Visual agnosia is the inability to recognise familiar objects presented visually. Most neuropsychologists distinguish between “apperceptive agnosia” and “associative agnosia”.
“Apperceptive agnosia” is a perceptual deficit, i.e. a failure of recognition due to impaired visual perception. An example of this would be a failure to distinguish between a square and a circle. See the case study of Mr.S.
“Associative agnosia” is when perceptual ability is intact but there is a failure of recognition because of difficulty in accessing the relevant knowledge from memory – a “normal percept stripped of its meaning” (Teuber 1968). An example of this would be having the ability to copy a line drawing of an object accurately, but not being able to name the object. See Rubens and Benson (1971).
One of the most fascinating examples of associative agnosia is Prosopagnosia. This is the inability to recognise faces despite intact intellectual functioning and even apparently intact visual recognition of most other stimuli (Farrah 2004). Prosopagnosia appears to be a failure of visual and associative memories to come together produce recognition. For example, a woman with Prosopagnosia might be able to describe the facial features of a person standing in front of her, but not recognise that this person is her daughter until she speaks (Palmer 1999). The clinical study undertaken by Young et al. (1993) that investigated face perception in 34 ex-servicemen is one example of a case study looking into Prosopagnosia. Here are more case studies into Prosopagnosia:
1. Case study of apperceptive agnosia (based on Benson and Greenberg 1969) – Mr S. was a young man who sustained brain damage after accidental carbon monoxide poisoning. His memory, spontaneous speech and comprehension of spoken language were intact. He could name colours, but was unable to name objects (or pictures of objects) if they were presented visually. He was also unable to copy letters or simple figures and could neither describe nor trace the outline of common objects.
2. Case study of associative agnosia (based on Rubens and Benson 1971) – The subject of this case was a middle aged man who had suffered brain damage, following a sudden drop in blood pressure. His mental and language abilities were normal, but he was unable to identify objects without touching them. When shown a stethoscope he could not name it, describing it as “a long cord with a round thing at the end”. He was never able to describe or demonstrate the use of an object if he could not name it. He found it difficult to recognise pictures of objects but, remarkably, he could copy line drawings accurately.
3. Case study of Prosopagnosia (McNeil and Warrington 1993) – W.J. was a middle aged professional man who developed Prosopagnosia, following a series of strokes. When he was shown the photograph of a famous person, together with the pictures of two unknown people, he was unable to select the famous one. After developing Prosopagnosia, W.J. changed his career and went into sheep…