thumb|250px|Picture of the ventral and dorsal streams. The ventral stream is depicted in purple and the dorsal stream is depicted in green.
Agnosia is a neurological disorder characterized by an inability to process sensory information. Often there is a loss of ability to recognize objects, persons, sounds, shapes, or smells while the specific sense is neither defective nor is there any significant memory loss. It is usually associated with brain injury or neurological illness, particularly after damage to the occipitotemporal border, which is part of the ventral stream. Agnosia affects only a single modality, such as vision or hearing. More recently, a top-down interruption is considered to cause the disturbance of handling perceptual information.
Types
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! Name !! Description
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| Akinetopsia|| Also known as cerebral akinetopsia, this is associated with the inability to see visual motion. One cause of cerebral akinetopsia is lesions outside the striate cortex.
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| Allotopagnosia || Patients cannot point at external targets located outside their own body, whether other persons' body parts or objects, but they perfectly point at their own body parts.
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| 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 is 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 of some of their impairments but completely unable to perceive others.
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| Apperceptive visual agnosia || Patients are unable to distinguish visual shapes and so have trouble recognizing, copying, or discriminating between different visual stimuli. Unlike patients with associative agnosia, those with apperceptive agnosia are unable to copy images.
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| Associative visual agnosia || Patients can describe visual scenes and classes of objects but still fail to recognize them. They may, for example, know that a fork is something you eat with but may mistake it for a spoon. Patients with associative agnosia are still able to reproduce an image through copying.
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| Astereognosis || Also known as somatosensory agnosia, it 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.
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| Auditory agnosia || Auditory agnosia has been recognized since 1877.
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| Auditory verbal agnosia || Also known as pure word deafness (PWD). This presents as a form of meaning "deafness" in which hearing is intact but there is significant difficulty recognising spoken words as semantically meaningful.
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| Autotopagnosia || Is associated with the inability to orient parts of the body, and is often caused by a lesion in the parietal part of the posterior thalmic radiations.
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| Cerebral achromatopsia || A difficulty in perceiving colors in which the world may appear drab or in shades of gray. Cerebral achromatopsia is caused by neurological damage. There are two regions of the brain which specialize for color recognition, areas V4 and V8. If there is a unilateral lesion to area V4, a loss of color perception in only half of the visual field may result known as hemiachromatopsia.
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| Cortical deafness || Refers to people who do not perceive any auditory information but whose hearing is intact.
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| Environmental agnosia || It is the inability to locate a specific room or building that one is familiar with, as well as the inability to provide directions for how to arrive at a particular location. These individuals experience difficulty with learning routes. This form of agnosia is often associated with lesions to the bilateral or right hemisphere posterior regions. It is also associated with prosopagnosia and Parkinson's disease.
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| 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.
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| Pure alexia || Inability to recognize text. Patients with pure alexia often have damage to their corpus callosum, as well as damage to the left visual association areas. However, individuals with pure alexia show a frequency effect. They are able to read high frequency words better and faster than they can read low frequency words.
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| 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).
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| 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.
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| Simultagnosia || The inability to process visual input as a whole. The person instead processes faces, bodies, objects, rooms, places, pictures in a bit-by-bit fashion. When looking at a picture they can describe the parts of the picture but struggle to comprehend the picture as a whole. Simultagnosia occurs in Bálint syndrome but may also occur in brain injury. This condition can also be described by only seeing one object at a time. An example is having two cups in your visual field and only being able to see one at a time.
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| Tactile agnosia || Impaired ability to recognize or identify objects by touch alone.
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| Topographical disorientation || Also known as topographical agnosia or topographagnosia, 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 recognize objects. Nevertheless, they may still have an excellent capacity to describe the visual layout of the same place. Patients with topographical agnosia have the ability to read maps, but become lost in familiar environments.
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| Visuospatial dysgnosia || This is a loss of the sense of "whereness" in the relation of oneself to one's environment and in the relation of objects to each other. It may include constructional apraxia, topographical disorientation, optic ataxia, ocular motor apraxia, dressing apraxia, and right-left confusion.
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| Visual agnosia || Is associated with lesions of the left occipital lobe and temporal lobes. Many types of visual agnosia involve the inability to recognize objects.
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Visual agnosia
Visual agnosia is a broad category that refers to a deficiency in the ability to recognize visual objects. Visual agnosia can be further subdivided into two different subtypes: apperceptive visual agnosia and associative visual agnosia.
Individuals with apperceptive visual agnosia display the ability to see contours and outlines when shown an object, but they experience difficulty if asked to categorize objects. Apperceptive visual agnosia is associated with damage to one hemisphere, specifically damage to the posterior sections of the right hemisphere.
Speech agnosia
Speech agnosia, or auditory verbal agnosia, refers to "an inability to comprehend spoken words despite intact hearing, speech production and reading ability". Patients report that they hear sounds being produced, but that the sounds are fundamentally unrecognizable or untranslatable.
- EXAMINER: What did you eat for breakfast?
- PATIENT: Breakfast, breakfast, it sounds familiar but it doesn't speak to me. (Obler & Gjerlow 1999:45)
Despite an inability to process what the speaker is saying, some patients have been reported to recognize certain characteristic information about the speaker's voice (such as being a man or woman).
The effect of damage to the superior temporal sulcus is consistent with several types of neurolinguistic deficiencies, and some contend that agnosia is one of them. The superior temporal sulcus is vital for speech comprehension because the region is highly involved with the lexical interface. According to the 1985 TRACE II Model, the lexical interface associates sound waves (phonemes) with morphological features to produce meaningful words. This association process is accomplished by lateral inhibition/excitement of certain words within an individual's lexicon (vocabulary).
History
The term agnosia comes from the Ancient Greek (), . It was introduced by Sigmund Freud in 1891: "For disturbances in the recognition of objects, which Finkelnburg classes as asymbolia, I should like to propose the term 'agnosia'." Prior to Freud's introduction of the term, some of the first ideas about agnosia came from Carl Wernicke, who created theories about receptive aphasia in 1874. He noted that individuals with receptive aphasia did not possess the ability to understand speech or repeat words. He believed that receptive aphasia was due to lesions of the posterior third of the left superior temporal gyrus. Due to these lesions, Wernicke believed that individuals with receptive aphasia had a limited deafness for certain sounds and frequencies in speech.
After Wernicke, came Kussmaul in 1877 who attempted to explain why auditory verbal agnosia, also known as word deafness, occurs. Contrary to Wernicke's explanations, Kussmaul believed auditory verbal agnosia was the result of major destruction to the first left temporal gyrus. Kussmaul also posited about the origins of alexia (acquired dyslexia) also known as word blindness. He believed that word blindness was the result of lesions to the left angular and supramarginal gyri. During the time of Wernicke, Kussmaul and Lissauer there was little known about the cerebral cortex. Today, new neuroimaging techniques have made it possible to expand our knowledge of agnosia greatly.