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Agnosia (“a-gnosis”, “non-knowledge”) means a loss of ability to recognize objects, persons, sounds, shapes or smells while the specific sense is not defective nor is there any significant memory loss. It is usually due to temporal lobe[?] injury in the brain.

Visual agnosia is associated with lesions[?] of the left occipital lobe[?] and temporal lobes. Many patients have a severe visual field defects.

Object Agnosia is inability to recognize objects. Subtypes: Form agnosia: Patients perceive only parts of details, not the whole object.

Simultagnosia: Patients recognize objects or details but only one at the time. They cannot make out the scene they belong to or make out a whole image out of the details. They literally cannot see the forest for the trees.

Associative Agnosia: Patients can describe visual scenes and classes of objects but still fail to recognize them. He may, for example, know that fork is something you eat with but may mistake it for a spoon.

Prosopagnosia: Patients cannot consciously recognize familiar faces, maybe including their own. They can describe the face and its expression but cannot recognize whose face it is. They still show bodily emotion to the face (your hearth-throb still makes your heart throb). They may recognize a person through another cue, like familiar voice or clothing. It is especially likely after right temporal lobe damage.

Experts disagree about the causes of prosopagnosia. It may be object agnosia for specific faces, perception disorder[?] in face perception system or syndrome that causes separation of perception of face and memories associated of the face. Agnostic alexia: Inability to recognize text.

Color agnosia: There is a distinction between color perception versus color recognition. Central Achromatosia refers to deficiency in color perception

Auditory agnosia refers to similar symptoms with environmental, nonverbal auditory cues. This is separate from word deafness which is agnosia connected to auditory information. Receptive amusia is agnosia for music. Cortical deafness refers to people who do not respond to any auditory info but their hearing is intact.

Somatosensory Agnosia or Astereognosia is connected to tactile sense – that is, touch. Patient finds it difficult to recognize objects by touch based on its texture, size and weight. However, they may be able to describe it verbally or recognize same kind of objects from pictures or draw pictures of them. Thought to be connected to lesions or damage in somatosensory cortex[?].

Agnosia can result from strokes, dementia, or other neurological disorders. For all practical purposes, there is no direct cure. Patients may improve if information is presented in other modalities than the damaged one.

See also Aphasia, Apraxia.

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