Neuropsychology: The Seeing Brain

Question 1

What kind of process is seeing?

Answer 1

-Complex (hierarchical) process
-Constructive process
-Example: Kanisza illusion

Question 2

How is seeing a complex process?

Answer 2

-Eyes and brain play important roles
-Psychological and cognitive models: how does visual perception happen and which processes are involved

Question 3

How is seeing a constructive process?

Answer 3

Things are added to input and interpreted in certain way

Question 4

What is the Kanizsa illusion?

Answer 4

-Hard to perceive stimulus as three corners rather than triangle
-Triangle seen because stimulus gives impression of lines forming triangle
-Visual system in brain involved

Question 5

What is the hierarchical system of seeing?

Answer 5

-Start in retina
-Lateral geniculate nucleus (LGN) in thalamus
-Secondary visual cortex
-Third region
-Fourth region
-Fifth region (MT) and other regions at same time
-Parietal regions, inferior temporal regions and frontal regions (FEF, frontal eye field)
-Lastly hippocampus
(image)

Question 6

What are characteristics of the retina?

Answer 6

-First step in seeing
-Internal surface of eyes consisting of multiple layers
-Contains specialized photoreceptors to covert light into neural signals: rod cells and cone cells
-Optic nerves relay output of retinal ganglion cells to brain
-Blindspot: point at which optic nerve leaves eye, so no rods and cones present

Question 7

What are rod cells?

Answer 7

-In retina of eye
-Specialized for low levels of light intensity
-More active during night time
-Evenly distributed across retina, but not present in fovea

Question 8

What are cone cells?

Answer 8

-In retina of eye
-Specialized for detecting different wavelengths of light
-More active during day time
-Highest concentration in fovea

Question 9

What is the geniculostriate pathway?

Answer 9

-Dominant pathway from retina to brain: travels to primary visual cortex (V1) at back of brain
-Certain part of visual fields might no longer be seen when damage somewhere in pathway
-Lateral geniculate nucleus (LGN): transfers visual information through neurons inside
-Primary visual cortex (V1, striate cortex): extracts basic info from visual scene

Question 10

Which route does the geniculostriate pathway follow?

Answer 10

-Each retina has part for right stimuli and left stimuli
-Optic nerves receive info and half of them cross in optic chiasm
-Other parts of optic nerve stay on same side of brain and come together with opposite optic nerve in optic tract, so both hemispheres get input from both eyes
-In lateral geniculate nucleus (LGN), input from left and right eye stays separate in different layers
-Primary visual cortex combines input from both eyes and passes it on to further regions

Question 11

What kinds of conditions can be the result of damage somewhere in the geniculostriate pathway?

Answer 11

-Monocular blindness: damage to eye or optic nerve
-Bitemporal hemianopia: damage in crossing fibers of optic chiasm
-Right nasal hemianopia: damage in nerve that doesn’t cross
-Homonymous hemianopia: abscence of optic tract or lateral geniculate nucleus (LGN)
-Quadrantanopia: damage in part of optic tract or lateral geniculate nucleus (LGN)
-Macular sparing: damage in primary visual cortex

Question 12

What are characteristics of the lateral geniculate nucleus (LGN)?

Answer 12

-Transfers visual information through neurons inside
-6 layers: 3 for each eye
-Cells have center-surround receptive field
–>Respond to contrast: differences in light between center and surround of receptive field
–>Don’t like overall light or overall dark

Question 13

What are characteristics of the primary visual cortex (V1, striate cortex)?

Answer 13

-Extracts basic info from visual scene
-Info used by later stages of processing to extract info about shape, color, movement, etc.
-Hubel & Wiesel: single-cell recordings showed there’s hierarchy in processing in primary visual cortex
-Spatial arrangement of primary visual cortex: representation of whole visual field of one side combined in primary visual cortex
–>Retinotopic organisation
–>Damage to parts in primary visual cortex results in blindness for corresponding region of space

Question 14

What are the stages of cells in the primary visual cortex according to Hubel & Wiesel?

Answer 14

-Simple cells: derive response by combining responses of several LGN center-surround, respond to different orientations
-Complex cells: derived by combining responses of several simple cells, respond to orientation but have larger receptive field and require stimulation on entire length
-Hypercomplex cells: outside V1, derived by combining responses of several complex cells, sensitive to length AND orientation

Question 15

Which types of cells can be found in the primary visual cortex (V1)?

Answer 15

-Simple cells: derive response by combining responses of several LGN center-surround cells, respond to different orientations
-Complex cells: derived by combining responses of several simple cells, respond to orientation but have larger receptive fields and require stimulation on entire length
-Hypercomplex cells (outside V1): derived by combining responses of several complex cells, sensitive to length AND orientation

Question 16

What is retinotopic organisation?

Answer 16

-Different neurons will see different parts of visual field
-Spatial arrangement of light on retina retained in response properties of primary visual cortex-neurons, but inverted (top part of primary visual cortex is bottom of visual space and vice versa)

Question 17

What kind of blindness can be the result of damage to parts of the primary visual cortex?

Answer 17

-Hemianopia: fully damaged primary visual cortex in one hemisphere
-Scotoma: small damaged primary visual cortex in one hemisphere
-Quadrantanopia: half damaged primary visual cortex in one hemisphere

Question 18

Why is the place of damage important?

Answer 18

-Still residual vision if damage happens further in pathway, like primary visual cortex
-Reason: multiple pathways from eye to brain
–>Geniculostriate pathway most well understood and makes largest contribution to human visual perception
–>Other routes evolutionary more ancient

Question 19

What are the consequences of the geniculostriate pathway being the most important for conscious vision?

Answer 19

-Damage to pathway impairs conscious vision
-But other aspects spared
-Example: blindsight

Question 20

What is blindsight?

Answer 20

-Damage to V1: patient cannot consciously report objects presented in this region of space
-But patient still able to make visual discriminations in blind area (orientation, movement direction)

Question 21

What are characteristics of blindsight?

Answer 21

-Possible because of other routes from eye to brain: routes for unconscious vision
-More blindsight if damage is in both hemispheres
-Filling-in of blind regions: regions filled up with something expected to be there
-Low trust in tasks because can’t consciously see, but does task well
-Remaining vision not great, so small obstacles will probably make patient trip
-Example of how visual perception is constructed

Question 22

What other regions are involved in seeing?

Answer 22

-Prestriate regions: V2, V3, V3a, V4, V5/MT
-Non-visual cortical areas: temporal and parietal cortex
–>Ventral what stream: to temporal lobe
–>Dorsal where stream: to parietal lobe

Question 23

What is the V4 area?

Answer 23

-Main color center of brain
-Brain needs specialized processing system for color, although retina is already sensitive to different wavelengths of light
-Area V4 computes color of object taking into account variations in lighting conditions (color constancy)
-Damage in V4: achromatopsia

Question 24

Why does the brain need a specialized processing system for color, while the retina is already sensitive to different wavelengths of light?

Answer 24

-Problem: wavelength depends on composition of light source and color of object
–>Retina not specialized enough to compare wavelengths and discount effect of illumination
-Color constancy: V4 computes color of object taking into account variations in lighting conditions
–>Cells in V4 continue to respond to same surface color if light source is changed
–>Cells is V1 don’t respond to same surface color if light source is changed

Question 25

What is achromatopsia?

Answer 25

-Patients with damage in V4 -Fail to see all color: seeing world in black and white -Retina and cells in primary visual cortex still respond to different wavelengths of light -Not same as color blindness (due to cone deficiency) -Example of how visual perception is constructed

Question 26

What is the V5/MT area?

Answer 26

-Main movement center of brain -90% of cells in V5/MT respond to particular direction of movement (direction sensitivity) -Bilateral damage in V5/MT: akinetopsia -More regions involved in movement, especially in complex movements (biological motion) -->Involvement of posterior superior temporal sulcus

Question 27

What is biological motion?

Answer 27

-Possible to discriminate biological from random motion, given array of moving dots -Brain imaging and neuropsychology suggest that this may use additional regions/mechanisms beyond ones involved in determining overall direction of movement (system), including posterior superior temporal sulcus -Akinetopsic patients can discriminate biological motion

Question 28

What is akinetopsia?

Answer 28

-Bilateral damage in V5/MT -Fail to see movement: seeing world in series of still frames, not how frames change -Possible to detect movement with other senses -Quite rare

Question 29

What is needed beyond the visual cortex to see?

Answer 29

-Visual cortex (striate and extrastriate) extracts basic info (colors, movement, shapes, edges) -In order to be able to use info, needs to make contact with other types of info -->Where object is in space (can't be computed from retinal image alone) -->What object is -Leads to model of object recognition

Question 30

How is all the visual info put together to form a coherent perception according to the model of object recognition?

Answer 30

-1st: early visual processing (color, motion, edges, etc.) (already seen in previous part) -2nd: grouping of visual elements (gestalt principles, figure - ground segmentation) -3rd: matching grouped visual description to representation of object stored in brain (structural descriptions) -Attaching meaning to object (retrieved from semantic memory) (image)

Question 31

What consequences can damage in different stages of model of object recognition have?

Answer 31

-Damage in mid- or higher visual processes: visual agnosia -Damage in grouping visual elements: integrative and visual form agnosia -Damage in matching visual description to structural descriptions: object orientation agnosia -Apperceptive agnosia -Associative agnosia

Question 32

What is visual agnosia?

Answer 32

-Due to damage in mid- and/or higher-order visual processes -Problem in: mid- and/or high-order visual processes necessary to recognize objects based on vision -Intact: low-level visual processes (motion, color, etc.) -Intact knowledge about objects and thus intact recognition based upon other modalities (hearing voice, etc.) and intact alertness, intelligence, language -2 types: apperceptive agnosia and associative agnosia -->Clinically useful, but too simplistic (many stages in system)

Question 33

What is apperceptive agnosia?

Answer 33

-Type of visual agnosia -Disorder in forming coherent visual representation -2 types: integrative and visual form agnosia

Question 34

What is associative agnosia?

Answer 34

-Type of visual agnosia -Disorder in recognition and linking despite intact visual representation

Question 35

Which Gestalt principles are there to combine parts into a whole?

Answer 35

-Law of proximity -Law of similarity -Law of good continuation -Law of closure

Question 36

What is integrative agnosia?

Answer 36

-Damage in grouping visual elements (constitutes to 2nd stage of model of object recognition) -Type of apperceptive agnosia in which grouping principles are disrupted -Inability to form whole of visual image -Slightly different symptoms per patient possible -Spared: copying pictures, drawing from memory -Impaired: deciding if objects are real or not, naming objects, naming objects that overlap each other

Question 37

What is visual form agnosia?

Answer 37

-Damage in grouping visual elements -->Could be because of bilateral damage to lateral occipital cortex -Type of apperceptive agnosia (but borderline) -Inability to form representation -Impaired: unable to copy or recognize line drawings of objects -Spared: can recognize real objects based on color/texture, intact memory of objects (drawing from memory), can interact with objects (ex.: doesn't know what hammer is, but can use it)

Question 38

What is object constancy?

Answer 38

-Achieved by mapping potentially infinite number of visual depictions onto finite set of stored descriptions of object structure -Suggestion: brain stores objects in single viewpoint (canonical viewpoint that contains principle axis) -->Implication: object recognition involves view normalization from seen viewpoint to stored viewpoint (mental rotation) -Another suggestion: stored structural description accessed by matching feature-by-feature -Modern terminology: invariance/tolerance

Question 39

What are the neural substrates of object constancy?

Answer 39

-Monkey cells in inferotemporal cortex respond to very particular object attributes (corners, shapes), but less concerned with where located in space -->Ideal conditions for computing object constancy -fMRI in humans: inferotemporal regions respond to same object presented in different sizes: left region insensitive to viewpoint, right region sensitive to viewpoint -->Consistent with 2 different routes to object constancy

Question 40

What is object orientation agnosia?

Answer 40

-Right parietal lobe damage -Damage in matching visual description to structural descriptions -Inability to normalize viewpoints -Spared: able to recognize object in all viewpoints (unusual and canonical) -Impaired: unable to choose correct orientation for object -Provides evidence that principle axis is stored separately from other aspects of object recognition

Question 41

Which areas in the higher ventral vision pathway are associated with different object categories?

Answer 41

-Fusiform face area (FFA): for faces -Extrastriate body area (EBA): for bodies -Parahippocampal place area (PPA): for scenes -Visual word form area (VWFA): for word forms

Question 42

What is special about faces?

Answer 42

-Face recognition: within-category discrimination (meaning all faces look same), whereas other object recognition is between category (ex: distinguishing pen from cup) -Faces might use different types of processing -Faces might be/have been important from social/evolutionary perspective: domain specificity -->But might also be based upon experience -Evidence that faces are special: prosopagnosia

Question 43

Which different aspects of face processing did Bruce & Young find (1986)?

Answer 43

-Important proposals -->Distinction between processing familiar and unfamiliar faces (unfamiliar faces: direct route) -->Specific route for expression -->Specific route for facial speech -Different kinds of evidence found -Processing towars: face recognition units = structural descriptions (view-invariant)

Question 44

What kind of evidence is there for the Bruce & Young model of processing faces?

Answer 44

-Face constancy: double dissociation between recognizing familiar faces and matching unfamiliar faces across different viewing conditions -Face naming: often possible to retrieve semantic facts without retrieving name, but reverse pattern not found (name generation depends on semantic retrieval) -Double dissociation between recognizing familiar faces and recognizing emotion, age and sex -Double dissociation between recognizing familiar faces and using lip-reading cues

Question 45

What brain regions were connected to different processes of face recognition by Haxby et al. (2000)?

Answer 45

fMRI -Core system: for visual analysis -->Inferior occipital gyri: early perception of facial features -->Lateral fusiform gyrus (FFA): invariant aspects of faces - perception of unique identity -->Superior temporal sulcus (STS): changeable aspects of faces - perception of eye gaze, expression, and lip movement -Extended system: for further processing -->Intraparietal sulcus: spatially directed attention -->Auditory cortex: prelexical speech perception -->Amygdala, insula, limbic system: emotion -->Anterior temporal: personal identity, name and biographical information (image)

Question 46

What is prosopagnosia?

Answer 46

-Impairment in general face processing that doesn't reflect difficulties in early visual analysis -->Used specifically to refer to difficulty in recognizing previously familiar faces -Congenital or acquired -Impaired: failure in recognizing faces (even own family) -Spared: recognition by voice, clothes and making associations, able to match different views of faces and name other objects -Evidence for faces being special