7- Visual Processing and Agnosia Flashcards
Visual agnosia refers to inability to recognise objects despite intact sensory processing, what is Lissauer’s two-stage model?
Apperception: Conscious awareness of sensory impressions
Association: linking perceptions to stored memories and concepts.
Using Lissauer’s model, what is APPERCEPTIVE AGNOSIA? (visual form agnosia)
Deficit in processing/interpreting visual information to construct a coherent overall image.
Cannot generate a perceptual representation.
E.g. Problems with shape discrimination and drawing objects, matching objects to pictures. Challenges in integrating local features into global structures.
What can people with apperceptive agnosia do?
Name through other modalities
Use visual information to guide behaviour e.g. adapt grip when reaching for an object
What can’t people with apperceptive agnosia do?
Interpret sensory information to construct an overall image
Match shapes
Can’t copy
Draw objects
Match objects and pictures
Describe a visual image
Maybe overlapping shapes
How is apperceptive agnosia caused?
Damage to bilateral occipital lobes or posterior inferior temporal lobe (part of visual ventral stream) following CO poisoning or stroke
What is associative agnosia? (visual object agnosia)
Visual perception is intact but the connection to stored semantic knowledge is disrupted.
Cannot link representation to stored knowledge
Patients may identify shapes but fail to associate them with meaning or use.
What can people with associative agnosia do?
Name through other modalities
Copy pictures
Match objects
Draw objects
Describe the form of an object
Make same/different judgements
What can’t people with associative agnosia do?
Make sense of a visual picture
Tell you what they have drawn/copied
Match objects by function (categorization is impaired)
What causes associative agnosia?
Typically bilateral damage, especially to left parahippocampal cortex and fusiform gyrus
Perception what is the distal stimulus?
Actual object in the distance
What is the proximal stimulus?
Physical energy e.g. light waves reflected from the tree, impinging on the retina
What is percept?
Internal representation of an external stimulus
Serves as the basis for subsequent identification
What is the optic chiasm?
Pathway for optic nerve to primary visual cortex.
Both ipsilateral and contralateral divisions.
The visual cortex is split up in two:
Striate cortex = primary visual cortex
Extrastriate cortex = secondary visual cortex
Within these cortices, specialisation into..
Striate cortex contains..
VISUAL1= Retinotopic, basic features
Extrastriate contains..
VISUAL2= Colour, figure-ground (difference between background and object)
VISUAL3- Form, tracking motion/movement
VISUAL 4- Form, Colour
What is the ventral stream?
Ventral steam= the “what” pathway
INFERIOR TEMPORAL GYRUS
Specialised for identifying and recognising objects, colour and form.
Damage to ventral stream can lead to visual agnosia and prosopagnosia.
Slow and detailed processing
Prefers information from the fovea
What would damage to the ventral stream cause?
Can reach out for an object and pick it up but cannot describe it
Ventral stream: Specialist visual processing. Ventral pathway ends in specialised regions of inferior temporal lobe.
Preferential processing of:
Places
Parahippocampal Place Area
Faces
Fusiform Face Area
Words/Letter
Visual Word form area
What is the dorsal stream
the “where” pathway (spatial awareness and motion)
Posterior Parietal cortex (particularly right parietal lobe in visuospatial neglect)
Processes spatial information, motion, object location
Integrates vision and action (guides reach, grasp movement towards object)
Quick, often independent of conscious thought
Takes information from anywhere on the retina.
What would damage to dorsal stream cause?
Can describe an object but cannot approach it or pick it up.
Optic ataxia (difficulty coordinating hand movements with visual input)
Explain bottom-up processing
Starts with stimulus sensation that sends info to the brain
Perception instructs cognition
Explain top-down processing
Using prior knowledge and experience to interpret a stimulus
Perception is constructed by cognition
What is visual agnosia?
Disorder of recognition confined to the visual modality. Despite adequate primary visual functions and continued ability to make sense of the world through other modalities.
Birmingham Object Recognition Battery
Basic visual features (Feature detection)
Figure-Ground segregation
Viewpoint invariance
Visual representation (visual memory)
Amodal representation
What is figure-ground segregation
Process of segmenting visual display into objects vs background surfaces
Figure= object of focal interest nearby/sharper/brighter
Ground= background
further away, fuzzy, dull
What is viewpoint invariance?
Objects can look different from different perspectives, the proximal stimulus can vary while distal stimulus remains the same. NEED TO RECOGNISE AN OBJECT REGARLDESS OF ITS DISTANCE, ORIENTATION, MOTION
How do we achieve viewpoint invariance?
Distributed representation in Inferior Temporal (IT) cortex
Groups of a few hundred neurons respond to objects regardless of viewpoint (Majaj et al., 2012)
Receptive fields are large and include the fovea.
what is visual representation/memory?
Matching perception to stored information
What cortical regions does visual memory rely on?
Occipito-temporal regions (visual ventral)
Inferior temporal cortex
Fronto-parietal regions
(visual imagery particularly relies on parietal regions)
Hippocampus/ entorhinal cortex
What is amodal representation
Sensory-specific information feeding into the idea of something. Multiple components eg. tactile, auditory, visual, taste, smell, eppisodic memories, semantic info, associations.
Humphreys and Riddoch 5-way distinction according to stages. 1= SHAPE AGNOSIA
Shape agnosia is when you cannot recognise shapes, cannot copy (Lissauer’s apperceptive)
Humphreys and Riddoch’s 5-way distinction according to stages. 2= INTEGRATIVE AGNOSIA
Integrative agnosia can recognise features but cannot put together or identify whole. Can copy but slow and laborious. Can identify objects feature by feature.
Humphreys and Riddoch’s 5-way distinction according to stages. 3= TRANSFORMATIONAL AGNOSIA
Cannot recognise unusual/less frequent perspectives. To do with viewpoint invariance.
Cannot match to more normal view
Humphreys and Riddoch’s 5-way distinction according to stages. 4= SEMANTIC AGNOSIA
Difficulty with visual memory.
Can describe, match, copy
Cannot access stored structural knowledge about objects.
Cannot draw from memory
Fail at object decision test “Is this a real object”.
Humphreys and Riddoch’s 5-way distinction according to stages. 5= SEMANTIC ACCESS AGNOSIA
Lissauer’s associative agnosia: difficulty linking the visual system with the semantic system.
Can see, describe match, copy, vague idea what it is but cannot access detailed knowledge
If asked to describe a picture of a dog might say “its an animal” but not which.
Fail at ‘item match’.
What is prosopagnosia?
“Face-blindness”. Disorder of face recognition, can’t recognise people from their facial features but can still through their hairstyle, voice, clothes.
Damage to which part of the brain causes prosopagnosia?
Fusiform Face Area (bilateral inferior temporal lobe) Damage to ventral visual pathway.
Which are the main cortical areas responsible for vision and where they located?
Primary visual cortex/ striate cortex which in in the occipital lobe. It receives input from the retina via the lateral geniculate nucleus (LGN) of the thalamus. Processes basic visual info like orientation, edges, light-intensity.
Secondary Visual Cortex (adjacent to V1 in occipital lobe). Receives info from V1 and integrates features like depth etc.
Dorsal and Ventral V3- Dorsal involved in motion perception and spatial processing (part of dorsal stream)
Ventral contributes to shape perception and form recognition (part of ventral stream)
V4= ventral occipital lobe and extending into temporal lobe. It processes colour perception and object form
What are the differences between the dorsal and ventral visual processing pathways?
Dorsal= “where” extends to posterior parietal lobe
Ventral = “what” extends to inferior temporal gyrus
What problems arise from damage to the dorsal and ventral processing pathways?
Damage to dorsal= optic ataxia, difficulty coordinating hand movements with visual input. Can describe an object but cannot reach out to grab it.
Ventral stream dame= visual agnosia (cannot recognise object) and prosopagnosia (cannot recognise faces)
In apperceptive agnosia (Lissauer’s taxonomy) what can a person do and not do?
In associative agnosia (Lissauer’s taxonomy) what can a person do and not do?
What is achromatopsia?
Syndrome where patient loses ability to see colours after cortical damage. Sees world in black and white but no difficulty recognising objects.
Deficit in motion perception
. Severe damage to middle temporal gyrus.
She could not perceive actual movement, so that when objects did move (eg, cars or people), they seemed to jump from one position to the next. She found that
understanding language was problematic (people’s lips seemed to hop up and down), as was meal preparation (she had difficulty in pouring and measuring liquids because they appeared frozen like a glacier).
Are the processes of figure-ground perception and figure perception interlinked or separate?
Separate. Can be impaired in one but not the other.
Because in figure-ground perception, figures can be distinguished based on on colour, depth, motion.
What do Riddoch & Humphries argue against Lissauer about?
Lissauer was not correct in his assumption that some form of perceptual deficit underlies the recognition problem in all patients with visual agnosia. We suggest that there exist relatively pure forms of associative agnosia, without a concomitant apperceptive deficit
Riddoch and Humphrey’s suggest two forms of stored knowledge serving object recognition, what are they?
Stored structural descriptions (of the structural properties of the object)
Stored semantic knowledge (information about object function and interobject associations)
