VISUAL AGNOSIAS (using them to explain perception)

Research into agnosia is used to provide us infomation with how the normal perceptive recognition process works.

Apperceptiive Agnosia:

Apperceptive agnosia is a failure to recognise objects due to an inability to percieve its form. Apperceptive agnosia is a specific problem with the perceptual system. An example may be failure to distinguish between a cricle and a square.

Case study of Apperceptive agnosia: 

Greenberg 1969: Case study of a man who could not recognise faces, match letters or objects,however could identify colours.

Associative Agnosia:

Associative agnosia is different to apperceptive agnosia as it is not a direct problem with your perception, but failure to recognise objects due to an innability to retrieve relative infomation from memory. An example of this is being able to copy objects by drawing them but being unable to name them. 

Case studies of Associative Agnosia: 

HJA- could copy and match objects but was unable to name them. Another interesting case was JBR- JBR had 80% accuracy when recognising non living things vs 6% accuracy when recognising living things.

Prosopagnosia:

Prospagnosia is an inability to recognise faces despite the ability to recognise most other stimuli. It is thought to be a failure of the perceptual and associative systems to come together to produce recognition. 

Case studies of prospagnosia: 

VA could recognise objects but could not name famous faces. 

Mr. W was a prosopagnosic farmer who could recognise indivual cows but could not recognise human faces.

Prosopagnosia case studies can be used in support of bruce and youngs theory of face recognition, as it suggests a seperate system for recognising human faces to objects is used.

1. Specificty 

Some case studies on agnosias do suggest a difference between living and non living recognition, suggesting different modules in the brain deal with the different infomation. However this could be due to the specifcity of the task. Identifying an object such as a mammal as a 'lion' is much more specific than having to identify an object such as a hat for example. This may be used to explain prosopagnosia because recognising faces involves the identification of specific features. 

This explanation of prospagnosia provides us with infomation about our perceptual abilities as it suggests there are not seperate modules in the brain for types of recognition. 

2. Recognition lies on a continuum

Farah 1991: suggested that different agnonsias are not seperate conditions, but result in a distruption of two proceses that lie of continuum. These processes are configural processes (ability to decode structure) and a hollistic process (ability to commute relationships between parts). Therefore prosopagnosia would be an example of the a disfunction of the hollistic process. 

Such evidence suggests there is not different modules in the brain for different kinds of recogniton. Use as AO2 against prosopagnosia suggesting there are different processes. 

Case studies (IDA) 

Reearch using case studies on brain damaged patients are difficult to generalise to the wider population for a number of reasons:

-We do not know how good their perception was before they became brain damaged. We could be drawing invalid conclusions about how normal perception works by looking at what part of their brain is damaged, when infact their poor perceptual abilities may have been present before they became brain damaged. 

-Only a very small number of individuals have been studies making findings unrepresentative of a larger ' normal' population and difficult to generalise.

High levels of individual differences lead to contradictory evidence. For example some propspagnosic farmers cant recognise their cows suggesting that human face recoginiton is not special, contradicting other research where propsopagnosics can recognise their cows.Thus it is difficult to draw conclusions about how our perceptual system works