Definition of Agnosia

In the absence of memory deficits and sensory systems lesions, agnosia is a tactile, visual and / or acoustic perception of a sensory discriminatory nature. In simple words, we speak of agnosia when the subject is unable to recognize and identify a given object, perfume, form, person or entity, despite maintaining its perceptive capacity unchanged.

The term agnosia derives from the Greek a-gnosis, which literally means not knowing .

Sensory system:

has the function of receiving information from receptors, then transmitted to the CNS. The sensory system preserves the functions of movement control, perception, maintenance of alert status and regulation of 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. Not surprisingly, in fact, we talk about the lesional origin of agnosias: on the basis of the type of sensitiveness affected, agnosia is characterized by different names.

Classification

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

  1. Agnosia appercettiva : only the perceptual functionality of the patient is compromised. That particular object is badly recognized precisely because of a deficit concerning the processing of perceptual data, according to a given modality (visual, tactile, acoustic). In the apperceptive form, the elementary functions (such as for example the recognition of the color and the size of an object) remain unaltered, to the detriment of the ability to copy an image onto a sheet, to draw a movement with the fingers and to distinguish an object from other similar ones. There are three forms of apperceptive agnosia, classified according to the sense (or mode ) involved.

    In the transformational form, the agnostic patient is able to recognize a given object only when located in a particular context: if the object is upside down, smaller, larger or prospectively different, he will not recognize it.

    The agnosia by form is an agnostic typology in which the patient is able to analyze the individual characteristics of the object, but is not able to trace its ideal conformation: in practical terms, the agnostic patient by form is not able to associate objects of the same shape, nor to report the object in question on a drawing.

    Finally, integrative agnosia denies the patient's ability to integrate the different parts of an object: he is able to describe the individual parts that make up a given object, animal or person, but fails to integrate them (very similar to agnosia by form).

  1. 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 modality . 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 not able to recognize the object, to remember the name, nor its correct use. The perceptive analysis remains unchanged: to translate the theory into practice, an example is reported: the subject perceives a glass, recognizes it, but is not able to remember its name, use, method of use. When, on the other hand, the patient is verbally asked what a glass is and how it is used, he responds correctly (confirmation of the fact that perceptive abilities remain such). Associative agnosia, therefore, refers only to the visual sphere: only the impossibility of accessing the memory following a visual stimulation occurs.

This classification was first described 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

Visual agnosities make it impossible to recognize certain objects, although the visual abilities are not harmed or damaged. Generally, the disorder becomes even more marked when the affected subject is in a poorly lit place. It is right to point out that a visual agnostic is not blind: often, in fact, this category of patients is mistakenly mistaken for blind.

Despite being able to draw an object under visual stimulation, the patient subsequently fails to recognize his own design.

Among the visual agnoses, we recall:

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

Auditory or acoustic agnosias

Those suffering from auditory agnosia are denied the possibility of recognizing sounds and noises of more or less strong intensity; both the apperceptive and the associative forms exist. The patient is not able to sing a known melody, nor to recognize the person's voice.

They are almost rare and less disabling types of agnostics than visual agnosia.

Tactile agnosia

The subject is unable to recognize an object through touch: this means that by touching an object with closed eyes, the tactile agnosic cannot imagine what it is. The tactile agnosia is classified, in turn, in:

  • Amorfognosia : the patient does not recognize the shape and size of the object in his hand
  • Asimboliatattile : incapacity / difficulty of naming 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 obvious problems in processing spatial information perfectly: the simple operations of writing, drawing and sometimes even reading, are very problematic for these patients.

Digital agnosia

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

Unilateral spatial neglect

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