Apperceptive Agnosia - Theories of Causation

Theories of Causation

No two apperceptive agnosic patients are the same, but case studies have been used to make theories on what causes the object recognition deficits. While it is established that semantics plays a large role in apperceptive agnosia deficits, it is not agreed upon how semantics alter recognition processes.One theory proposes that semantic memories are divided into differential semantic categories. Brain damage leads to apperceptive agnosia because there is damage to a particular semantic category. Another theory proposes that there are no different categories but there are different areas of the brain that stores different types of features. Living things are thought to be identified by their perceptual features and man-made objects are identified by their functional features. In this theory, damage to one particular area might cause a loss of ability to differentiate living things or non-living things depending on the area of damage. Yet another theory suggests that the pattern of deficit arise from independent impairments to a particular input modality and a single non perceptual semantic system that is organized by category. Deficits are largely due to semantics, however many categories are related perceptually as well. Objects that are biologically similar are likely to have physical resemblance to each other as well. Perceptual confusion arises because of structural similarity contributes or accounts for some modality specific deficit.

Object processing is said to occur by two processes. There is first a stage of object perception. In this step there is mapping of visual description from the stimulus to a set of stored structural descriptions onto a set of structural descriptions of familiar objects. In the second stage, there is object identification. In this step the structural description is mapped onto the semantic representations giving rise to a full specification of the object. Researchers differ in their believe of how perceptual knowledge has an effect. Some believe that the loss of perceptual attributes should always accompany structural similarity. Others observe that perceptual and structural information often accompany each other but they believe that the information can occur independently from each other. Based on patient information it seems that objects belonging to a category with many structurally similar neighbours would be vulnerable to this semantic access impairment.