Agnosia - A Short Overview

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To understand what agnosia is, we must first understand what gnosia is. Gnosia, according to the dictionary is the perceptive faculty enabling one to recognize the form and the nature of the persons and things.

So agnosia is a loss of ability to recognize different things like: objects, persons, sounds, shapes, and even smells. However, the sense organ is not defective and there is also no significant memory loss involved. The whole thing is actually associated with brain injury and neurological illness, mostly occipitotemporal border (a part of the ventral stream). 

Agnosia will usually result from strokes, dementia, head injury, brain infection, but it can also be hereditary. There are some forms of agnosia that are genetic, for example dyslexia.

There are many types of agnosia which can be divided into 4 groups:

1- Auditory agnosia

This form of agnosia manifests in the inability to recognize or differentiate between sounds, but it has no connection with the hearing organ. There are three main types of auditory agnosia:

Classical auditory agnosia – the person cannot process environmental sounds such as animal noises or industrial noises.

Interpretative or receptive agnosia – the inability to understand music, it is also called amusia.

Linguistic agnosia – the people cannot comprehend words but they are capable of speech and they usually understand through sign language and reading books. However, when they are spoken to, they cannot find meaning in the sounds associated with the words.

2 - Tactile agnosia

This form of agnosia is manifested through an inability to recognize objects when touching, with the eyes closed. They are usually linked to injuries of the parietal lobe and the supramarginal gyrus.

Primary: The patients often cannot recognize the material used for the object or recognize the form and dimension of the object.

Secondary: the patients can recognize all of the above but they cannot recognize the object.

3 – Visual agnosia

The person cannot “see” objects although their sense organ is not injured. There are different types of visual agnosia:

-         objects

-         images

-         spatial

-         graphic symbols

-         associative

-     prosopagnosia or face blindness
-     blind sight

If you’ve read The man who mistook his wife with a hat by Oliver Sacks you will recognize visual agnosia.  

4- Body image agnosia

The person who suffers from this type of agnosia will lose their perception, recognition and representation of the existence of his body. This can be a total body image agnosia or a partial one.

-         asomatognosia – the person cannot recognize parts of their body

-         autotopagnosia - inability to localize and orient different parts of the body (it is linked to the parietal lobe of the left cerebral hemisphere)

-         finger agnosia – inability to distinguish the fingers on the hand (present in lesions of the dominant parietal lobe and a component of Gerstmann syndrome) 

Further reading:

http://www.psychology.sbc.edu/redwine1.htm

http://brainmind.com/Agnosia.html

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