Week 9 The Visual System

Question 1

Vision tings

Answer 1

• Far sense – light travels fast ~300,000km/sec
• Stimulus is EM energy – light source is usually the sun
• Allows us to identify, locate and react to things in the environment; allows
  communication
• Process of inverse optics – emitted light itself seldom the message of interest, the
  reflection of light off objects is more relevant
  o When light hits an object, some of it is absorbed and some is reflected
• Amount of light reflected
  o Off an object gives us a perception of lightness
• Pattern of light reflected
  o Gives us a perception of shape, texture

Question 2

Stimulus for vision

Answer 2

• Electromagnetic energy within the visible spectrum
• Stimulus properties
  o Intensity
• Perceptual submodalities
  o Intensity – gives brightness
  o Wavelength – gives perception of colour
   Visible spectrum – nM ~ 400-750nM
• Measure
  o Watt/m^2 or lumen/W or candelas/m^2
  o Log scales because expanded stimulus::intensity relationship
• Property of object
  o Reflected light = lightness measured in candelas

Question 3

The human eye

Answer 3

• 3 layers
  o Fibrous tunic
   Protection and shape – sclera (white of eye)
   Anterior portion is thinner (cornea) – helps refract light
  o Vascular tunic
   Vascular nourishment
   Pigmentation reduces light scatter – pigment determines eye colour
   Manufactures aqueous humour by ciliary body
  o Retina
   Stimulus detection and signal transduction
   Where sensory neurons are located
• 2 chambers
  o Anterior chamber
   Filled with aqueous humour made by ciliary bodies
   Pressure is important – ciliary bodies continually making liquid, filling up – need to drain the old fluid via ducts
   If pressure builds up it creates pain in the eye, pushing on the lens and optic nerve
   Glaucoma – imbalance in pressure
   Can permanently damage eye
  o Vitreous chamber
   Filled with vitreous fluid
   Can see when stuff gets built up in it

Question 4

Puplis

Answer 4

• Can accommodate (dilate, constrict) to alter amount of light entering eye
  o Bright – gets smaller
  o Dark – bigger
• Influenced by
  o Light levels
  o Autonomic nervous system
   Sympathetic nervous system – gets bigger to potentially see anything in environment to be ready for
   Parasympathetic – gets smaller, less likely to be something in enviro you need to respond to
   Emotional state
   Shows in pupils
   Paying attention, excited, romantic
  o Drugs
   Opioids – tiny pupils, impaired vision
   Cholinergic – dilate
  o Age
   As you get older you lose pupillary accommodation ability
   Change is smaller vs young people
   Lose ability – contributor towards presbyopia
   Sight of old age

Question 5

Presbyopia

Answer 5

• Sight of old age
• Lose pupillary accommodation
  o Things look darker – turn on all the lights
• Lose lens accommodation
  o Things become out of focus
• Increase in lens thickness
  o Things become blurry, out of focus
  o Reduced visual acuity
• Lose spatial packing of rods and cones
  o Become more spread out as you age
  o Lose visual acuity

Question 6

Visual Focus

Answer 6

• The optics of the eye function to refract waves of light onto retina
  o Cornea provides initial refraction
   Shape of cornea must be correct to get right refraction
   Can reshape cornea to bend light in a different manner to improve vision
  o Lens provides further refraction, ciliary muscles control accommodation
   Muscles alter shape of lens, pull it taught or make it fatter – contract and relax to accommodate
• Need to bend the light for maximum visual acuity
  o Bends through cornea and again through lens so the light converges on the fovea to have the sharpest resolution vision we can have
• In addition to refraction
  o The eyeball must be the right shape and size so the focused image falls onto the retina exactly
  o Eyeball needs to be the right length for the optics of the eye you’re born with
   If born with optical power from cornea and lens of a set amount, that doesn’t match eyeball shape, you might be near sighted or far sighted
   Eyeball too long – vision point of focus falls before fovea
   Eyeball too short – focus falls behind fovea
  o Put contact lens in front of eye to shape the refraction so it falls on the right part of the eyeball
• Lens accommodation decreases with age
  o Second contributor to presbyopia

Question 7

Lens

Answer 7

• Can accommodate (make more or less convex) to improve refraction of light

Question 8

Lens transparency is essential

Answer 8

o Reduced transparency = cataract
 Cumulative UV exposure – lens acts as filter to protect from UV, can
be damaged and lead to cataract – wear sunglasses
 Congenital – detrimental to visual cortex development
 Needs to be removed or visual pathways doesn’t develop fully o Cataract lens can be surgically removed
 Compensate with strong optical spectacles or replace with synthetic lens
 Refraction power of lens can be replaced, but accommodation lost

Question 9

When lens is taken out

Answer 9

o Some people start to have capabilities in UV light
o Usually UV is outside of visible spectrum – if take lens out, people can start to see UV, adds additional spectrum
o Things have glows to them – neural wise, brain has inbuilt capabilities to see UV light, but don’t see it as lens blocks it from entering eye

Question 10

The lens grows all throughout your life

Answer 10

o From age 0-90, there is !4x increase in thickness
o Third contributor to presbyopia
o Reduced visual acuity
o Things become blurry – also because of lost accommodation
o Need glasses to assist

Question 11

Retina

Answer 11

• Two odd features
  o Photoreceptors are located at the back of the retina
  o Light decreases the amount of neurotransmitter release
   There is baseline level of NT release from photoreceptors, when light hits them, it reduces or stops sending NT out

Question 12

Retina

2 subtypes

Answer 12

o Rods
 Peak absorption at ~500nm
 Activated between 400-600nm
 Very sensitive – can be activated by 1 photon
 Low acuity – rough, pixilated vision
 Key for night/scotopic vision
 Allows to see little specs of light
 Key for peripheral vision
o Cones
 Peak at !440, 530, 560nm
 Have different spectrums
 Less sensitive – need 100+ photons to activate
 High acuity
 Key for high-res colour/photopic vision
 Concentrated in macula and fovea
- Best visual acuity in the macula, worst acuity (no acuity) in the blind spot
o Where axons of retinal ganglion cells exit the eye at the optic disk

Question 13

Dark adaptation

Answer 13

• You can gain activity in scotopic vision if you are patient
  o Can make rods higher acuity if you give them more time in dark environment
• Any exposure to light breaks the dark adaptation of the rods – sudden return to poor
  acuity

Question 14

Cones

dark adaptation

Answer 14

o Absolute threshold – minimum light needed to activate
o Start at say 75 and drop to about 50 – need 50 photons of light to activate the cones still
 Won’t drop more than that – if less then 50 they won’t activate at all – will think it is dark there

Question 15

Rods (dark adaptation)

Answer 15

o As you spend more time in the dark the absolute threshold drops
o To a point where minimal amounts of light will be detected – sensitivity increases
o Can see in a room that was initially pitch black
o Any bit of light in the room will break the adaptation
 Go back to reliant on cones
 Except one form of light

Question 16

Long wave light

Answer 16

o Red spectrum light
 rods are not activated by EM energy that is >650nm (red)
o if these wavelengths are the only ones in your environment; only getting red light
 cones detect the presence of this energy and allow you to see
 Rods don’t detect this energy – they go into adaptation mode and
prep you for scotopic vision
 Cycle of dark adaptation occurs and get rod sensitivity
o Uses
 Training who need to go out in the dark in red light conditions
 Fighter pilots need to be able to fly at night
o Red spectrum light
 rods are not activated by EM energy that is >650nm (red)
o if these wavelengths are the only ones in your environment; only getting red light
 cones detect the presence of this energy and allow you to see
 Rods don’t detect this energy – they go into adaptation mode and
prep you for scotopic vision
 Cycle of dark adaptation occurs and get rod sensitivity
o Uses
 Training who need to go out in the dark in red light conditions
 Fighter pilots need to be able to fly at night
 Sit in a red lit room – rods accommodate but cones still see
 Eyes are ready to see in the dark if called to fly

Question 17

Oguchi’s disease

Deficits in dark adaptation

Answer 17

 Rare disorder where rod adaptation takes 3-4 hours
 Conformational change is slow
 Get impaired night vision, delayed scotopic vision

Question 18

Retinitis pigmentosa

Deficits in dark adaptation

Answer 18

 Eye disorder associated with genetic aetiology
 Lose visual acuity – dying cells
 Rods are initial victims – first to die off
 Impaired night vision and peripheral vision
 Lose two facets dependent on rod function
 Visual scene gets tunnel vision – progressively until can only see in
macula
 Eventually cones die off as well – completely blind

Question 19

Variations in retina

Answer 19

• Rod/cone distribution
  o Cones are concentrated in macula
  o Macula appears dark on scan thing because of concentration
• Thickness
  o Thinnest in specific region of macula – fovea
  o Foveal pit

Question 20

Spatial arrangement of rods/cones

Answer 20

o For best acuity, photoreceptors need to be tightly packed in an ordered fashion
o If not packed – spaces in a loose arrangement
 You get low resolution vision
 Diffuse looking scenes
 Pixilated almost, depending on how diffuse arrangement is
o Spatially diffuse in infancy, mature in 4 years, decline with age
 Rods and cones lose spatial packing – get more spread out as you age
 4th contributor to presbyopia
o Albinism exhibits permanent spatially diffuse photoreceptors
 Lack melanin – pigment in tunic later that helps light scatter
 Lack light scatter
 Rods and cones never become tightly packed also – things stay
pixilated from childhood

Question 21

Photoreceptor distribution

Answer 21

• Cones highest in fovea region of macula - give macula highest acuity for vision
• Macular degeneration most common cause of blindness over 50 years of age
  o Lose central visual field and start to get scotoma in centre of field
  o Absence of visual scene, lack of info
  o Blind spot that follows where you’re trying to focus on
  o Shifts with eyes
  o Gets bigger

Question 22

Blind spot

Answer 22

• Edme mariotte in dissection noticed variation in retina at site of optic disk
  o No photoreceptors on this spot
• Filling in perceptual completion demonstrated by Wallis, and Ramachandran
  o Simple patterns/textures/background are filled in
  o Don’t notice the spot
  o Complex items are not filled in
  o Illustrates top-down educated guesswork of brain
   Use info you have, to make a guess – can only make inference so far
   Can’t fill in a table or a face but can fill in pattern on a wall or floor  Fill it in with whatever is around it
  o Enables smooth, stable perception despite constant interruption in the incoming signal
   Perceptual stability – gaps in incoming signal but don’t perceive them

Question 23

Perceptual stability

Answer 23

• Frequent changes/gaps in sensory stimuli are not perceived due to compensatory
  neural mechanisms; perception remains stable/unchanged
• Examples
  o VOR
  o Blind spot filling
  o Blink suppression

Question 24

Blinks

Answer 24

• Clean and moisten the eye, reflexively protect
• Involuntary or voluntary
• Blink rates indicate interest/attention, increase during social interaction
• Last about 1/3 of a second
  o For half this time eyes are completely closed
  o We don’t experience brief blackouts every time we blink
   Not filling in
• Brain is suppressing visual processing before each blink
   It knows blink is coming – compensatory mechanism
   So that you don’t perceive the interruption in info

Question 25

Process of vision

Answer 25

- 3 part relay – trisynaptic - Convergence of info o Start with ~127 million photoreceptor cells converging to 1.25 million ganglion cells o Much fewer ganglion o Send somewhat already processed signal to brain 1. Electromagnetic energy hits photoreceptor cell o Signal transduction o Horizontal cell – signal to noise amplification 2. Bipolar cells detect the change in neural signal o Relays signal to ganglion cells o 2 types of bipolar – increment responsive and decrement responsive o Amacrine cells – signal to noise amplification 3. Ganglion cells send signal to brain o Via CNII

Question 26

Ganglion cells

Answer 26

- Each shows a receptive field for a specific region of space in the visual field - Single unit recording (1 fibre of optic nerve = 1 axon of 1 ganglion cell) o In dark, AP still being recorded – baseline activity o In lit room, as the stimulus moves across the screen, APs increase in one part of the grid and decrease in other parts

Question 27

Centre surround receptive fields

Answer 27

o Some are on centre – when light is in the centre the cell firing increases  For optimal FR – light in centre, dark in surround  Cell firing decreases when light in surround  Cell firing stays at baseline at most other times  If light everywhere it cancels out – increase activity but simultaneously decrease activity so net result is 0 o Some are off centre – when light is in the centre the cell decreases firing  For optimal FR – light in surround, dark in centre o Vision need changes in light, edges, differences in light and dark

Question 28

Ganglion receptive fields

Answer 28

o The centre and surround work in an antagonistic manner | o Optimal FR changes occur only when circumscribed light/dark regions detected

Question 29

Data processing in the retina

Answer 29

o Photoreceptor cells provide raw data on light levels in visual field o The raw data is overwhelming and not that useful/informative by itself o What is useful/informative is where significant differences occur in that data  More efficient if info going to brain has already been processed a bit  Want to know differences, not similarities – where receptive field picks up a higher level and lower in surround  A change occurs here  Ganglion cells send info when there is a change  Figure out where differences lie  Helps to find edges and boarders – allow to distinguish thing in visual field o Ganglion cells work to collate the photoreceptor data  127 mil bits of data - 1.2 mil ‘results’ o What we’re really interested in is differences in light levels within that visual field  Edges/boards/contrast - shapes - features  Centre surround processing helps pick up edges of objects instantaneously

Question 30

Basic visual processing

Answer 30

1. Process of vision 2. Retinal relay o Photoreceptor-bipolar-ganglion 3. Neural signal travels along CNII o Rearrangement at optic chiasm 4. 20% signals sent to superior colliculus – midbrain o Old visual pathway o Responsive to sudden shifts in light o Serves to orient head/eyes to visual stimuli o Shift attention and movement of eyes to sudden changes in visual environment o SC connects to muscles that controls eyes o Respond to sudden changes – can detect and shift attention - 80% of signals sent to lateral geniculate nuclei –thalamus o The LGN has distinct layers, each containing a retinotopic map o Spatial arrangement from retina maintained 5. Signal sent to V1 primary visual cortex o Retinotopic map maintained – left V1 receives info from right visual field (L+R eyes) and vice versa

Question 31

Visual cortex key features

Answer 31

- Retinotopic maps maintained in V1 - Hierarchical processing o Starts with basic info sent from V1-V2 etc to add on more complexity of info, building from bottom up - Concurrent processing o V1-V2-V3 etc. o Send info up hierarchy and at each step send info back that modulates further incoming signals going up o V1 also sends projections back to LGN thalamus  Circular processing

Question 32

Visual processing

Answer 32

- Visual processing occurs along two streams: - Ventral ‘what’ stream o What we are looking at – object perception, colour perception - Dorsal ‘where/how’ stream o Spatial – how is movement integrated

Question 33

Damage to V1

Answer 33

o Blindsight  Can't perceive what is in blind hemifield but can identify where an object is  Unconscious sight o Old SC pathway still active – up to dorsal stream  Helps you orient and gives conscious perception of where things are in the environment  Not necessarily attending to them but still processing

Question 34

David Hubel and Torsten Wiesel | Feature detection in V1

Answer 34

o Shone light across visual field of cat – recordings along visual pathway o Centre surround firing responses in ganglion cells and LGN thalamus  Ganglion cells have a bigger centre surround o V1 cells didn’t respond to patterns of light moving across  Nothing worked – different shapes  Accidentally left recording on during slide switching  Cell fired to switching of slide  Respond to movement going downward but not back up

Question 35

Cells in V1 | Feature detection in V1

Answer 35

o Found that V1 cortical cells show specific responses to specific stimuli  V1 cells exhibit feature detection o Some cells exhibit e.g.  Ocular dominance  They respond more strongly to stimulation in the left eye vs the right eye or vice versa  Orientation selectivity  They respond more strongly to horizontal bar stimuli or vertical bar stimuli  Direction selectivity  They respond more strongly to moving stimuli left or right or vice versa, or up to down or vice versa  Any many other forms of selectivity

Question 36

V1 cortical development | Feature detection in V1

Answer 36

o Feature detection in V1 is influenced by experience  Experience dependent plasticity e.g. monocular deprivation, selective rearing o Proper V1 development required visual input early on in life – critical phase  Multiple facets of vision mean multiple critical phases  Some critical phases for anatomy, some for functionality  Have to have experience in first few months of life of those features so the V1 cells become specific for feature detection  Don’t get exposure in critical time – won't develop feature detectors for what was lacking in environment o Need visual experience, exposure o Deficits in the eye or early visual pathway in infancy can impair proper develop of V1  Lazy eye – one eye doesn’t have as much acuity  Recognise one eye is weak and correct with glasses early on to teach weak eye to get visual input so hemifield develops

Question 37

V1 vision – a Cezanne painting

Answer 37

- V1 provides feature detection but this is not vision as we know it o V1 cells are very basic things o How do we take orientation and movement and lines and perceive objects and people and landscapes instantaneously? - Feature detection o Edges, boarders, contrast, shading o In black and white

Question 38

How do we add colour

Answer 38

o Incoming signal from retina contains coded info that relates to nM differences in the surrounding EM energy  Tri- di- monochromatic codes  Dichromatic – colour blind, see different colour spectrum  Mono – black and white o V1-V2-V4  V2 – contrast, contour, shading  V4 – decodes colour message o V4 is needed to translate this code into a colour qualia  Things become recognisable – can make assumptions about what you're looking at

Question 39

Damage to V4

Answer 39

o Cerebral achromatopsia = cortical colour blindness o Without colour vision at cerebral level – lost ability to decode colour messages

Question 40

How do we recognise objects

Answer 40

o Can make assumption about what you're seeing based on colour  Can detect colour differences to find things – can find bananas, tell if they are rotten or ripe

Question 41

Object perception | Bottom up

Answer 41

- Light detection-edge detection-feature detection-object discrimination- ? - object identification - Bottom up? o Pattern recognition o Do we put together all the basic elements/features/geons and recognise the pattern  Pattern X = object X  Take all the things and add together into a full object o Pro?  Logical – fits physiology and idea of hierarchical processing  Putting increasingly complex things together to build the whole object o AI robot vision  Our current best hope for developing robotic vision  Getting from discrimination to identification – what is the step in between?  Lack of AI visual perception o Con  Doesn’t account for speed of object perception  Don’t have to build the stages of what we’re seeing  We process very fast, if we had to build things up each time it would not be this quick

Question 42

Object perception | Top Down

Answer 42

o Theories for pattern recognition:  Brain formulates constant hypotheses about patterns present in environment  (stemming from experience)  Makes an educated guess  Then checks essential details to confirm o Would allow for rapid, efficient object perception  By allowing us to skip some of the tedious steps of visual data processing  Haven’t processed every bit of info but can still see the scene  Allows fast and fluid perception o In many instances we do seem to skip over the precise visual data  Perceptual set processing  Have a hypothesis already, check minor details, move on very fast  Make a guess about what the word is based on first and last letter  Can make sense of muddled information  Don’t pay attention to your own mistakes as you know what you're trying to say – other people can see them  Inverted face phenomenon  Eyes and mouth in wrong positions  Everything askew in actual sensory info but you don’t perceive it o Skipping over data  We have coherent visual percepts even if the exact sensory stimuli are:  Missing/wrong/ambiguous  Different in different situation o Size constancy  Interpretation is solid even if size is different  Have understanding based on knowledge of depth perception that a person far away is a normal sized human o Shape constancy  Turn an object to profile – know that when things turn the shape goes linear  Skip over visual data and have a coherent recognition even if things are wrong within visual scene  This suggests there is a strong top down influence on vision

Question 43

Object perception | Inference

Answer 43

o Based on previous knowledge gained through a lifetime of experience, we can make a best guess and this enables rapid visual perception o Most of our reality is our brain’s best guess  As long as it is close enough not to impact our survival it lets us function in the world  Allows fluid functioning

Question 44

Object perception | AI

Answer 44

o Robots lack this part of perception o Top down processing is hard to build  How to give them knowledge base  Need every single object in every shape and size and with obscurities o Let them learn these things  Play with things, pull them apart, move them around  Integrate with motor robotics to learn perceptual tactics o Merging motor and AI  Have to learn all these things throughout lifetime  Build a baby robot and teach from infancy about perceptual world

Question 45

Motion perception

Answer 45

- How do we add motion? Especially complex motion oSuperior colliculus-dorsal visual stream gives some info o V1 feature detection gives some info o V1-V2-V5/MT  V5 is needed to bind feature motion into global motion  STS – biological motion

Question 46

Damage to V5

Answer 46

o Lack fluid motion perception o Can't see movement in environment  See frames – brain updates position of things but can't see the movement

Question 47

Motion + Biological motion

Answer 47

1. Motion o Is there something moving in our visual scene o The pattern of motion helps with recognition o V5/MT 2. Biological motion o Is there something animate moving in our visual scene o Is it alive vs a tree o The pattern of motion helps with recognition o What is the critical components of the pattern: limb movement o Superior temporal sulcus STS

Question 48

Biological motion

Answer 48

- Recognising movement as animate/alive vs inanimate - Point light motion experiments o Stationary frames difficult to comprehend, but joint based motion almost immediately enhances our perceptual understanding of biological motion  STS activation o Can even discriminate identity, gender, emotion in dancing point light participants o Attention to biological motion develops ~4months, and does not decline with age  Inter-species characteristic - Applications o Motion capture tech and facial capture tech for actors  Makes the movie seem more real  Lights up STS – it is moving how a human would move

Question 49

Eyes in constant motion

Answer 49

- Yet we don’t perceive constant motion - Our eyes are moving o When we move, our eyes move with our body  Visual scene stays still  VOR o When we turn our head fast our eyes move fast  We don’t see a blur in visual field  VOR o When we dart our eyes around to read/drive/watch TV  We don’t see motion  Saccades, not VOR

Question 50

Saccades

Answer 50

o Rapid eye movements that occur between visual fixation points o Our eyes saccade around a scene – looking at a face we look everywhere  We don’t see motion - Why don’t we experience blurred vision during saccades? o Maybe they are too past to perceive ?  Saccadic movements ~20cm/sec – quite slow  But we can detect fast things o Maybe brain suppresses visual shift when due to a saccade  Motion sensitive cells in MT are silenced during saccades suggests brain monitors self-induced shifts in visual field

Question 51

Saccadic suppression

Answer 51

o Gives you perceptual stability o You don’t see the motion, sudden changes in stimuli are not perceived due to V5/MT suppression o Motion perception suppressed

Question 52

Examples of perceptual stability

Answer 52

- VOR - Blind spot filling in - Blink suppression - Saccadic suppression

Question 53

Driving and visual perception

Answer 53

- Driving itself is a massive feat of visual and sensori-motor integration - Self-driving cars o Can’t build a car that has the function of human visual perception o Can't tell the difference between a cat and a leaf - Driving involves a lot of visual motion processing o Constant saccades to scan scene o Tracking to track cars/pedestrians o Calculating distances, speeds, ect. o Predicting changes in distance, speed, etc. - Alcohol o Slows initiation and velocity of saccades o Impedes tracking o The brain tries to catch up by initiating fast saccades = jerky eye movements  Perceptual instability o Not only is your motor response time impaired but your ability to visually detect events in the first place is impaired

Question 54

Case Studies One

Answer 54

o Deets  79, male  Age related macular degeneration, early cataracts, significant vision loss  For last 6 months vivid hallucinations in scotomas  Road maps, wreaths, faces  Acknowledges not real – finds amusing o Evaluative tests  Visual acuity – vision 20/400 and 20/200  Macular degeneration  CT scan fine  MMSE 23/29  No drugs other than prescription o Diagnosis  Charles Bonnet Syndrome  Damage to any part along the visual pathway can lead to abnormal region of blindness in the visual field = scotoma  Levels of scotoma depend on where damage is in the pathway  Can have hallucinations populate tunnel vision as well – just not CBS  Specific type of hallucination  Visual hallucination in established scotoma  A release hallucination – a response to sensory deprivation  Lose input – those neurons become unmasked, released o Baseline levels no longer suppressed  When the brain doesn’t get sensory activity is starts to make its own  Wants input to come up to a normal level  Give yourself a perceptual world  Endogenously – complex hallucinations

Question 55

Case Studies Two

Answer 55

o Deets  38, male  Over last 8 months, episodes of macropsia and micropsia, including lilliputianism  Changes in shapes and sizes of objects, traces of objects left even after it is gone  More noticeable during stressful weeks at work, starting to impair function at worl  Acknowledges not real – but unnerving  Initially said this was recent but admitted it has been happening for years o Evaluative tests  Visual acuity – 20/30 and 20/20  Normal ophthalmic exam  CT scan fine  MMSE 29/30  Drug trace – alcohol and cannabis o Diagnosis  Alice in wonderland syndrome  Transient alteration in visual object perception due to transient changes in neural processing  Micropsia and macropsia – illusion/distortions  In the sensory cortices, changes in neuronal activity and neural processing can alter S&P  Alterations could be caused by  Migraine SD, epilepsy o Hyperactivity in brain regions  Neuronal injury o Lose perceptual capacity  Neurdegeneration  Electrical stimulation o Make neurons active to cause perception  Drugs o Heaps of drugs can cause alteration in S&P particularly at high doses, but only one drugs class does this intentionally – psychedelics

Question 56

Week 9 The Visual System