Visual agnosia is an acquired neurological condition characterized by difficulty recognize and visually 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.
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 changes. 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 ability of the eye to perceive objects persists, however it does not have the capacity to recognize its characteristics and therefore to integrate them as an operative mental representation.
Visual agnosia occurs when the visual process is performed irregularly. This process involves the participation of retinal receptors, which is an extension of the central nervous system, along with nervous circuits and cells, as well as photoreceptor cells called rods and cones. These react to light and transmit the message to other cells which 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, neurons are distributed in different areas depending on the stimuli they process, and largely responsible for analyzing the attributes of visual images. All of the above helps form a first representation of objects and their characteristics, Which results in a specific perception of the observer, then by a recognition step centered on the object and its semantic information (by making the nomination).
In these later stages, some difficulties that cause visual agnosia have been identified.
Context and first definitions
In 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: perceptual 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 make sense of them.
It was in 1891 that Sigmund Freud, who in addition to being a psychoanalyst was a neurologist, baptized this condition “agnosia”. The word agnosia comes from the Greek “gnosis” which means knowledge, and from the prefix “a” which means “absence of”, because it refers to a condition characterized by an “absence or lack of knowledge”.
6 types of visual agnosia
From the first definitions, several types of visual agnosia have been identified. For example, it is referred to as pure visual agnosia when it manifests only through the sensory channel of vision, but in many cases it is also related to tactile or auditory channels (tactile agnosia and auditory agnosia).
In all cases, some of the major subtypes of visual agnosia are aperceptive 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, so it is a condition that affects the discriminating stage of visual identification, which ultimately affects the inability to represent these stimuli. For example, the person may have serious difficulty representing or matching objects through drawings and pictures.
It is usually caused by damage to 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.
Perceptual agnosia and associative agnosia are usually 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 pictures, but has difficulty naming them. Likewise, the person can use the objects shown to him, but he has trouble telling which object it is.
Prosopagnosia is the difficulty in recognizing faces. It is caused by the specific functioning of the spindle area, which is a region of the brain associated specifically with facial recognition. Prosopagnosia can occur, for example, in people with Alzheimer’s disease and other neurodegenerative diseases.
Achromatopsia is characterized by difficulty recognizing the colors of objects. In some cases, there is color recognition but it is not possible to name them. It is associated with lesions in the V4 region of the brain and is linked to the regions responsible for the regulation of language activity.
Alexia is having difficulty visually recognizing words. Sometimes people can speak and write without too much difficulty, but they problems telling what word it is once they see it written.
Acinetopsia is characterized by difficulty in recognizing motor activity. This means that the person has difficulty 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 are the causes of agnosia? Accessed June 22, 2018. Available at https://www.healthline.com/symptom/agnosia.
- Maritza, J. (2010). Visual agnosia. Science and technology for vision and eye health. 8 (1): 115-128.