agnosia and prosopagnosia

Question 1

agnosia - when object recognition fails

Answer 1

• Agnosia - typically occurs after damage to the occipital or inferior temporal cortex.
  
  • “The impairment of visual object recognition in people who possess sufficiently preserved visual fields, acuity and other elementary forms
    of visual ability to enable object recognition, and in whom the object recognition impairment cannot be attributed to… Loss of knowledge
    about objects.” Farah (1999)
  • Cant recognise objects.
• Different types of agnosia reveal important clues concerning the processes underlying object recognition.

Question 2

apperceptive agnosia

Answer 2

• Able to move about and negotiate obstacles without difficulty.
  
  • Their grasp reveals knowledge of size and shape (Goodale and Milner, 2004).
  • Low-level binding of feature appears to be absent.
  • Unable to perform basic copying and matching tasks.
• Damage in stage 2 leads to apperceptive agnosia

Question 3

associative agnosia

Answer 3

• Copying and matching skills are unimpaired - can copy.
  
  • Patient unable to name the object despite intact knowledge.
  • Involves a failure in accessing knowledge about the objects.
• Damage in stage 3 leads to associative agnosia

Question 4

prosopagnosia

Answer 4

• Profound loss in ability to recognise faces usually due to a right inferotemporal lesion.
  
  • Though unable to recognise previously familiar faces via visual input, recognition by other modalities remain intact. Thus, individuals can be identified by their voices.
• The ability to recognise faces is tested through overt behavioural measures.

Question 5

covert recognition

Answer 5

• Peak skin conductance responses (SCRs) occur 1-5 seconds after a face has been presented (red arrows).
  
  • Peak amplitudes are larger for a familiar relative to an unfamiliar face (Tranel et al, 1985).
• Similar patterns have been observed for prosopagnosic patients (Ellis et al, 1993).

Question 6

recognition without feelings - capgras delusion

Answer 6

• Patients with Capgras delusion both recognise a face and yet deny the identity of the individual.
• For instance Alan Davies, after a car crash insisted that his wife died in the crash and that the women living with him was an imposter.

Question 7

prosopagnosia vs capgras syndrome

Answer 7

• Yellow route is the covert dorsal route through the superior temporal sulcus and inferior parietal lobe.
  
  • Red route is the overt ventral route.
  • Loss in the ventral stream can result in prosopagnosia.
• Loss in the dorsal stream can result in Capgras delusion.

Question 8

summary

Answer 8

• Visual agnosia is a deficit of visual object recognition.
  
  • Typically results from damage to the occipital or inferior temporal cortex (early in the ventral stream).
  • Lissaur (1890) first distinguished different types of agnosia:
  • Apperceptive - low-level binding of features is compromised.
  • Associative - access to associated knowledge is compromised
  • This dissociation is supportive of modularity in the process of visual object recognition.
• Modularity is central to the cognitive neuropsychology approach and while useful is typically an over simplification.