Lecture 2

Question 1

What happens when light reaches the retina?

Answer 1

• Reception = absorption of physical energy
• Transduction = physical energy converted into electrochemical pattern in neurones
• Coding = correspondence between physical stimulus and resultant nervous system activity

Question 2

Receptors in the eye

Answer 2

• Cones: colour vision and sharpness of vision

- Rods: vision in dim light and movement

Question 3

Colour vision

Answer 3

• Visible light part of EM spectrum that cones/ rods respond to
• Most sensitive to light in green wave

Question 4

Trichromatic Theory

Answer 4

• 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)

Question 5

Problem of trichromatic theory

Answer 5

• Negative afterimage

- Green square shown = no red light hitting retina so why do we perceive red?

Question 6

Opponent-process theory

Answer 6

• 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)

Question 7

Colour constancy

Answer 7

• 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

Question 8

The two different pathways

Answer 8

• 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

Question 9

Pathway from eye to brain

Answer 9

1. Retina
2. Optic nerve
3. Optic chiasm
4. Lateral Geniculate Nucleus (LGN)
5. Cortical area V1
   - Left visual cortex comes from left side of the 2 retinas

Question 10

Properties of visual neurones

Answer 10

• 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

Question 11

Lateral Geniculate Nucleus

Answer 11

• 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

Question 12

Primary visual cortex (V1)

Answer 12

• Second stop after the eye
• Extracts info from the visual scene (shape, colour, movement)
• Maintains retinotopy

Question 13

Damage to V1

Answer 13

• 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

Question 14

Important visual pathways

Answer 14

• 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

Question 15

Functional specialisation theory: Zeki

Answer 15

• 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

Question 16

V4

Answer 16

• 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

Question 17

V5

Answer 17

• 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

Question 18

Challenges for functional specialisation - the binding problem

Answer 18

• 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

Question 19

Model of object recognition

Answer 19

1. Early visual processing (colour, motion)
2. Perceptual segregation: group visual elements (known as figure-ground segmentation)
3. Match visual description onto representation of object stored in brain = structural descriptions
4. Attach meaning to object based on prior knowledge

Question 20

Perceptual organisation

Answer 20

• 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

Question 21

Gestalt laws of perceptual organisation

Answer 21

• 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)

Question 22

Figure-group segregation

Answer 22

• 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

Question 23

Criticisms in Gestalt Psychology

Answer 23

• Most evidence descriptive
• Relies heavily on introspection and evidence from 2D drawings
• Some segmentation occurs by top-down prior knowledge which is ignored

Question 24

Object recognition deficits

Answer 24

• Agnosia = impairment in object recognition

- Different impairments arise depending on what stage object recognition damaged

Question 25

2 types of Agnosia

Answer 25

• Apperceptive agnosia

- Associative agnosia

Question 26

Apperceptive agnosia

Answer 26

• 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

Question 27

Associate agnosia

Answer 27

• 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

Question 28

Problems with face recognition

Answer 28

• Within-category discrimination = all faces look similar

- Faces important in evolutionary terms so may have specific mechanism

Question 29

Prosopagnosia

Answer 29

• 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

Question 30

Fusiform face area

Answer 30

-Part of ventral stream that recognises faces

Question 31

Holistic/configurable processing

Answer 31

• 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

Question 32

Visual expertise

Answer 32

• 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