Explanation of visual, aditory and tactile agnosia. Theories, models, symptoms, causes and terms all discussed.

What is agnosia? What are the main aspects of visual (apperceptive & associative), auditory and tactile agnosias? What models have been developed to explain agnosias and what explanations are there for prosopagnosia?

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...tergrated agnosia', this disorder was exhibited by their patient HJA.

CASE STUDY
HJA was a businessman who had suffered a bilateral stroke. In one instance HJA took six hours to copy a simple drawing, after which he was unable to name the picture he had drawn. He was unable to bring together elements of a stimulus into a meaningful unit.

Humphreys & Riddoch suggested that one of two processes necessary for visual processing is defective. They have argued that visual perception involes (1) the establishment of the global form of the stimulus and (2) a process whereby individual features of a stimulus are percieved and 'bound' together.

HJA's problem appears to be an inability to appreciate 'intergrative local grouping cues'.

ASSOCIATIVE VISUAL AGNOSIA
Patients with this disorder are characterised by poor object recognition. There are degrees of severity associated with this disorder. Often the identification of line drawings are more difficult to identify than real life objects.

This disorder is associated with left hemisphere lesions (Alexander & Albert, 1983).

This type of agnosia is rare and is often seen in combination with other agnosias, such as the inability to identfy faces and colours.

Colour agnosia is the inability to name colours despite the ability to discriminate between them. Sittig (1921) found that patients were unable to colour pictures appropriately. Three colour agnosia syndromes have been identified:

1). Central achromatopsia (dyschromatopsia)
2). Colour anomia
3). Specific colour aphasia

THEORIES OF VISUAL AGNOSIA
There are 4 main models developed to explain visual agnosia. These were developed by Bauer (1993).

1. STAGE MODELS - Lissauer (1880) proposed that recognition comprises of two stages of processing, 'apperception' and 'association'.

2. DISCONNECTION MODELS - Geschwind (1965) argued that agnosia results from a disconnection between two processes, for example visual and verbal processes.

3. COMPUTATIONAL MODELS - Marr (1982) suggests that there are three types of representation in the perceptual world, thesea are the 'primary sketch', '2 1/2 D' sketch and the 'object centred 3-D sketch'. Thses different experiences need to be combined together to construct the 'whole'.

4. COGNITIVE NEUROPSYCHOLOGICAL MODEL - These models are slightly different and view cognitive and emotional operations as dissociable. The disorder is regarded in terms of 'components' and 'moduels'.

There has been much controversy surrounding visual agnosia research, two main problems have been identified:

1). Are apperceptive and associative agnosias ...