Agnosia (a-gnosis, "non-knowledge") is 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 associated with brain injury or neurological illness, particularly after damage to the temporal lobe.
Visual agnosia is associated with lesions of the left occipital lobe and temporal lobes. Many patients have severe visual field defects.
Object Agnosia is the 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 a fork is something you eat with but may mistake it for a spoon.
Mirror Agnosia: Patients cannot recognize objects or activity on either their left or right field of view. Impairment can vary from mild inattention to complete inability to perform spatial reasoning with regard to the afflicted side. The disorder takes its name from an experiment in which a patient was shown objects reflected in a mirror and saw them, but was unable to find them when prompted.
Prosopagnosia: Patients cannot consciously recognize familiar faces, sometimes even including their own. Impairment may vary from 'faces making no sense' to being able to perceive faces but not connect them with any semantic information, such as the person's identity, name or occupation. Curiously, despite being unable to consciously recognise people, studies have shown that people with prosopagnosia can show an emotional response to familiar faces. Affected people may be able to recognise a person through another cue, like familiar voice or clothing. It is especially likely after bilateral (both sides) or right temporal lobe damage.
Experts disagree about the causes of prosopagnosia. It may be object agnosia for specific faces or a specific face perception disorder.
See prosopagnosia entry for more details.
Agnostic alexia: Inability to recognize text.
Color agnosia: There is a distinction between color perception versus color recognition. Central Achromatopsia 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 information but whose 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.