Source: http://www.stir.ac.uk/postgrads/psychology/pgm1/Net_Presentation/sld017.htm
The earliest researchers of agnosia theorized that visual agnosia was the result of bad eyes sight or problems in memory (Vecera & Gilds 1998). How could a person who could see not be able to identify something that they had seen before? Either they could not see correctly or they had forgotten what they had seen before. This theory is often termed the sensory-deficit account. As supporting evidence, researchers Bender and Feldman (1998) examined all hospital records of people diagnosed with visual people with agnosia within a twenty-year period. They discovered that every case had some evidence of either low-level visual processing problems (simple vision problems) or mental dysfunction (memory problems) (Vecera & Gilds1998). However, this early theory was problematic because it did not identify a specific cognitive mechanism that could have been damaged and thereby causing the memory problems. The theory also failed to explain the different patterns of behavior displayed by different patients (Farah et al. 1991). There are two main patterns of behavior exhibited by agnosia patients.
There are two stages of visual agnosia:
· Appreceptive agnosia
· Associative agnosia
Another more recent explanation is the "peppery mask" account. Some researchers suggested that patients with visual agnosia experience "visual noise" or dimmed or blocked out areas of their vision. The scientists thought that air bubbles or blood clots in the blood vessels behind the eye could be blocking complete vision (Farah et al. 1991). As a result, random areas of dimmed vision varying in size and dimness would be scattered throughout a patients visual field. Therefore, a person with visual agnosia sees the world through a "peppered mask" that degrades their visual processing (Vecera & Gilds 1998). However, while the peppery mask account explains the degraded low-level visual processing, the theory offers no explanation for the various grouping disorders that tend to appear in visual people with agnosia. The serious difficulty encountered in the peppery mask account is that the presence of detrimental random visual noise peppering the visual field would seem to increase reliance upon perception of the whole picture despite the broken pieces. However, visual people with agnosia seem to perceive the parts without being able to identify the whole (Sajda & Finkle 1995).
In an experiment by Vecera & Gilds (1998), researchers covered different displays trying to mimic the "peppery mask" condition for people with normal vision. However, people looking at these displays did not react like a person with agnosia would act (Vecera & Gilds 1998).
Later, researchers conducted another experiment to explore what they called "the grouping-deficit account". Basically they removed many of the main cues that help with identification, such as making two parallel lines not look parallel. They also took points where two lines meet and both of them end and made it difficult to tell what actually happens there. Volunteers who looked at these displays acted similar to people with agnosia, in that they had a hard time identifying objects (Vecera & Gilds, 1998).
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