M2, T2, disorders of visual processing and object recognition

Question 1

Disorders of visual processing and object recognition

Answer 1

• impairment in early visual processing
• visual agnosia and sub-types of visual agnosia
• evidence for disorders at the different stages of object recognition processing/models
• semantic system problems, implications for object recognition

Question 2

Disorders of object recognition

Answer 2

Problems in object recognition can arise due to deficits within or between any of these stages in the framework (Ellis and Young model, Riddoch and Humphreys model)

Patterns of impairment in object recognition in brain damaged patients reveal the relationship between these different stages and the internal organisation of each stage

Question 3

Impaired shape/form perception

Answer 3

• Efron (1968) - patients cannot identify or copy simple geometric shapes, can’t tell things about size, orientation and shape
• Intact colour, motion, location of object in space but misperceives the form of an object
• presented with a circle and a square, patient cannot judge if the two stimuli have the same shape or not
• reports of patients with impaired form or shape not always clear within literature

Question 4

Impaired depth perception

Answer 4

• impairments in depth perception have been reported for both monocular and binocular cues
  -> after parietal occipital damage…
  “to the patient the chair is flat, though he knows from experience that his visual impressions are cheating him… yet he knows from light and shade”
  -> the exact link to depth perception to the different visual channels is not clear

Question 5

Impaired colour perception - colour blindness

Answer 5

• Colour Blindness: Person who has a genetic defect resulting in an abnormality in the photoreceptor system
• Dichromats: people with only two photopigments
• > red-green colour blind – missing photosensitive pigment for medium or long wavelengths
• > blue-yellow – short wavelengths
• Anomalous trichromat: have all 3 photopigments but one of them has abnormal sensitivity

Question 6

Impaired colour perception - damage to CNS

Answer 6

• Colour Perception deficit in CNS
• Cerebral achromatopsia: patient cannot perceive colours of object or scenes.
• Vision is based on shades of grey.
• Due to damage in the colour-analysing regions of the cortex (V4- lingual and fusiform gyri, could extend to V8 – ventral areas)
• Patient was able to perceive colours normally before brain damage
• Often occurs alongside prosopagnosia (impaired face recognition)

Question 7

Impairments of colour processing - 3A’s

Answer 7

• Cerebral Achromatopsia – perceptual deficit
• Colour Agnosia: central and general loss of knowledge about colour
• Colour Anomia: inability to produce colour names
  Present each of these patients with a colour patch, none could name the colour of the patch
  BUT for three different reasons.
  -> defective colour perception
  -> defective colour knowledge
  -> defective colour name production

Question 8

Impaired motion perception

Answer 8

Akinetopsia: selective loss of motion perception

• Patient LM (Zhil et al., 1983).
• Normal visual acuity, no visual field loss, can perceive colour, shape and location of an object
• Difficulties with object movement
• Instead of continuous motion this type of patient sees objects as jumping discontinuously from one position to the next.
• View the world as series of snapshots - e.g. pouring coffee into a cup, seen as static brown column

Question 9

Patient LM and impaired motion processing - brain info

Answer 9

• CT scans - bilateral lesions of the temporoparietal cortices, lesions included posterior and lateral portions of the middle temporal gyrus
• Bilaterally symmetric brain injury with damage consistent with the location of area dorsal stream/V5-MT and its surroundings in primates and humans
• Some motion processing remained intact:
  Zihl et al. (1983) – she could see simple things that moved slowly
  Could also detect biological motion (McLeod, Dittrich, Driver, Perret and Zihl, 1996)

Question 10

Visual agnosia

Answer 10

• Visual Agnosia: object perception or recognition failure with intact shape, colour and motion processing abilities
• Agnosia = means failure of knowledge and recognition
• Adequate low-level vision, intellectual function, language
• Impairment for object perception/recognition for the visual modality
• Able to identify objects via other modalities (e.g., auditory, tactile)
• Different types of sensory modality agnosia e.g., tactile

Question 11

Visual agnosia - patient PT had a stroke

Answer 11

• normal language function
• no co-ordination problems
• normal visual acuity, intact colour, shape and motion perception
• severe object recognition problems
• colour name to definition
• could identify objects using other sensory modalities
• Therefore deficit modality specific

Question 12

Apperceptive agnosia

Answer 12

Lissauer (1890; 1907) distinction between apperceptive agnosia and associative agnosia
Apperceptive agnosia: problems with distinguishing figure from ground, or inability to form a proper visual description of an object from visual descriptions of its parts
Corresponds to the stages of “perceptual” processing.
Associated with damage to perceptual mechanisms
- problems within viewer-centred and object-centred des (Ellis and Young)

Question 13

Associative agnosia

Answer 13

Lissauer (1890; 1907) distinction between apperceptive agnosia and associative agnosia

• Associative Agnosia: problem (difficulty) occurs at a later stage of processing
• problem associating a visual configuration with information about its meaning, normal visual representations but cannot use this information to recognise things
• problems within object recog units and semantic system (Ellis and Young)

Question 14

Geschwind (1965) - disconnection

Answer 14

Geschwind (1965) - a disconnection between an intact perceptual representation and its stored association

• > Simplistic explanation for this
• > Disconnection between visual and speech areas of the brain
• Patient has injury to the posterior part of the left cerebral hemisphere, which deprives the left hemisphere speech area of visual input
• Also disconnection of the corpus callosum so visual information cannot be processed from the right hemisphere to the left speech area

Question 15

Incomplete letters task and Gollin picture task

Answer 15

• apperceptive agnosia tests
• can patients recognise objects in degraded format?
• viewer-centred representation

Question 16

Warrington (1985) - Gollin and Incomplete letters tasks

Answer 16

• 80 Patients with unilateral cerebral lesion in either the left or right cerebral hemispheres
• Patients with right hemisphere lesions performed more poorly than control subjects or patients with lesions in the left hemisphere
• But majority of left hemisphere patients had greater/severe language problems
• Perhaps the issue is a problem with object constancy (object centred representation)
• > Tested hypothesis using the Unusual Views Objects Test

Question 17

Warrington (1985) - unusual views naming and matching test

Answer 17

Test Object Naming -> controls – very few errors

• Patients with right hemisphere lesions - poor at unusual views, ok from the usual viewpoint
• > so problem not loss of visual knowledge

Test Same/Different Matching Task - photos of pairs of objects, same object different views or different objects

• Patients with posterior right hemisphere damage worse performance than healthy controls or patients with posterior left hemisphere damage
• > because area linked to perceptual categorisation (problem object-centred representation) *look up image

Question 18

Associative agnosia example, patient FRA (McCarthy & Warrington, 1986)

Answer 18

• Infarct to left posterior cerebral artery, lesion occipital region of the left hemisphere extended to posterior temporal cortex
• Could copy shapes, could segment a complex drawing into parts and could colour common objects
• Could only name or describe the function of half the line drawings of common objects
• Poor at size judgements of animals from the same category but could do size judgements to spoken names
• Problem - impaired ability to recognise and name objects from visually presented information

Question 19

Matching by function test

Answer 19

• Three pictures and patient points to the two functionally similar
• Requires patients to categorise stimuli based on semantic properties
• Patients with posterior lesions in either hemisphere impaired on this task
• Warrington - this is for two different reasons
• > Patients with right hemisphere damage - fail to recognise the object
• > Patients with left hemisphere damage - recognise objects in isolation but cannot make a functional connection between objects

Question 20

Loss of semantic system access

Answer 20

• Test semantic access or function using objects, written and spoken words
• Marin (1987) lady with pre-senile dementia
• > Turns pictures to their correct orientation (stored visual knowledge)
• > Couldn’t do picture matching
• > Unable to do categorisation tasks on the basis of functional or associative knowledge
• > General loss of semantic access, since the problem occurred regardless of the modality of input

Question 21

Impairment at semantic representation level - category specific agnosia

Answer 21

JBR: severe associative agnosia

• > Picture naming worse for living (6% correct) than non-living things (90% correct)
• > Dense amnesia, memory loss, word finding difficulties
• > Normal performance on tests of apperceptive agnosia but severe associative agnosia
• > Category specific disorder - selective loss of knowledge in semantic system for living things