Agnosia (a-gnosis, "non-knowledge", or loss of 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.
Form agnosia: Patients perceive only parts of details, not the whole object.
Simultanagnosia: Patients can recognize objects or details in their visual field, but only one at a 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. Simultanagnosia is a common symptom of Balint's syndrome.
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. Patients suffering from associative agnosia are still able to reproduce an image through copying.
Apperceptive agnosia: Patients are unable to distinguish visual shapes and so have trouble recognizing, copying, or discriminating between different visual stimuli. Unlike patients suffering from associative agnosia, those with apperceptive agnosia are unable to copy images.
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.
Semantic agnosia: Those with this form of agnosia are effectively 'object blind' until they use non-visual sensory systems to recognise the object. For example, feeling, tapping, smelling, rocking or flicking the object, may trigger realisation of its semantics (meaning).
Color agnosia: There is a distinction between color perception versus color recognition.
Prosopagnosia also known as faceblindness and facial agnosia: Patients cannot consciously recognize familiar faces, sometimes even including their own. This is often misperceived as an inability to remember names.
Social emotional agnosia Sometimes referred to as Expressive Agnosia, this is a form of agnosia in which the person is unable to perceive facial expression, body language and intonation, rendering them unable to non-verbally perceive people's emotions and limiting that aspect of social interaction.
Topographical agnosia This is a form of visual agnosia in which a person cannot rely on visual cues to guide them directionally due to the inability to recognise objects. Nevertheless, they may still have an excellent capacity to describe the visual layout of the same place
Verbal auditory agnosia This presents as a form of meaning 'deafness' in which hearing is intact but there is significant difficulty recognising spoken words as semantically meaningful.
Visual verbal agnosia This is where the difficulty comprehending the meaning of words effects comprehension of the written word. The capacity to read is usually intact but the comprehension of what is read will be impaired.
Auditory agnosia With Auditory Agnosia there is difficulty distinguishing environmental and non-verbal auditory cues including difficulty distinguishing speech from non-speech sounds even though hearing is usually normal.
Amusia or Receptive amusia is agnosia for music. It involves loss of the ability to recognize musical notes, rhythms, and intervals and the inability to experience music as musical.
Cortical deafness refers to people who do not perceive any auditory information but whose hearing is intact.
Phonagnosia is the inability to recognize familiar voices, even though the hearer can understand the words used.
Somatosensory agnosia or Astereognosia[clarify] 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.
Pain agnosia Also referred to as Analgesia, this is the difficulty perceiving and processing pain; thought to underpin some forms of self injury.
Integrative agnosia This is where one has the ability to recognize elements of something but yet be unable to integrate these elements together into comprehensible perceptual wholes.
Anosognosia This is the inability to gain feedback about one's own condition and can be confused with lack of insight but is caused by problems in the feedback mechanisms in the brain. It's caused by neurological damage and can occur in connection with a range of neurological impairments but is most commonly referred to in cases of paralysis following stroke. Those with Anosognosia with multiple impairments may even be aware off some of their impairments but completely unable to perceive others.
Time Agnosia is the loss of comprehension of the succession and duration of events.
Apraxia is a form of motor (body) agnosia involving the neurological loss of ability to map out physical actions in order to repeat them in functional activities. it is a form of body-disconnectedness and takes several different forms; Speech-Apraxia in which ability to speak is impaired, Limb-Kinetic Apraxia in which there is a loss of hand or finger dexterity and can extend to the voluntary use of limbs, Ideomotor Apraxia in which the gestures of others can't be easily replicated and can't execute goal-directed movements, Ideational Apraxia in which one can't work out which actions to initiate and struggles to plan and and discriminate between potential gestures, Apraxia of Gait in which co-ordination of leg actions is problematic such as kicking a ball, Constructional Apraxia in which a person can't co-ordinate the construction of objects or draw pictures or follow a design, Oculomotor Apraxia in which the ability to control visual tracking is impaired and Buccofacial Apraxia in which skilled use of the lips, mouth and tongue is impaired.
Agnosia can result from strokes, dementia, or other neurological disorders. It may also be trauma-induced by a head injury, brain infection, or hereditary. Some forms of agnosia have been found to be genetic.
For all practical purposes, there is no direct cure. Patients may improve if information is presented in other modalities than the damaged one. In some cases, occupational therapy or speech therapy can improve agnosia, depending on its etiology.