Neuroscience of perception

Question 1

Why is perception tough to find out about?

Answer 1

We are not aware of the processes that go on behind perception in the brain. We only have access to the results of these complex processes

Question 2

What is perception for?

Answer 2

Evolutionary adaptionist view: shaped by environment to optimize fitness
- Animals have undergone evolutionary changes in their brain to better deal with their environment and perceiving it

Question 3

How have animal perception changed evolutionary?

Answer 3

Olfactory sensitivity in dogs (“smell” in nose and brain)
Bat ‘vision’ - echolocation
Bird ‘vision’ - magnetic fields

Question 4

What do evolutionary changes help us to do?

Answer 4

To work with the environment specific to whatever has to be fulfilled

Question 5

What is perception for?

Answer 5

Have to ask: What is the problem that has to be solved?

• Have to work out what problem needs to be solved regarding perception e.g. light, noise
• E.g. for humans, perceiving faces may be more important than for a more solitary creature

Question 6

How is the visual system important?

Answer 6

• 2/3 of human brain is related to vision – important and vital for survival
• Humans are diurnal (daytime) - so vision is a vital sense
• We can perceive information around us at a distance
  o E.g. noticing predators, food
• But vision is much better at distance than other senses e.g. seeing a lion before licking it

Question 7

Does perception start with the stimulus?

Answer 7

o Not exactly
o If a tree falls in the forest…
o Don’t see things behind your head?
o Must being looking at a stimulus: “Attention”

Question 8

What happens as light passes through the lens of the eye?

Answer 8

• The image is inverted and focused onto the back surface of the eye – retina
• The stimulus must fall on receptors
• The information goes to the back of the eye and then goes back through where the information is being passed through the environments
• But we don’t “perceive” what’s on receptors e.g. the retinal image is inverted

Note: If you could re-design the eye, you wouldn’t do it like that. It’s opportunistic, not very intuitive. But it works

Question 9

What does the visual system do?

Answer 9

• Translates discrete points of light falling onto photoreceptors in the retina into meaningful objects we recognise
• Discriminates objects from other aspects of the visual scene i.e. background environment
• It recognises these objects in different orientations, even if it has only ever seen a car from the front view (even if only seen partially)

Question 10

How does visual information crossover?

Answer 10

• Information from the LVF projects to the Right Hemisphere
• Information from the RVF projects to the Left Hemisphere
• ~ 90% of optic nerve fiber’s project from retina=> LGN=>cortex
• ~ 10% of fiber’s go to subcortical structures like pulvinar nucleus and superior colliculus

Question 11

Where does visual processing start?

Answer 11

The back of the brain

Question 12

Where and why is visual information crossed over?

Answer 12

It gets crossed between the eyes so the left and right brain can work independently but also together

Question 13

Can you have eye dominance?

Answer 13

Yes. Extending the arms and triangle with hands check. You can have an ‘eyedness’

Question 14

How can you work out perceptual brain regions?

Answer 14

Intercranial stimulation

• Epilepsy patients (e.g. Wilder Penfield operations)
• Can activate sensations of smell, sight, touch, hearing, by stimulating the brain regions directly
• Perception happens in the ‘brain’, not in the receptors
• When you want to cut out parts of the brain which are linked with the epilepsy, but not the important parts
  o Stimulate the area, find out what it does, number them and move on
  o Stimulation would activate perception for individuals (e.g. smelling burned toast)

Question 15

What are the visual levels within the brain?

Answer 15

• Once visual signals enter the brain, perceptual mechanisms code various aspects of the information
• Low-level (edges)
• Higher-level (shapes)
• Complex-level (faces)

Question 16

What is retinotopicity?

Answer 16

• Size of receptive fields increases further along ventral stream
  o An early neuron might respond to edges only in specific point of space (more specific)
  o A later face neuron might respond to head direction anywhere attended (can be within a wider spot of the map you’re looking at)
• The stimulus needed for optimal cell activation becomes more complex along the ventral visual stream

Question 17

What are the 2 theories of visual processing?

Answer 17

1) Hierarchy hypothesis

2) Specialisation hypothesis

Question 18

What is the hierarchy hypothesis?

Answer 18

• Each successive visual area elaborates on the representation from previous areas
• Representations become more complex as they move along the visual system (lines => shapes => objects)
• However, a simple hierarchy doesn’t seem to exist in brain
  o Our perceptual systems are too complex
  o Multiple feedback loops, crosstalk, convergence, and divergence = multiple pathways and networks

Question 19

What is the specialisation hypothesis?

Answer 19

• Each visual area ‘represents’ different type of information – thus each area is specialised
  o V3 = form
  o V4 = colour
  o V5/MT = motion
• Together the visual areas form a ‘map’ of stimulus
  o Neurophysiological and neuropsychological evidence ‘generally’ supports this hypothesis
• Therefore, processing is both distributed and specialised

Question 20

What brain problems are there leading to colour deficits?

Answer 20

• Achromatopsia (‘without hue’) – damage to V4/colour
• Not the same as being colour-blind
  o The retina photoreceptors are working ok, but the brain isn’t processing the colour information
• Other visual aspects are intact (depth, motion, etc) so they can still ‘see’ and recognise objects normally

Question 21

What is achromatopsia?

Answer 21

• The world is drained of colour
• Like looking at a black and white TV
  o Shape and size seem normal
  o Sparing of retinal colour processing (eyes process colour information okay)
  o Object recognition, reading may be spared/unaffected
  o Problems with foods, plants etc. due to the loss of colour (food looks ‘bad’)

Question 22

What are neuropsychology double dissociations?

Answer 22

Colour vs Form
- Patient Mr. S (Efron, 1968)
- Unable to recognise common objects by sight
o But could tell about them from names or tough
o Presented with a ‘form perception’ deficit alongside intact knowledge about the items
o Could identify colours, perceive luminance contrasts, match surface textures, describe fine details, judge direction of motion, and localise objects in space
o Therefore, memory for objects and colour processing were intact, alongside a deficit in form perception

Question 23

What do clinical double dissociations suggest?

Answer 23

That there a differences in mechanisms and brain areas.
E.g. Achromatopsia: Colour=impaired, form=spared
Form Agnosia Colour=spared form=impaired
Supports the idea that these can be differentially impaired or intact, and are different brain regions

Question 24

How can motion deficits occur?

Answer 24

• Akinetopsia (‘without motion’) – damage to V5
• Patient M.P. had a selective deficit in motion perception – saw the world as a series of snapshots
  o But both colour and form processing were intact
  o So not a general perception problem
  o More apparent for faster moving stimuli vs slower
• Could see the objects in the environment around them, just not the movement of things
• More of a problem for faster moving stimuli than slower moving stimuli

Question 25

Who was patient LM?

Answer 25

o Colour vision and acuity remained normal
o There was no difficulty recognizing faces or objects, and no difficulty with stereo vision
o But LM cannot see coffee flowing into a cup
 It appears frozen like a glacier and they do not perceive the fluid rising - often lets the coffee spill or overflow
o Had damage to large area of posterior cortex outside PVC
o Damage included area V5

Question 26

What do fMRI’s of motion perception show?

Answer 26

• Shown moving (M) and stationary (S) dot displays, then subtract M – S brain activation to see area for Motion (V5)
• Activation also seen for apparent motion (Larsen et al, 2006)

Question 27

How to selective brain lesions affect specific function?

Answer 27

V1: If everything else in intact, people act like they are blind but visual input still gets through to other areas (V2) ‘BlindSight’
V2: Like V1 but less severe
V3: Perception of form us affected
V4: Vision in shades of gray (i.e. a colour deficit)
V5: Can’t perceive objects in motion

Question 28

What potential limits does the specialisation theory face?

Answer 28

• However, not all neurons within specialised areas respond selectively to that specific type of stimulus
• And some neurons within areas respond to multiple aspects (e.g. colour and form in V4)
• Maybe focus should be on function of the different brain regions and not description on what they respond to?

Question 29

What can neuronal adaptions demonstrate?

Answer 29

• We can see the sensitivity of specialised neurons for specific types of information through the use of behavioural adaptation paradigms
• The basic idea is that there are neural populations coding for specific types of visual information
  o But our ‘perception’ is comprised of a ‘balance’ of the neural activity across different populations
• If we can adapt certain neurons (i.e. make them less sensitive) then it should result in changes to our subsequent perception of that information

Question 30

What does the tilt after effect show?

Answer 30

• Perceiving left titled lines causes the neurons coding for left line orientation to fire more at first
• But over short time the neurons in early visual cortex coding for left tilted lines become ‘fatigued’
• They subsequently fire less, because after a while nothing interesting about them was going on for the brain!
• So your ‘perceptual balance’ in the brain is briefly shifted - dominated more by neurons coding for right line orientation (or shifted away from centre…)
• Neurons coding for right tilt orientation are not fatigued!

Question 31

What does the colour after effect show?

Answer 31

• The colour after effect is a perceptual illusion that gives impression of seeing the ‘shadow’ or ‘negative’ of the image you just fixated on
• Why does this happen?
• Theory has to do with the ‘opponent processing’ for colour information coming into the brain
• Red Green
• Blue Yellow
• When ‘red’ coding is fatigued, the perception is then briefly dominated more by activity of the opponent green coding neurons
• You would see red if adapted to green (opposite)

Question 32

Bottom-up versus Top-down processing

Answer 32

• Information coming from sensory receptors moving ‘upstream’
• Knowledge is “higher-order” - receptors are “lower-order”
• Computer vision
• Goal: perception based entirely on bottom up analysis
• Still problematic: biggest problem might be scene understanding

Question 33

How is perception knowledge-dependent?

Answer 33

o Can be drawn in the minds eye when missing parts of knowledge
o Gestalt grouping
o Continuation
o Computers find this task difficult to do – don’t have the top-down processing to fill in the gaps
- Perception is more than the stimulus
o It’s a constructive creative process

Question 34

Can we dissociate perception and recognition?

Answer 34

o Yes and no
o Brain damage: Visual agnosia
 Vision without knowledge
 People with agnosia can copy objects they don’t even recognise
 Their perceptual systems are intact to copy it, but they do not know what they are copying

Question 35

What is perceptual modularity?

Answer 35

• Certain bottom-up perceptual processes are not normally influenced by knowledge e.g. flash of light

Question 36

What us cognitive impenetrability?

Answer 36

Visual illusions still occur despite knowing they are illusions

Question 37

What is cortical blindness?

Answer 37

• lesions to the Primary Visual Cortex results in blindness
• The extent of the blindness and where it is in visual field, relates to the location and size of the cortical damage
• blindness occurs on opposite side of visual field to damage (crossing)
• patients report not being able to see anything within the blind area

Question 38

What is blindsight?

Answer 38

• Even though a patient is ‘phenomenologically’ blind
• They can often point to stimuli they can’t see if forced to e.g. motion, threat
• And they are better than chance when guessing about stimuli

Question 39

What can blindsight show?

Answer 39

• Blindsight illustrates the existence of parallel circuitry for processing visual information - can guide our actions
• Implies we don’t need to be conscious of stimuli in order to act on that stimulus

Question 40

Is awareness necessary for perception?

Answer 40

• Subcortical visual pathway
  o ~ 10% of visual fibers go to other subcortical structures like:
   pulvinar nucleus
   superior colliculus
  o More ancient visual system - preconscious?
  o So, we don’t apparently need awareness for perception!

Question 41

How can we ‘see’ without awareness?

Answer 41

• Video: Man with complete visual lobe destruction couldn’t ‘see’ any part of the visual field but managed to not walk into any objects in the corridor
• Could be using other senses (However, they tried to control for this)
• It is not necessarily the case that blindsight is mediated solely by subcortical brain regions
• Perhaps some residual activity or connections still available in ‘part’ of the visual cortex?
• Perhaps some visual information is passed to the visual cortex beyond V1 from other brain regions