Brain injuries often cause cognitive deficits that affect a particular sensory modality, such as sight or touch.
When these problems are due to lack of stimulus recognition, the diagnosis of “agnosia” is used. In this article we will describe the 5 types of agnosia: visual, auditory, tactile, body and motor .
What are agnosias?
Agnosia is defined as the inability to process certain types 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.
Agnosias generally appear as a consequence of brain injuries; they are very frequent in those resulting from ischemic strokes, neurological disorders such as dementias or head injuries, particularly when the regions where the occipital and temporal lobes meet are involved.
In these disorders the perception of stimuli is not altered in itself , but the problem is located in a higher stage of the perceptive process: it is associated with the memory (more specifically, with the recovery) of specific keys that allow us to relate the information we perceive with the information we have stored in the memory.
The term “agnosia” comes from the classical Greek and 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 previous decades.
Main types of agnosias
Most commonly, agnosias occur in only one sensory mode. It is therefore useful to divide the main classes of agnosias and their corresponding subtypes according to whether they fundamentally affect the perception of visual information, sound information, touch, motor patterns or the perception of our own body.
Most 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 mistook his wife for a hat”, the best-seller by neurologist Oliver Sacks.
This type of agnosia is usually caused by injury to the left occipital lobe or any of the temporal lobes. We can divide visual agnosias into two categories: the aperceptive ones, in which there are problems in categorizing objects, and the associative ones , characterized by the inability to name them.
Prosopagnosia, which consists of a deficit in face recognition (but is also associated with problems in naming animals or car brands, for example), is the best known type of associative visual agnosia. As for aperceptive agnosia, it is worth mentioning simultagnosia, in which the elements are properly processed but the whole is not.
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 be of a general nature, the most common is that they only affect one type of auditory stimulation, such as receptive language or music (amusia) .
Auditory-verbal agnosia, also known as “pure word deafness”, is one of the most relevant agnosias within this category. In these cases the affected person has difficulty distinguishing speech sounds from other auditory stimuli that he or she perceives, or in recovering the meaning of phoneme sets.
3. Tactile or somatosensory (asthereognoses)
Tactile or somatosensory agnosia can be defined as an inability to identify objects through touch , based on aspects such as texture or size, despite the fact that no sensory alterations are present. Another name for this type of disorder is “asthereognosia”.
A clinically significant subtype is digital agnosia, which specifically affects recognition through the fingers. It usually occurs in conjunction with agraphia, acalculia and left-right disorientation in the context of Gerstmann syndrome, caused by lesions in the lower parietal lobe.
4. Motor (apraxia)
The term “motor apraxia” is used very rarely due to the great popularity of one of its synonyms: “apraxia”. Apraxia consists of difficulties in remembering and executing motor patterns learned intentionally; however, the same movements may appear spontaneously in their natural contexts.
Three main types of motor agnosia have been described: the ideational apraxia, in which there are difficulties in carrying out sequences of movements; the ideomotor apraxia, characterized by the inability to execute actions in response to a verbal request; and the constructive apraxia, which consists of problems in constructing forms or drawing figures.
In body agnosias, there is an inability to identify one’s own body . We speak of somatognosy when the problems are related to the whole body, of hemiasomatognosy if they appear only in one of its halves and of autotopagnosy in cases where the person is able to spatially locate the parts of his body.