Brain damage often causes cognitive deficits that affect a specific sensory modality, such as sight or touch.
When these problems are due to a lack of recognition of the stimulus, the diagnosis of “agnosia” is used. In this article we will describe the 5 types of agnosia: visual, auditory, tactile, bodily and motor.
What are agnosias?
Agnosia is defined as inability to process certain types of sensory stimulation. Depending on the cognitive systems involved, 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 usually appear as a result of brain injury; they are very common in those resulting from ischemic strokes, neurological disorders such as dementia or cranioencephalic trauma, particularly where the junction regions of the occipital and temporal lobes are involved.
In these troubles perception of stimuli is not altered per seBut the problem is at a higher stage of the perceptual process: it is associated with the memory (more precisely with the retrieval) of specific keys that allow us to relate the information we perceive with the information we have stored in memory.
The term “agnosia” comes from the classical Greek i can be translated as “lack of knowledge” or “recognition”. It was invented by Sigmund Freud, the father of psychoanalysis, in 1891, but pioneers of neuropsychology such as Carl Wernicke and Heinrich Lissauer had theorized similar concepts in previous decades.
Main types of agnosia
The most common is that agnosias occur in a single sensory modality. This is why it is useful to divide the main classes of agnosia and their corresponding subtypes depending on whether they fundamentally affect the perception of visual information, that of sounds, touch, motor patterns or the perception of our own body.
Most visual agnosias consist of deficits in the recognition of objects by sight without affecting the sensory organs. Perhaps the most famous case of visual agnosia is the one that gave the title to “The Man Who Mistaken His Wife for a Hat”, the best-selling book by neurologist Oliver Sacks.
This class of agnosia usually occurs due to damage to the left occipital lobe or one of the temporal lobes. We can divide visual agnosias into two categories: aperceptivas, in which problems arise in categorizing objects, and associatives, Characterized by the inability to name them.
Prosopagnosia, which is a lack of facial recognition (but also associated with problems with naming animals or car brands, for example), is the most well-known type of associative visual agnosia. As far as the aperceptivas it is necessary to emphasize the simultagnosia, in which the elements are treated correctly but not thus the whole.
2. Hearing aids
In auditory agnosias, recognition problems arise in relation to sound stimuli. Although when the brain damage causing agnosia is very severe, the deficits can be general in nature, the most common is that they only affect a type of auditory stimulation, such as receptive language or music (fun).
Verbal auditory agnosia, also known as “pure word deafness”, is one of the most relevant agnosias in this category. In these cases, the affected person has difficulty distinguishing the sounds of speech from other auditory stimuli that they perceive, or finding the meaning of sets of phonemes.
3. Tactile or somatosensory (asterognosias)
Tactile or somatosensory agnosia can be defined as a inability to identify objects by touch, Based on aspects such as its texture or size, although there is no sensory alteration. Another name for this type of disorder is “asterognosia”.
A clinically significant subtype is digital agnosia, which specifically affects finger recognition. It usually occurs with stapling, acalculia, and left-right disorientation against the background of Gerstmann syndrome, caused by lesions in the lower parietal lobe.
4. Motors (apraxia)
The term “motor apraxia” is very rarely used due to the great popularity of one of its synonyms: “apraxia”. Apraxias consist of difficulty remembering and executing motor patterns learned intentionally; however, the same movements can appear spontaneously in their natural contexts.
Three main types of motor agnosia have been described: ideational apraxia, in which it is difficult to achieve sequences of movements, ideomotor, characterized by the inability to perform actions in response to a verbal request, and constructive, which consists of construction problems. shapes or drawing of figures.
In bodily agnosias, we are given inability to identify one’s own body. We speak of somatognosia when the problems are related to the whole body, hemiasomatognosia if they only appear in one of its halves and autotopagnosia in cases where the person is able to spatially locate parts of his body.