Бір мезгілде көруге қабілетсіздік: Симультанагнозия ауруы

Simultanagnosia (or simultagnosia) is a rare neurological disorder characterized by the inability of an individual to perceive more than a single object at a time. This type of visual attention problem is one of three major components (the others being optic ataxia and optic apraxia) of Bálint's syndrome, an uncommon and incompletely understood variety of severe neuropsychological impairments involving space representation (visuospatial processing). The term "simultanagnosia" was first coined in 1924 by Wolpert to describe a condition where the affected individual could see individual details of a complex scene but failed to grasp the overall meaning of the image. Simultanagnosia can be divided into two different categories: dorsal and ventral. Ventral occipito temporal lesions cause a mild form of the disorder, while dorsal occipito parietal lesions cause a more severe form of the disorder.

Simultanagnosia can be divided into two different types: dorsal and ventral, with each taking its name from the dorsal and ventral circuits concerned with the perception of objects' shapes and locations, respectively. These two forms of simultanagnosia are associated with different symptoms as well as damage to separate areas of the brain.

Dorsal simultanagnosia results from bilateral lesions to the junction between the parietal and occipital lobes. It is also possible for simultanagnosic symptoms to develop from degenerative disorders. For example, one study found that four patients with progressive dementia eventually developed symptoms of simultanagnosia as well as components of Gerstmann's syndrome and transcortical sensory aphasia. In addition, patients with Huntington's disease have been found to exhibit visual impairments similar to those of simultanagnosia.

It is likely that damage to any of several cognitive mechanisms could result in simultanagnosia. Furthermore, if the second word was shown after a long delay following the first word, identification of the second word was easier. These results indicate that patients with simultanagnosia have difficulty processing objects presented in rapid succession, and the patient is unable to shift his attention rapidly enough between successive stimuli since a certain amount of time is required for the patient to shift his attention from the first word in order to be able to identify the second word.

It is also possible that impairments in mechanisms that register spatial locations lead to simultanagnosia. In order to identify objects, focused attention is required to bind perceptual representations of objects being viewed with features in their proper locations. Parietal lesions damage the master map of locations, and as a result, a variety of deficits can occur, including simultanagnosia. If space is necessary to distinguish objects, then deficits in explicit access to spatial information located in the master map leads to the inability to perceive more than one object at a time. One study developed a computer model of high level visual processing, which contrasts with low level visual processing in that it involves the use of previously stored information to identify objects and navigate. When the spatiotopic mapping subsystem of the model was partially damaged, simultanagnosic symptoms resulted. In the model simulation of simultanagnosia, the same location was assigned to all stimuli, therefore preventing the model from identifying multiple objects at once. Either the model "locked" onto the first object and was unable to disengage attention, or once recognition of the first object was completed, it "disappeared" from sight to be replaced by the second object. Coslett and Saffran studied one patient who was unable to maintain location information for more than one shape. Since only a single explicit binding could occur between spatial and shape information, the patient was incapable of perceiving more than one object at a time. The results suggest that impairments in parsing, such as the process by which important regions are extracted from the retinal image, or difficulty in discriminating elementary visual features led to simultanagnosia.

Finally, simultanagnosia may result from deficits in spatial indexing. Once these features have been extracted, they can be indexed, which allows them to function as anchor points for additional visual routines; visual routines are sequences of elemental operations, such as visual search or texture segregation, which define the spatial relationships among objects as well as their properties. Saliency of a feature facilitates the ease with which it can be indexed. In another study a participant showed an improvement 18 months after stroke induced ventral simultanagnosia, this "represents the usual partial recovery from an early ventral simultanagnosia/pure alexia".