Agnosia, a neurological term of Greek origin (a þ Greek gnosis), signifies a lack of knowledge and is virtually synonymous with an impairment of recognition.
Patients with agnosia disorder cannot recognize or identify objects, sound, or persons despite having normally functioning senses. There is no deficit in attention, language, familiarity to the stimulus, or memory. The disease occurs due to damage to the brain areas that deal with spatial processing, visual and motor information processing and attention. As these areas are mainly located in the parietal cortex and occipito temporal area, head injury is often a reason behind agnosia.
Agnosia is when recognition impairments are confined to one sensory modality. Impairments can include vision, or audition, or touch. If there are two or more modalities defective, the appropriate word is amnesia. For example, Agnosia is the correct term when the patient can not recognize a previously familiar person, or cannot recognize a cup by sight, but can tell its color or function when they touch it.
Two types of agnosia are recognized: Associative and Apperceptive.
According to the Encyclopedia of Neuroscience, Associative agnosia is when a patient fails to recognize due to defective activation of information pertinent to a given stimulus. Apperceptive agnosia refers to a disturbance of the ‘integration’ of otherwise normally perceived components of a stimulus.
Other classification based on the type of senses involved are Visual, Tactile and Auditory Agnosia. Among these, Visual Agnosia is the most common and best understood.
Agnosia can be due to strokes, dementia, and trauma induced by a head injury. It can also be idiopathic, with no obvious cause.
Pure agnosia being very rare, it is usually diagnosed by ruling out other causes of symptpms, I.e. a differential diagnosis. A patient may undergo CT, MRI, and other physical and mental tests.
Treatment focuses on improving the quality of the patient's life. A multidisciplinary approach is taken. Very few people regain their sensory function.