7- Visual Processing and Agnosia

Question 1

Visual agnosia refers to inability to recognise objects despite intact sensory processing, what is Lissauer’s two-stage model?

Answer 1

Apperception: Conscious awareness of sensory impressions
Association: linking perceptions to stored memories and concepts.

Question 2

Using Lissauer’s model, what is APPERCEPTIVE AGNOSIA? (visual form agnosia)

Answer 2

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.

Question 3

What can people with apperceptive agnosia do?

Answer 3

Name through other modalities
Use visual information to guide behaviour e.g. adapt grip when reaching for an object

Question 4

What can’t people with apperceptive agnosia do?

Answer 4

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

Question 5

How is apperceptive agnosia caused?

Answer 5

Damage to bilateral occipital lobes or posterior inferior temporal lobe (part of visual ventral stream) following CO poisoning or stroke

Question 6

What is associative agnosia? (visual object agnosia)

Answer 6

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.

Question 7

What can people with associative agnosia do?

Answer 7

Name through other modalities
Copy pictures
Match objects
Draw objects
Describe the form of an object
Make same/different judgements

Question 8

What can’t people with associative agnosia do?

Answer 8

Make sense of a visual picture
Tell you what they have drawn/copied
Match objects by function (categorization is impaired)

Question 9

What causes associative agnosia?

Answer 9

Typically bilateral damage, especially to left parahippocampal cortex and fusiform gyrus

Question 10

Perception what is the distal stimulus?

Answer 10

Actual object in the distance

Question 11

What is the proximal stimulus?

Answer 11

Physical energy e.g. light waves reflected from the tree, impinging on the retina

Question 12

What is percept?

Answer 12

Internal representation of an external stimulus
Serves as the basis for subsequent identification

Question 13

What is the optic chiasm?

Answer 13

Pathway for optic nerve to primary visual cortex.
Both ipsilateral and contralateral divisions.

Question 14

The visual cortex is split up in two:

Answer 14

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

Question 15

What is the ventral stream?

Answer 15

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

Question 16

What would damage to the ventral stream cause?

Answer 16

Can reach out for an object and pick it up but cannot describe it

Question 17

Ventral stream: Specialist visual processing. Ventral pathway ends in specialised regions of inferior temporal lobe.

Answer 17

Preferential processing of:
Places
Parahippocampal Place Area

Faces
Fusiform Face Area

Words/Letter
Visual Word form area

Question 18

What is the dorsal stream

Answer 18

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.

Question 19

What would damage to dorsal stream cause?

Answer 19

Can describe an object but cannot approach it or pick it up.
Optic ataxia (difficulty coordinating hand movements with visual input)

Question 20

Explain bottom-up processing

Answer 20

Starts with stimulus sensation that sends info to the brain
Perception instructs cognition

Question 21

Explain top-down processing

Answer 21

Using prior knowledge and experience to interpret a stimulus
Perception is constructed by cognition

Question 22

What is visual agnosia?

Answer 22

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.

Question 23

Birmingham Object Recognition Battery

Answer 23

Basic visual features (Feature detection)
Figure-Ground segregation
Viewpoint invariance
Visual representation (visual memory)
Amodal representation

Question 24

What is figure-ground segregation

Answer 24

Process of segmenting visual display into objects vs background surfaces
Figure= object of focal interest nearby/sharper/brighter

Ground= background
further away, fuzzy, dull

Question 25

What is viewpoint invariance?

Answer 25

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

Question 26

How do we achieve viewpoint invariance?

Answer 26

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.

Question 27

what is visual representation/memory?

Answer 27

Matching perception to stored information

Question 28

What cortical regions does visual memory rely on?

Answer 28

Occipito-temporal regions (visual ventral) Inferior temporal cortex Fronto-parietal regions (visual imagery particularly relies on parietal regions) Hippocampus/ entorhinal cortex

Question 29

What is amodal representation

Answer 29

Sensory-specific information feeding into the idea of something. Multiple components eg. tactile, auditory, visual, taste, smell, eppisodic memories, semantic info, associations.

Question 30

Humphreys and Riddoch 5-way distinction according to stages. 1= SHAPE AGNOSIA

Answer 30

Shape agnosia is when you cannot recognise shapes, cannot copy (Lissauer's apperceptive)

Question 31

Humphreys and Riddoch's 5-way distinction according to stages. 2= INTEGRATIVE AGNOSIA

Answer 31

Integrative agnosia can recognise features but cannot put together or identify whole. Can copy but slow and laborious. Can identify objects feature by feature.

Question 32

Humphreys and Riddoch's 5-way distinction according to stages. 3= TRANSFORMATIONAL AGNOSIA

Answer 32

Cannot recognise unusual/less frequent perspectives. To do with viewpoint invariance. Cannot match to more normal view

Question 33

Humphreys and Riddoch's 5-way distinction according to stages. 4= SEMANTIC AGNOSIA

Answer 33

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".

Question 34

Humphreys and Riddoch's 5-way distinction according to stages. 5= SEMANTIC ACCESS AGNOSIA

Answer 34

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'.

Question 35

What is prosopagnosia?

Answer 35

"Face-blindness". Disorder of face recognition, can't recognise people from their facial features but can still through their hairstyle, voice, clothes.

Question 36

Damage to which part of the brain causes prosopagnosia?

Answer 36

Fusiform Face Area (bilateral inferior temporal lobe) Damage to ventral visual pathway.

Question 37

Which are the main cortical areas responsible for vision and where they located?

Answer 37

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

Question 38

What are the differences between the dorsal and ventral visual processing pathways?

Answer 38

Dorsal= "where" extends to posterior parietal lobe Ventral = "what" extends to inferior temporal gyrus

Question 39

What problems arise from damage to the dorsal and ventral processing pathways?

Answer 39

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)

Question 40

In apperceptive agnosia (Lissauer's taxonomy) what can a person do and not do?

Question 41

In associative agnosia (Lissauer's taxonomy) what can a person do and not do?

Question 42

What is achromatopsia?

Answer 42

Syndrome where patient loses ability to see colours after cortical damage. Sees world in black and white but no difficulty recognising objects.

Question 43

Deficit in motion perception

Answer 43

. 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).

Question 44

Are the processes of figure-ground perception and figure perception interlinked or separate?

Answer 44

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.

Question 45

What do Riddoch & Humphries argue against Lissauer about?

Answer 45

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

Question 46

Riddoch and Humphrey's suggest two forms of stored knowledge serving object recognition, what are they?

Answer 46

Stored structural descriptions (of the structural properties of the object) Stored semantic knowledge (information about object function and interobject associations)