Agnosia is the loss of the ability to recognize objects, faces, voices, or places. It is a rare disorder. People with agnosia can still think, speak, and interact with the world normally.
Agnosia usually affects only a single information pathway in the brain. For example, people with visual agnosia wonât be able to name an object placed in front of them or describe its use. But they will still be able to reach for it and pick it up. Once they are holding the object, they will be able to use their tactile information pathwayâtheir sense of touchâto identify its use.
Agnosia occurs when the brain suffers damage along certain pathways. These pathways connect the primary sensory processing areas to the parts that store knowledge and information. Primary sensory processing areas include the visual and auditory cortices.
Agnosia is usually caused by lesions on the parietal and temporal lobes of the brain. These lobes are where semantic information and language are stored. Lesions can be created by strokes, head traumas, or encephalitis. Other conditions that damage or impair the brain can also cause agnosia. These conditions include dementia, carbon monoxide poisoning, and anoxia.
There are three main types of agnosia: visual, auditory, and tactile.
Visual agnosia is the most common type of agnosia. This might be because humansâ visual processing areas are so large and complex (Coslett, 2007).
Visual agnosia occurs when there is brain damage along the pathways that connect the occipital lobe of the brain with the parietal and temporal lobes. The occipital lobe assembles incoming visual information. The parietal and temporal lobes understand the meaning of this information.
Visual apperceptive agnosia is the difficulty in assembling parts of an image into an understandable whole. People with the condition may have difficulty understanding how objects are related to one another. When trying to copy a picture of a circle, an apperceptive agnosic might draw a series of concentric scribbles. People with this condition can still use vision to navigate their environment and pick up objects without trouble. Apperceptive visual agnosia is usually caused by lesions to the parietal or temporal lobes on both sides of the brain.
Associative visual agnosia is the inability to recall information associated with an object. This can include an objectâs name, use, or origin. Unlike an apperceptive agnosic, an associative agnosic can copy a picture without trouble. But he or she may not be able to name the object in the drawing. For example, an associative agnosic who is shown a padlock will be able to recognize it and use it, but wonât be able to say what it is.
Prosopagnosia is the inability to recognize faces. It is caused when there is damage to the fusiform face area (FFA), a highly specific region of the brain that recognizes faces. Difficulty with facial recognition can also occur in Alzheimerâs disease because brain deterioration damages this region.
Individuals with autism often experience difficulty recognizing faces. Autistic people do not use their FFA to see faces. Instead, each develops his or her own unique neural pattern for facial recognition (Pierce, et al., 2001).
Achromatopsia is the loss of color vision. It is caused by lesions in the V4 region of the brain. When a lesion separates V4 from the language areas, the result is color anomia. This is the inability to name colors despite being able to perceive them.
Pure alexia is the inability to recognize words visually. People with pure alexia cannot read. They can still speak fluently and can usually write without difficulty.
Akinetopsia is the inability to perceive motion. People with this condition see moving objects as a series of stills, like an object moving under a strobe light.
Pure word deafness is the inability to repeat or understand spoken language. It develops when the A1 sound-processing region of the brain is disconnected from its language centers. People with pure word deafness can still recognize environmental sounds. They can also still read and write.
Phnonagnosia is the inability to recognize and identify familiar voices. It develops when the brain suffers damage to a certain part of the sound association region. This region is located in the right half of the brain. People with this condition can still understand the words being spoken. They can also recognize environmental sounds or sounds made by objects.
Astereognosis is the inability to identify objects by touch. People with this condition cannot associate information regarding size, weight, and texture with the relevant words. They can still name objects by sight. They are also able to draw pictures of objects, as well as reach for them.
Autotopagnosia is when a person loses the ability to orient the parts of his or her own body. It is caused by damage to the left parietal lobe of the brain. Normally, a person has an awareness of his or her limbs are in space at all times, even with closed eyes. This awareness gets distorted when the brainâs internal representation of the body is damaged.