The 5 types of agnosia (visual, auditory, tactile, motor and bodily).

These strange phenomena prevent the person from recognizing stimuli even though his or her senses are functioning.

Brain lesions often cause cognitive deficits affecting a particular sensory modality, such as sight or touch.

When these problems are due to a lack of stimulus recognition, the diagnosis of "agnosia" is used. In this article we will describe the 5 types of agnosias: visual, auditory, tactile, bodily and motor..

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 language comprehension.

Agnosias generally appear as a consequence of brain lesions; they are very frequent in those resulting from ischemic strokes, neurological disorders such as dementia or traumatic brain injury, particularly when the regions where the occipital and temporal lobes meet are involved.

In these disorders the perception of stimuli is not altered per se, but the problem is localized.Rather, the problem is located at a higher stage of the perceptual process: it is associated with the recall (more specifically with the retrieval) of specific cues that allow us to relate the information we perceive with that stored in our memory.

The term "agnosia" comes from classical Greek and can be translated as "absence of knowledge" or "of can be translated as "absence of knowledge" or "absence 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 about similar concepts in previous decades.

Main types of agnosias

Most commonly, agnosias occur in a single sensory modality. It is therefore useful to divide the main types of agnosias and their corresponding subtypes according to whether they primarily affect according to whether they primarily affect the perception of visual information, the perception of sounds, touch, motor schemes or the perception of one's own body.

1. Visual

Most visual agnosias consist of deficits in the recognition of objects through sight. without any sensory organ involvement. 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 kind of agnosia is usually caused by lesions in the left occipital lobe or in any of the temporal lobes. We can divide visual agnosias into two categories: apperceptive, in which there are problems in categorizing objects, and associative agnosias.characterized by the inability to name them.

Prosopagnosia, which consists of a deficit in recognizing faces (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 apperceptive agnosia, it is worth mentioning symultagnosia, in which the elements are adequately processed but not the whole.

2. Auditory

In auditory agnosias, recognition problems occur in relation to sound stimuli. Although when the brain lesions causing the agnosia are very important, the deficits may be of a general nature, most commonly they affect only 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 difficulties in distinguishing speech sounds from the rest of the auditory stimuli he/she perceives, or in retrieving the meaning of sets of phonemes.

3. Tactile or somatosensory (astereognosias)

Tactile or somatosensory agnosia may be defined as a inability to identify objects by touch, based on aspects such as texture or size, even though there are no sensory alterations.based on aspects such as texture or size, despite the absence of sensory disturbances. Another name for this type of disorder is "astereognosia".

A clinically significant subtype is digital agnosia, which specifically affects finger recognition. It usually occurs together with agraphia, acalculia and left-right disorientation in the context of Gerstmann's syndrome, caused by lesions in the lower part of the parietal lobe.

4. Motor (apraxias)

The term "motor apraxia" is used very infrequently due to the great popularity of one of its synonyms: "apraxia". Apraxias consist of difficulties in remembering and executing intentionally learned motor schemes. learned intentionally; 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, ideomotor apraxia, characterized by the inability to perform actions in response to a verbal request, and constructional apraxia, which consists of problems in constructing shapes or drawing figures.

5. Bodily

In bodily agnosias there is an inability to identify one's own body. 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 its halves and of autotopagnosia in cases where the person is able to spatially locate the parts of his body.