Lecture 2 Flashcards
1
Q
What happens when light reaches the retina?
A
- Reception = absorption of physical energy
- Transduction = physical energy converted into electrochemical pattern in neurones
- Coding = correspondence between physical stimulus and resultant nervous system activity
2
Q
Receptors in the eye
A
- Cones: colour vision and sharpness of vision
- Rods: vision in dim light and movement
3
Q
Colour vision
A
- Visible light part of EM spectrum that cones/ rods respond to
- Most sensitive to light in green wave
4
Q
Trichromatic Theory
A
- Young said all colours produced by mixing 3 primary colours (red, blue and green)
- Must be 3 types of colour receptors responding to different wavelengths
- -> short (blue)
- -> medium (yellow-green)
- -> long (red)
5
Q
Problem of trichromatic theory
A
- Negative afterimage
- Green square shown = no red light hitting retina so why do we perceive red?
6
Q
Opponent-process theory
A
- Hering
- Perception of colour controlled by 3 receptor complexes:
- -> Red-green
- -> Blue-yellow
- -> Black-white
- Dual process theory = signals from the 3 cone types (Trichromatic Theory) are sent to the opponent cells (Opponent-process theory)
7
Q
Colour constancy
A
- Tendency for a colour to look the same under different viewing conditions
- What we perceive isnt entirely driven by the wavelengths of that light that hit our retina
8
Q
The two different pathways
A
- After light hits the retina there are 2 pathways:
- -> Parvocellular (P) pathway
- -> Magnocellular (M) pathway
P pathway:
- Sensitive to colour and fine detail
- Most input comes from cones
M pathway:
- Most sensitive to motion
- Most input comes from rods
9
Q
Pathway from eye to brain
A
- Retina
- Optic nerve
- Optic chiasm
- Lateral Geniculate Nucleus (LGN)
- Cortical area V1
- Left visual cortex comes from left side of the 2 retinas
10
Q
Properties of visual neurones
A
- Receptive fields = the region of sensory space where light will cause the neurone to fire
- Retinotopy = things in close proximity are processed in cells near to each other
- Lateral inhibition = reduction of activity in one neurone is caused by a neighbouring neurone
11
Q
Lateral Geniculate Nucleus
A
- First stop after the eye
- Part of the thalamus - sensory input and motor output
- Maintains a retinotopic map = mapping of visual input from retina to neurones
12
Q
Primary visual cortex (V1)
A
- Second stop after the eye
- Extracts info from the visual scene (shape, colour, movement)
- Maintains retinotopy
13
Q
Damage to V1
A
- Cortical blindness = patient cant report objects presented in region of space
- Can still make some visual discriminations in blind area as there are other routes from the eye to the brain
- Geniculostriate route is specialised for conscious vision
14
Q
Important visual pathways
A
- Parietal/dorsal pathways = concerned with movement processing (where)
- Temporal/ventral pathway = concerned with colour and form processing (what)
- Patient DF = lesion to lateral occipital cortex –> trouble locating and identifying objects = damage to the temporal pathway
15
Q
Functional specialisation theory: Zeki
A
- Parts of visual cortex specialised for different functions
- Central assumption = colour, form and motion processed in separate parts of visual cortex
- V1 and V2 = perception
- V3 and V3a = form
- V4 = colour
- V5 = visual motion
16
Q
V4
A
- Brain imaging PET study = V4 more active for colour than grey scale –> V4= specialised for colour. (Zeki)
- Patients with cortical achromatopsia = cant see colours, damage to V4 and often V2 and V3
- But can colour process
17
Q
V5
A
- V5 more active with moving dots compared with static dots –> V5 = specialised for motion
- Brain damage to VF = Akinetopsia
- Patient LM:
- -> Bilateral damage to V5
- -> Good at locating stationary objects
- -> Good colour vision
- -> Motion perception deficient
18
Q
Challenges for functional specialisation - the binding problem
A
- Dont perceive colour and shape separately but in brain processed separately
- Synchronisation hypothesis = perception depends on synchronised neural activity between brain regions dependent on attention
19
Q
Model of object recognition
A
- Early visual processing (colour, motion)
- Perceptual segregation: group visual elements (known as figure-ground segmentation)
- Match visual description onto representation of object stored in brain = structural descriptions
- Attach meaning to object based on prior knowledge
20
Q
Perceptual organisation
A
- Perceptual segregation
- -> Separating visual input into individual objects
- -> Thought to occur before object recognition
- Gestalt psychology:
- -> First attempt systematically study segregation
- -> Fundamental principle = ‘Law of Pragnanz’ = people will perceive ambiguous images in its simplest form
- Assumes set of rules that operate in early visual processing
21
Q
Gestalt laws of perceptual organisation
A
- Law of proximity
- Law of similarity
- Law of good continuation (perceive forms of similar shape, form, pattern)
- Law of closure (e.g. to us circle looks complete but its actually fragmented)
22
Q
Figure-group segregation
A
- Faces-goblet illusion = ambitious drawing that can be seen as 2 faces or as goblet
- Assumed more attention is paid to figure than the ground
23
Q
Criticisms in Gestalt Psychology
A
- Most evidence descriptive
- Relies heavily on introspection and evidence from 2D drawings
- Some segmentation occurs by top-down prior knowledge which is ignored
24
Q
Object recognition deficits
A
- Agnosia = impairment in object recognition
- Different impairments arise depending on what stage object recognition damaged
25
2 types of Agnosia
- Apperceptive agnosia
| - Associative agnosia
26
Apperceptive agnosia
- Damage to lateral occipital lobes
- Impairment in the perceptual process
- See parts but not the whole
- Patient HJA:
- -> Had it from extensive bilateral ventral-medial occipital damage
- -> Could recognise objects from touch but not from visual recognition
- -> Saw objects by parts
- -> Problems grouping info
27
Associate agnosia
- Impairment in the process which maps perceptual representation onto stored knowledge
- See the whole but not the meaning
- Associated with left occipto-temporal damage
- Patient LH:
- -> Damage to occiipito-temoral brain regions
- -> Could copy drawings of objects but couldn't name or show what they are for
28
Problems with face recognition
- Within-category discrimination = all faces look similar
| - Faces important in evolutionary terms so may have specific mechanism
29
Prosopagnosia
- Impairment of face processing t
- In study patient failed to recognise family but could do so by voice and could match different views of faces and name other objects --> impairment at the stage of matching to stored info
30
Fusiform face area
-Part of ventral stream that recognises faces
31
Holistic/configurable processing
- Faces processed holistically not in parts
- Slower and less accurate at identifying upside down faces
- Criticism = evidence shows that we use the horizontal contours around the eyes for recognition so its not hollistic
32
Visual expertise
- We have become experts at within category discriminations
- Criticisms:
- -> not all prosopagnosic patients are impaired on within-category discrimination
- -> Patient WJ = owned a flock of sheep and could distinguish between them
- -> Patient RM = could distinguish between his collection of 5000 miniature cars but was unable to identify famous faces or his/his wife face
