The 5 types of agnosia (visual, auditory, tactile, motor and corporal)
Brain injuries often cause cognitive deficits that affect a specific sensory modality, such as sight or touch.
When these problems are due to the lack of recognition of stimuli, the diagnosis of "agnosia" is used. In this article we will describe the 5 types of agnosias: visual, auditory, tactile, corporal and motor .
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What are agnosias?
Agnosia is defined as the inability to process certain kinds of sensory stimulation . Depending on the cognitive systems that are affected, deficits may appear in different functions, for example in the recognition of visual information, in the execution of sensorimotor patterns or in the understanding of language.
Generally agnosias appear as a consequence of brain injuries; they are very frequent in those that result from ischemic strokes, neurological disorders such as dementia or traumatic brain injuries, particularly when the regions where the occipital and temporal lobes join are involved.
In these disorders the perception of stimuli is not altered in itself , but the problem is located at a higher stage of the perceptual process: it is associated with memory (more specifically, recovery) of specific keys that allow us to relate the information we perceive with that which we have stored in memory.
The term "agnosia" comes from the classical Greek and it can be translated as "absence of knowledge" or "of recognition" . It was coined by Sigmund Freud, the father of psychoanalysis, in 1891, but pioneers of neuropsychology such as Carl Wernicke or Heinrich Lissauer had theorized around similar concepts in the previous decades.
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Main types of agnosias
The most common is that agnosias are given in a single sensory modality. It is therefore useful to divide the main classes of agnosias and their corresponding subtypes depending on whether they fundamentally affect the perception of visual information, the perception of sounds, touch, motor patterns or the perception of our own body.
The majority of visual agnosias consist of deficits in the recognition of objects through sight without affecting the sensory organs . The most famous case of visual agnosia is probably the one that gave the title to "The man who confused his wife with a hat", the best-seller of neurologist Oliver Sacks.
This kind of agnosias usually occur because of injuries in the left occipital lobe or in any of the temporal lobes. We can divide the visual agnosias into two categories: the apperceptives, in which there are problems to categorize objects, and associative ones , characterized by the inability to name them.
Prosopagnosia, which consists of a deficit in the recognition of faces (but is also associated with problems to name animals or car marks, for example), is the best known type of associative visual agnosia. Regarding the apperceptives, it is worth highlighting the simultagnosia, in which the elements are processed properly but not the whole.
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In auditory agnosias, recognition problems occur in relation to sound stimuli. Although when the brain lesions that cause agnosia are very important, the deficits may have a general character, the most usual is that they affect only a type of auditory stimulation, such as receptive language or music (amusia) .
Auditory verbal agnosia, also known as "pure deafness for words," is one of the most relevant agnosias within this category. In these cases, the affected person has difficulty distinguishing the speech sounds from the rest of the auditory stimuli that he perceives, or to recover the meaning of the phoneme sets.
3. Touch or somatosensory (astereognosia)
Tactile or somatosensory agnosia can be defined as a inability to identify objects through touch , based on aspects such as its texture or size, although there are no alterations of sensory type. Another name for this type of disorder is "astereognosia".
A significant subtype from a clinical point of view is digital agnosia, which specifically affects recognition through the fingers.It usually occurs together with agraphia, acalculia and left-right disorientation in the context of Gerstmann syndrome, caused by lesions in the lower part of the parietal lobe.
4. Motorboats (apraxias)
The term "motor apraxia" is used very infrequently due to the great popularity of one of its synonyms: "apraxia". Apraxias consist of difficulties to remember and execute motor schemes intentionally learned; however, the same movements may appear spontaneously in their natural contexts.
Three main types of motor agnosia have been described: ideational apraxia, in which there are difficulties in carrying out sequences of movements, the ideomotor, characterized by the inability to execute actions in response to a verbal request, and the constructive, consisting of in trouble to build shapes or draw figures.
In body agnosias there is a inability to identify one's own body . We speak of somatognosia when the problems are related to the whole body, of hemiasomatognosia if they appear only in one of their halves and of autotopagnosia in the cases in which the person is able to spatially locate the parts of his body.