Visual Agnosia
13 important questions on Visual Agnosia
What skills are visual agnosia patients capable of and what are they not capable of?
- low-level vision i.e. intact primary visual pathways (they can physically see the object)
- memory i.e. they know what the object is
- object knowledge
- language i.e. they can describe the object
- object recognition from touch i.e. they cannot recognise objects in vision but they can through touch
Incapable of:
- objection recognition from sight
= pure deficit in object recognition
What are the subtypes of agnosia?
- Apperceptive
- Associative (integrative)
What are mild deficits of Apperceptive Agnosia?
Deficits:
- Incomplete drawings of objects (Street, 1931)
- Ghent overlapping figure test (De Renzi et al., 1969)
- Unusual views test (Warrington & Taylor, 1973) - i.e. they can only recognise the object from a typical viewpoint
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What is converging evidence for the role of lateral occipital complex (LO or LOC) in shape processing?
- Increased fMRI activation during object and shape processing - (Review: Grill-Spector, Kourtzi & Kanwisher , 2001)
- TMS over LO disrupts the processing of a shape discrimination task (Ellison & Cowey, 2006)
Regarding Appercetive Agnosia, is the problem primary visual processing?
- Patient SDV with apperceptive agnosia as determined based on typical profile of impairment
- Central visual field defect + severe problem with line orientation processing
---> Primary visual processing deficit could underlie some cases of apperceptive agnosia
Who is the classic Associative Agnosia patient?
- Awoke one day and was unable to read.
- Well preserved language, spatial, visual and perceptual abilities
- Impaired recognition of visually presented common objects
---> Damage to left fusiform gyrus
What is the problem with single case studies?
What is are 2 distinctions between apperceptive agnosia and associative agnosia?
Associative agnosia - intact drawing and matching
Lesion:
Apperceptive agnosia - bilateral lesion to lateral occipital cortex
Associative agnosia - left fusiform gyrus
Describe everyday life with visual agnosia?
- Intact memory, cognition, speech, etc.
- Relatively good obstacle avoidance
- Better object recognition with real objects than pictures
Examples of difficulties:
- shopping
- cannot read
- driving
Who was Unteroffizier S (Bodamer, 1947)?
- Could distinguish the elements of a face, but he had no sense that the features made a face
= Suggests a disorder of basic face perception
What do patients with prosopagnosia rely on for person identification?
Who is the classic prosopagnosia patient?
- Anterograde amnesia and achromatopsia
- Prosopagnosia
---> Good at recognising 3D and everyday 2D objects. Some difficulty with more obscure objects and silhouettes.
---> Cannot recognise any faces
- Difficulty recognising gender, age...
- Impaired at deciding if two faces are of the same person
Lesion
Lesion to undersurface of occipital lobes bilaterally and to left hippocampus:
- Damage to bilateral fusiform face area (FFA) (prosopagnosia)
- Left V4 (achromatopsia)
- Left hippocampus (amnesia)
Do agnosia and prosopagnosia frequently co-occur?
There are patients with preserved face recognition and impaired object recognition (agnosia), and patients with preserved object recognition and impaired face recognition (prosopagnosia)
---> This suggests different brain regions dedicated to face and object recognition
Converging Evidence:
- Single-cell recording in monkey area IT in response to different stimuli (Bruce, Desimone and Gross, 1981)
- fMRI activity in response to faces relative to objects (Kanwisher et al., 1997)
- Ishai et al. (1999) - see slide 2, p. 9 lecture notes
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