual agnosia , what were their first definitions and what are their main manifestations.
What is visual agnosia?
Visual agnosia is an acquired difficulty in identifying objects through vision. It occurs 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 colours, shapes and movements.
In other words, it is a condition in which the ocular capacity to perceive objects persists, but the ability to recognize their characteristics and, therefore, to integrate them as an operative mental representation is missing.
Visual agnosia occurs when the visual process is carried out in an irregular manner. Such processes involve receptors on the retina, 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 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 calcareous cleft. 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 are roughly in charge of analyzing the attributes of the visual images. All of the above helps to form an initial representation of the objects and their characteristics , which is translated into a specific perception by the observer, and then into a recognition stage centred on the object and its semantic information (we proceed to the nomination).
It is in these last stages that some difficulties have been identified that cause visual agnosia.
Background and early 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: aperceptive and associative. In his theory, strongly based on recognition systems, agnosia is a consequence of the disorganization of the processes necessary to carry out 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 as “agnosia”. The word agnosia comes from the Greek word “gnosis” which means knowledge, and the prefix “a” which means “absence of”, as far as a condition characterized by an “absence or lack of knowledge” is concerned.
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 itself 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 aperceptive agnosia, associative agnosia, prosopagnosia, achromatopsia, alexia and acinetopsy.
1. Aperceptive visual agnosia
Aperceptive visual agnosia is characterized by a difficulty in connecting the parts of an image into an understandable whole. This translates into a difficulty in 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 results in the inability to represent such stimuli . For example, the person may have serious difficulties in representing or matching objects through drawings and images.
It is usually caused by injury to the temporal lobe or parietal lobe, in both hemispheres of the brain.
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 aperceptive agnosia and associative agnosia are often assessed, for example, based on a person’s ability to copy drawings. In this case, the person can perform tasks such as drawing or image matching, but has difficulty in naming them. Similarly, the person can use the objects shown to him or her, but has difficulty saying which object it is .
Prosopagnosia is the difficulty in recognizing 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.
Achromatopsia is characterized by difficulties in recognizing the colors of objects. In some cases there is recognition of colours 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 in charge of regulating linguistic activity.
Alexia is the difficulty in visually recognizing words. Sometimes people can speak and write without much difficulty, but they have trouble saying what word it is once they see it written .
Acinetopsy is characterized by a difficulty in 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 to varying degrees. When the condition is severe, the person may lose the ability to recognize any type of movement.
- Healthline (2018). What causes agnosia?. Retrieved June 22, 2018. Available at https://www.healthline.com/symptom/agnosia.
- Maritza, J. (2010). Visual agnosia. Science and Technology for Visual and Ocular Health. 8(1): 115-128.