Perception 2: High level vision Flashcards
(35 cards)
What is object agnosia?
Difficulty visually perceiving objects
What tests can be done to identify object agnosia?
Presenting an object in front of a patient and them being unable to recognise it
Name an object and ask them to draw it - distinguish between memory and perception
Show them two similar objects and two dissimilar objects - ask them which are more similar (this does not test high level ability of naming objects however)
Need to check lower level vision - there could be damage to visual field - object recognition is complex and hierarchical
Lissauer was the first to identify a visual agnostic patient - they distinguished between two stages of recognition which present as two types of agnosia - what are these?
1) Apperceptive
2) Associative
What is associative agnosia?
Impairment in recognition or assigning meaning to a stimulus that is accurately perceived
No impairment in perceiving object, just cannot recognise it
(can replicate a drawing)
Failures in recognition due to failures of memory
What is apperceptive agnosia?
Failures in recognition due to failures of perception
Is there a true distinction between apperceptive and associative agnosia in humans? - Patient HJA
Riddoch and Humphreys (1987) would argue no:
- Patient HJA passes tests used to diagnose apperceptive agnosia - could conclude he has no issues of perception
- But nonetheless shows higher order impairments that are perceptual in nature - when using different tests
So does not fit into either boundary
Does agnosia mean impairment?
No - not complete impairment, just takes longer and is more difficult
Could patient HJA discriminate between real and unreal objects?
No
What helped patient HJA to improve in discriminating real/unreal objects?
When they were just silhouettes, his performance was better
Less interference - fewer things to decide whether to integrate or not
Different from controls, in whom performance gets worse for silhouettes
Less information so is harder
Riddoch and Humphreys (1987) tests and results on Patient HJA suggest something more complex than a simple apperceptive/associative dissociation
The term integrative agnosia was introduced to describe patients like HJA
What is this?
Describes a high-level perceptual impairment in integrating the form and features of an object
- Percept not devoid of meaning, but still impaired at level of perception and integration, not association
- Take a long time - have to use deliberative process to pull info together
What is the Birmingham, object recognition battery?
Series of tests designed to identify the level of processing at which a recognition impairment exists
- Tests get progressively more high-level - at some point ppt will drop off
Is agnosia likely to be a continuum?
Yes
Ettlinger (1956) tested whether associative agnosia was due to lower level impairments in a comprehensive test of basic visual functions in three groups of patients with cerebral lesions.
Group A - no visual field deficit or agnosia
Group B - visual field deficit and no agnosia
Group C - visual field deficit and agnosia
How did the groups differ in their basic visual functions?
Group C did not perform significantly worse than Group B - only one patient would today be considered to have visual agnosia (face blindness)
Showed that impairments in visual sensory abilities were associated with visual field defects…
… but not the presence of higher perceptual disorders (only one of which had “agnosia”).
What is an issue with Ettlinger’s result that agnosia was attributable to visual field deficit not perception?
The tests Ettlinger (1956) used don’t fully account for the functional organisation of the visual system (and he didn’t focus on object agnosia)
There are a number of dissociable visual “features”
- Lightness
- Colour
- Movement
- Texture
- Shape
Etc.
Without proper tests of basic visual functions, it isn’t possible to distinguish associative from apperceptive agnosia
Cannot argue that they had intact low level perception as tests did not properly measure this
DeHaan et al (1995) repeated Ettlinger’s (1956) study, (using more appropriate tests of visual function) focussing on agnosia and taking the following visual abilities into consideration:
Shape discrimination
Location discrimination
Colour discrimination
Lightness discrimination
Shape from motion
Texture discrimination
Line orientation
What did they find?
No evidence that these visual functions (even shape perception) are necessary or sufficient to cause agnosia
Agnosia had fewer impairments in these abilities compared to non-agnosia controls
Shows not all failed lower level tests are necessary to cause agnosia - agnosia may be higher level perception
What is visual form agnosia?
Type of apperceptive agnosia
Impairment is at level of recognising objects based on shape - lower level abilities intact
Benson and Greenberg studied Mr S, who had visual form agnosia.
Unable to match objects, faces or letters - the impairment seemed specific to visual form (or shape) perception
Can discriminate objects with different luminance or colour, so when testing for shape perception, you need to control for these things
When keeping low level features the same, Mr S shows specific high level impairment
What does this show about agnosia?
Shows it is likely an issue of higher level perception - very specific and selective impairment
What are the four stages of Marr’s model of object representation? What parts would someone with visual form agnosia be able to do?
Gray-level representation
Primal sketch
2.5D sketch
3D model
Someone with visual form agnosia would only be able to do gray-level representation
Campion & Latto (1985)
Measured contrast sensitivity in an agnostic (patient RC)
How did contrast thresholds differ between agnostic and controls?
Abnormal thresholds relative to controls - not U shaped, and on average could only perceive much higher contrasts than controls
Indicates a possible sensory deficit (dependent on spatial frequency and orientation)
Campion and Latto (1985) suggested an account based on peppery field defects - a ‘masking’ account
- Assessed using fine grained perimetry
- Subject rates brightness of single dot presented in 1° portions of visual field
Was agnosia explained by peppered field defects? (scotoma)
Yes
Loss of small parts of conscious experience in the visual field
What is function of occipital and temporal cortices?
Object recognition
Ventral stream - what pathway
What is the function of the parietal cortex?
Visual spatial processing - spatial judgement
Dorsal stream - where pathway
David Milner and Mel Goodale suggested a different dissociation to the dorsal/ventral streams for spatial judgment and object recognition
They studied patient DF:
- Profound visual form agnosia
- Accompanied by other deficits (brightness, motion, depth) but largely intact low level vision
Memory intact - visual form representation was the main impaired thing
DF does not have conscious access to shape info, but can use this for other tasks
When rotating a card as if you would post it, vs actually posting a card, how does DF perform?
When rotating card as if you were to post it through the slot, DF finds it impossible
When actually posting, DF can do it
Suggests conscious knowledge of orientation is impaired
Can people with apperceptive agnosia copy?
No
Those with associative agnosia CAN copy
