Agnosia

Definition of Agnosia

In the absence of memory deficits and lesions affecting the sensory systems, agnosia presents itself as a disturbance of perception of a sensory discriminatory nature, of a tactile, visual and / or acoustic type. In simple terms, we speak of agnosia when the subject is unable to recognize and identify a given object, perfume, shape, person or entity, despite maintaining its perceptive capacity unaltered.
The term agnosia derives from the Greek a-gnosis, which literally means do not know.

 

Sensory system:
it has the function of receiving information from the receptors, which is then transmitted to the CNS. The sensory system preserves the functions of controlling movement, perception, maintaining the alert status and regulating the correct functionality of internal organs

Causes

In the vast majority of cases, agnosia is directly related to brain injury: depending on the severity of the trauma, agnosia will be more or less severe. It is no coincidence, in fact, that we speak of lesional origin of agnosias: on the basis of the type of sensitivity affected, agnosia is connoted with different names.

Classification

As mentioned above, there is no "single form of agnosia, since the disease can attack one sense or all of them. A first general classification distinguishes the perceptive from the associative forms:

1. Apperceptive agnosia: only the patient's perceptual functionality is compromised. That particular object is badly recognized precisely because of a deficit regarding the processing of perceptual data, as a function of a given mode (visual, tactile, acoustic). In the apperceptive form, the elementary functions (such as, for example, the recognition of the color and size of an object) remain unchanged, to the detriment of the ability to copy an image on a sheet, to draw a movement with the fingers and to distinguish an object There are three forms of apperceptive agnosia, classified according to sense (or mode) interested.
   In the transformational form, the agnostic patient is able to recognize a given object only when it is located in a particular context: if the object is upside down, smaller, larger or prospectively different, he will not recognize it.
   Agnosia by form is an agnostic typology in which the patient is able to analyze the individual characteristics of the object, but is unable to trace its ideal conformation: in the practical level, the agnostic patient by shape it is not able to associate objects with the same shape, nor to reproduce the object under examination on a drawing.
   Finally, integrative agnosia denies the patient's ability to integrate the different parts of an object: he is indeed able to describe the individual parts that make up a given object, animal or person, but fails to integrate them (very similar all "agnosia for form).

2. Associative agnosia: the patient is not able to associate a meaning to a given object, therefore this is not recognized on the basis of a precise mode. In the associative forms of agnosia, there is a comparison between the perceived object and the knowledge accumulated by the subject in semantic memory, during the course of life: in such situations, the patient is unable to recognize the object, to remember its name, nor its correct use. The perceptual analysis remains unchanged: to translate the theory into practice, an example is given: the subject perceives a glass, recognizes it, but is unable to remember its name, use, method of use. When instead the patient is asked verbally what a glass is and how to use it, he answers correctly (confirms that the perceptive abilities remain such). The "associative agnosia, therefore, refers only to the visual sphere: only the impossibility occurs. to access the memory following a stimulation visual.

This classification was described for the first time around the end of the nineteenth century by a well-known German neurologist of the time and is still considered a reference model.

Visual agnosia

The visual agnosias involve the impossibility of recognizing some objects, although the visual abilities are not damaged or impaired. Generally, the disturbance becomes even more marked when the affected subject is in a poorly lit place. It should be pointed out that a visual agnostic is not blind: in fact, this category of patients is often mistakenly mistaken for blind.
Despite being able to draw an object under visual stimulation, the patient subsequently fails to recognize his own drawing.
Among the visual agnosies, we remember:

• Prosopoagnosia: typical perceptual agnostic disorder, in which patients cannot identify people's faces. In the case of severity, prosopoagnosia prevents the affected subject from recognizing himself in the mirror.
• Topographical agnosia: inability to recognize or remember typical familiar or domestic places. Often times, topographical agnosia is associated with prosopoagnostic one.
• Agnosia for colors: inability to recognize colors (not to be confused with color blindness).

Auditory or acoustic agnosias

Patients with auditory agnosia are denied the possibility of recognizing sounds and noises of varying intensity; there is both the apperceptive and the associative form. The patient is unable to sing a known melody, nor to recognize the person's voice.
They are agnostic types that are almost rare and less disabling than visual agnosia.

Tactile agnosia

The subject is unable to recognize an object through touch: this means that by touching an object with his eyes closed, the tactile agnosic is unable to imagine what it is. Tactile agnosia is classified, in turn, in:

• Amorphognosy: the patient does not recognize the shape and size of the object he is holding
• Asymboliatactile: inability / difficulty to name that object through touch (tactile agnosia proper)
• Ailognosia: with touch, the patient does not understand the material with which that object is made, neither the weight nor the temperature

Visuospatial agnosia

The patient shows evident problems in "perfectly processing spatial information: the simple operations of writing, drawing and sometimes even reading, are very problematic for these patients.

Digital agnosia

Digital agnosia patients are unable to identify, distinguish or simply name their hands. Digital agnosia can sometimes involve other body sites.

One-sided spatial neglect

Rather frequent form of agnosia, in which the subject is unable to identify a part of the space that surrounds him, following lesions in some areas of the brain (damage to the optic pathways).

Other articles on "Agnosia"

1. Agnosia: diagnosis and treatment
2. Agnosia in Brief: Summary of Agnosia