What is agnosia?

Agnosia is a condition in which the brain cannot recognize something even though the senses detect it. Cleveland Clinic explains that a person's eyesight or hearing may work fine, but brain damage stops them making sense of what they see, hear, smell, taste or touch. For example, someone may see a cat and know what cats are, yet be unable to say it is a cat.

In dementia, agnosia means a person loses the ability to recognize familiar objects, people or places. The National Institute of Neurological Disorders and Stroke defines it as the inability to recognize and identify objects or persons using one or more senses, caused by brain damage from conditions including dementia. It notes this does not necessarily mean the person has problems with vision or thinking ability.

Agnosia is caused by damage to the parts of the brain that process information from the senses. Cleveland Clinic lists stroke, Alzheimer's disease and other dementias, brain tumors, head injuries, lack of oxygen to the brain, infections such as encephalitis, and toxins such as carbon monoxide. Agnosia that appears suddenly, especially with signs of stroke, needs immediate medical attention.

Agnosia itself is not curable, but it can often be managed, and some underlying causes can be treated. Cleveland Clinic says therapy teaches people to work around it with another sense, such as recognizing people by their voice. At home, labeling objects, keeping things in the same place every time and reducing clutter can help someone who no longer recognizes items by sight.

Agnosia is a problem with recognizing, and apraxia is a problem with doing. The National Institute of Neurological Disorders and Stroke defines agnosia as the inability to recognize objects or people using the senses, and apraxia as the loss of skilled movements and gestures, such as the steps of bathing or dressing. Both are caused by brain damage, including dementia, and can occur together.