Ассоциативная зрительная агнозия: нарушение узнавания объектов при сохранном восприятии.

Associative visual agnosia is a form of visual agnosia. It is an impairment in recognition or assigning meaning to a stimulus that is accurately perceived and not associated with a generalized deficit in intelligence, memory, language or attention. The disorder appears to be very uncommon in a "pure" or uncomplicated form and is usually accompanied by other complex neuropsychological problems due to the nature of the etiology. Associative visual agnosia refers to a subtype of visual agnosia, which was labeled by Lissauer (1890), as an inability to connect the visual percept (mental representation of something being perceived through the senses) with its related semantic information stored in memory, such as, its name, use, and description. This is distinguished from the visual apperceptive form of visual agnosia, apperceptive visual agnosia, which is an inability to produce a complete percept, and is associated with a failure in higher order perceptual processing where feature integration is impaired, though individual features can be distinguished. In reality, patients often fall between both distinctions, with some degree of perceptual disturbances exhibited in most cases, and in some cases, patients may be labeled as integrative agnostics when they fit the criteria for both forms. caused by ischemic stroke, head injury, cardiac arrest, brain tumour, brain hemorrhage, or demyelination. Most cases have injury to the occipital and temporal lobes and the critical site of injury appears to be in the left occipito temporal region, often with involvement of the splenium of the corpus callosum. The etiology of the cognitive impairment, as well the areas of the brain affected by lesions and stage of recovery are the primary determinants of the pattern of deficit. Goldberg suggested that the associative visual form of agnosia results from damage to the ventral stream of the brain, the occipito temporal stream, which plays a key role in object recognition as the so called "what" region of the brain, as opposed to the "where," dorsal stream. described the associative agnostic as having a "percept stripped of its meaning," because the affected individual cannot generate unique semantic information to identify the percept, since though it is fully formed, it fails to activate the semantic memory associated with the stimulus. offered that the problem lies in impaired access to generic engrams (memory traces) that describe categories of objects made up of a multitude of similar elements. The fact that agnosias are often restricted to impairments of particular types of stimuli, within distinct sensory modalities, suggests that there are separate modality specific pathways for the meaningful representation of objects and pictures, written material, familiar faces, and colors.

Visual object agnosia (or semantic agnosia) is the most commonly encountered form of agnosia. The clinical "definition" of the disorder is when an affected person is able to copy/draw things that they cannot recognize. Individuals often cannot identify, describe or mimic functions of items, though perception is intact, since images of objects can be copied or drawn. Individuals may retain semantic knowledge of the items, as exemplified during tasks where objects are presented through alternate modalities, through touch or verbal naming or description. Some associative visual object agnostics retain the ability to categorize items by context or general category, though unable to name or describe them. Diffuse hypoxic damage is the most common cause of visual object agnosias.

Category specific agnosias are differential impairments in subject knowledge or recognition abilities pertaining to specific classes of stimuli, such as living things vs. non living things, animate vs. inanimate things, food, metals, musical instruments, etc. Some of the most common category specific agnosias involve recognition impairments for living things, but not non living things, or human faces, as in prosopagnosia. This type of deficit is typically associated with head injury or stroke, though other medical conditions have been implicated, such as, herpes encephalitis.

Prosopagnosia (or "face blindness") is a category specific visual object agnosia, specifically, impairment in visual recognition of familiar faces, such as close friends, family, husbands, wives, and sometimes even their own faces. Individuals are often able to identify others through alternate characteristics, such as, voice, gait, context or unique facial features. This deficit is typically assessed through picture identification tasks of famous persons. This condition is associated with damage to the medial occipito temporal gyri, including the fusiform and lingual gryi, as the suggested location of the brain's face recognition units.

Two subtypes are distinguished behaviorally as being associative or apperceptive in nature. Associative prosopagnosia is characterized by an impairment in recognition of a familiar face as familiar; however, individuals retain the ability to distinguish between faces based on general features, such as, age, gender and emotional expression. This subtype is distinguished through facial matching tasks or feature identification tasks of unknown faces.

Pure alexia ("alexia without dysgraphia" or "pure word blindness") is a category specific agnosia, characterized by a distinct impairment in reading words, despite intact comprehension for verbally presented words, demonstrating retained semantic knowledge of words. Perceptual abilities are also intact, as assessed by word copying tasks.

thumb|Color agnostics fail to identify abnormally colored objects or pictures|alt=Two adjacent photos of bananas. The left is in sunlight; the right is under ultraviolet light. Cerebral achromatopsia, also known as Color agnosia, is a category specific semantic impairment pertaining to semantic color associations, such that individuals retain perceptual abilities for distinguishing color, demonstrated through color categorization or hue perception tasks; however knowledge of typical color object relationships is defective. Color agnostics are assessed on performance coloring in black and white images of common items or identifying abnormally colored objects within a set of images. For example, a color agnostic may not identify a blue banana as being improperly colored.

This deficit should be distinguished from color anomia, where semantic information about color is retained, but the name of a color cannot be retrieved, though co occurrence is common. Both disorders linked to damage in the occipito temporal cortex, especially in the left hemisphere, which is believed to play a significant role in color memory.

A recognition disorder is not considered to be agnosia unless there is a lack of aphasia, dementia, or other generalized defect that affects any stage of the object recognition process, such as, deficiencies in intelligence, linguistic ability, memory, attention or sensory perception. Therefore, individuals must be assessed for language ability, auditory comprehension, fluency, repetition, praxis, and reading and writing. thumb|A Flowchart for Clinical Assessment of Forms of Visual Agnosia|alt=Flowchart for Assessment of Visual Agnosias.

Sensory modality testing allows practitioners to assess for generalized versus specific deficits, distinguishing visual agnosias from optic aphasia, which is a more generalized deficit in semantic knowledge for objects that spans multiple sensory modalities, indicating an impairment in the semantic representations themselves.

The distinction between visual agnosias can be assessed based on the individual's ability to copy simple line drawings, figure contour tracking, and figure matching. Apperceptive visual agnostics fail at these tasks, while associative visual agnostics are able to perform normally, though their copying of images or words is often slavish, lacking originality or personal interpretation.