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Flashcards

Week 9 The Visual System

Imogen Taylor English 61 cards

Cards in this deck (61)

  1. 1QuestionAnswer
  2. 2Vision tings- 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; allowscommunication- Process of inverse optics – emitted light itself seldom the message of interest, thereflection of light off objects is more relevanto When light hits an object, some of it is absorbed and some is reflected- Amount of light reflectedo Off an object gives us a perception of lightness- Pattern of light reflectedo Gives us a perception of shape, texture
  3. 3Stimulus for vision- Electromagnetic energy within the visible spectrum- Stimulus propertieso Intensity- Perceptual submodalitieso Intensity – gives brightnesso Wavelength – gives perception of colour Visible spectrum – nM ~ 400-750nM- Measureo Watt/m^2 or lumen/W or candelas/m^2o Log scales because expanded stimulus::intensity relationship- Property of objecto Reflected light = lightness measured in candelas
  4. 4The human eye- 3 layerso Fibrous tunic Protection and shape – sclera (white of eye) Anterior portion is thinner (cornea) – helps refract lighto 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 chamberso 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
  5. 5Puplis- Can accommodate (dilate, constrict) to alter amount of light entering eyeo Bright – gets smallero Dark – bigger- Influenced byo Light levelso 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, romantico Drugs Opioids – tiny pupils, impaired vision Cholinergic – dilateo 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
  6. 6Presbyopia- Sight of old age- Lose pupillary accommodationo Things look darker – turn on all the lights- Lose lens accommodationo Things become out of focus- Increase in lens thicknesso Things become blurry, out of focus o Reduced visual acuity- Lose spatial packing of rods and coneso Become more spread out as you age o Lose visual acuity
  7. 7Visual Focus- The optics of the eye function to refract waves of light onto retinao 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 visiono 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 acuityo 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
  8. 8Visual Focus #2o The eyeball must be the right shape and size so the focused image falls onto the retina exactlyo 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 foveao 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 ageo Second contributor to presbyopia
  9. 9Lens- Can accommodate (make more or less convex) to improve refraction of light
  10. 10Lens transparency is essentialo Reduced transparency = cataract Cumulative UV exposure – lens acts as filter to protect from UV, canbe 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
  11. 11When lens is taken outo Some people start to have capabilities in UV lighto Usually UV is outside of visible spectrum – if take lens out, people can start to see UV, adds additional spectrumo 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
  12. 12The lens grows all throughout your lifeo From age 0-90, there is !4x increase in thicknesso Third contributor to presbyopiao Reduced visual acuityo Things become blurry – also because of lost accommodation o Need glasses to assist
  13. 13Retina- Two odd featureso Photoreceptors are located at the back of the retinao 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
  14. 14Retina2 subtypeso 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 visiono 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 spoto Where axons of retinal ganglion cells exit the eye at the optic disk
  15. 15Dark adaptation- You can gain activity in scotopic vision if you are patiento 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 pooracuity
  16. 16Cones (dark adaptation)o Absolute threshold – minimum light needed to activateo 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
  17. 17Rods (dark adaptation)o As you spend more time in the dark the absolute threshold dropso To a point where minimal amounts of light will be detected – sensitivity increaseso Can see in a room that was initially pitch blacko Any bit of light in the room will break the adaptation Go back to reliant on cones Except one form of light
  18. 18Long wave lighto 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 andprep you for scotopic vision• Cycle of dark adaptation occurs and get rod sensitivityo Uses Training who need to go out in the dark in red light conditions Fighter pilots need to be able to fly at nighto Red spectrum light rods are not activated by EM energy that is >650nm (red)
  19. 19Long wave light #2o 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 sensitivityo 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
  20. 20Oguchi’s disease(Deficits in dark adaptation) Rare disorder where rod adaptation takes 3-4 hours Conformational change is slow Get impaired night vision, delayed scotopic vision
  21. 21Retinitis pigmentosa(Deficits in dark adaptation) 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 inmacula Eventually cones die off as well – completely blind
  22. 22Variations in retina- Rod/cone distributiono Cones are concentrated in maculao Macula appears dark on scan thing because of concentration- Thicknesso Thinnest in specific region of macula – foveao Foveal pit
  23. 23Spatial arrangement of rods/coneso For best acuity, photoreceptors need to be tightly packed in an ordered fashiono If not packed – spaces in a loose arrangement You get low resolution vision Diffuse looking scenes Pixilated almost, depending on how diffuse arrangement iso 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 presbyopiao 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 staypixilated from childhood
  24. 24Photoreceptor distribution- Cones highest in fovea region of macula - give macula highest acuity for vision- Macular degeneration most common cause of blindness over 50 years of ageo Lose central visual field and start to get scotoma in centre of field o Absence of visual scene, lack of infoo Blind spot that follows where you're trying to focus ono Shifts with eyeso Gets bigger
  25. 25Blind spot- Edme mariotte in dissection noticed variation in retina at site of optic disko No photoreceptors on this spot- Filling in perceptual completion demonstrated by Wallis, and Ramachandrano Simple patterns/textures/background are filled in o Don’t notice the spoto Complex items are not filled ino 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 ito Enables smooth, stable perception despite constant interruption in the incoming signal Perceptual stability – gaps in incoming signal but don’t perceive them
  26. 26Perceptual stability- Frequent changes/gaps in sensory stimuli are not perceived due to compensatoryneural mechanisms; perception remains stable/unchanged- Exampleso VORo Blind spot filling o Blink suppression
  27. 27Blinks- 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 secondo For half this time eyes are completely closedo 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
  28. 28Process of vision- 3 part relay – trisynaptic- Convergence of infoo Start with ~127 million photoreceptor cells converging to 1.25 million ganglion cellso Much fewer gangliono Send somewhat already processed signal to brain1. Electromagnetic energy hits photoreceptor cello Signal transductiono Horizontal cell – signal to noise amplification2. Bipolar cells detect the change in neural signalo Relays signal to ganglion cellso 2 types of bipolar – increment responsive and decrement responsive o Amacrine cells – signal to noise amplification3. Ganglion cells send signal to brain o Via CNII
  29. 29Ganglion cells- 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 activityo In lit room, as the stimulus moves across the screen, APs increase in one part of the grid and decrease in other parts
  30. 30Centre surround receptive fieldso 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 0o Some are off centre – when light is in the centre the cell decreases firing For optimal FR – light in surround, dark in centreo Vision need changes in light, edges, differences in light and dark
  31. 31Ganglion receptive fieldso The centre and surround work in an antagonistic mannero Optimal FR changes occur only when circumscribed light/dark regions detected
  32. 32Data processing in the retinao Photoreceptor cells provide raw data on light levels in visual fieldo The raw data is overwhelming and not that useful/informative by itselfo 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 fieldpicks 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 fieldo 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
  33. 33Basic visual processing1. Process of vision2. Retinal relayo Photoreceptor-bipolar-ganglion3. Neural signal travels along CNIIo Rearrangement at optic chiasm4. 20% signals sent to superior colliculus – midbraino Old visual pathwayo Responsive to sudden shifts in lighto Serves to orient head/eyes to visual stimuli o Shift attention and movement of eyes to sudden changes in visual environmento 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 –thalamuso The LGN has distinct layers, each containing a retinotopic mapo Spatial arrangement from retina maintained5. Signal sent to V1 primary visual cortexo Retinotopic map maintained – left V1 receives info from right visual field (L+R eyes) and vice versa
  34. 34Visual cortex key features- Retinotopic maps maintained in V1- Hierarchical processingo Starts with basic info sent from V1-V2 etc to add on more complexity of info, building from bottom up- Concurrent processingo V1-V2-V3 etc.o Send info up hierarchy and at each step send info back that modulates further incoming signals going upo V1 also sends projections back to LGN thalamus  Circular processing
  35. 35Visual processing- Visual processing occurs along two streams:- Ventral ‘what’ streamo What we are looking at – object perception, colour perception- Dorsal ‘where/how’ streamo Spatial – how is movement integrated
  36. 36Damage to V1o Blindsight Can't perceive what is in blind hemifield but can identify where an object is Unconscious sighto 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
  37. 37David Hubel and Torsten Wiesel(Feature detection in V1)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 surroundo 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
  38. 38Cells in V1(Feature detection in V1)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 vsthe right eye or vice versa Orientation selectivity They respond more strongly to horizontal bar stimuli or verticalbar stimuli Direction selectivity They respond more strongly to moving stimuli left or right orvice versa, or up to down or vice versa  Any many other forms of selectivity
  39. 39V1 cortical development(Feature detection in V1)o Feature detection in V1 is influenced by experience Experience dependent plasticity e.g. monocular deprivation, selective rearingo 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 featuredetectors for what was lacking in environment o Need visual experience, exposureo 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
  40. 40V1 vision – a Cezanne painting- V1 provides feature detection but this is not vision as we know ito V1 cells are very basic thingso How do we take orientation and movement and lines and perceive objects and people and landscapes instantaneously?- Feature detectiono Edges, boarders, contrast, shadingo In black and white
  41. 41How do we add colouro 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 whiteo V1-V2-V4 V2 – contrast, contour, shading V4 – decodes colour messageo V4 is needed to translate this code into a colour qualia Things become recognisable – can make assumptions about whatyou're looking at
  42. 42Damage to V4o Cerebral achromatopsia = cortical colour blindnesso Without colour vision at cerebral level – lost ability to decode colourmessages
  43. 43How do we recognise objectso 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
  44. 44Object perceptionBottom up- Light detection-edge detection-feature detection-object discrimination- ? -object identification- Bottom up?o Pattern recognitiono 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 objecto Pro? Logical – fits physiology and idea of hierarchical processing Putting increasingly complex things together to build the whole objecto AI robot vision Our current best hope for developing robotic vision Getting from discrimination to identification – what is the step inbetween? Lack of AI visual perceptiono 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 itwould not be this quick
  45. 45Object perception Top Downo 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 confirmo Would allow for rapid, efficient object perception By allowing us to skip some of the tedious steps of visual dataprocessing Haven’t processed every bit of info but can still see the scene• Allows fast and fluid perceptiono 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 veryfast• Make a guess about what the word is based on first and lastletter
  46. 46Object perception Top Down #2• Can make sense of muddled information• Don’t pay attention to your own mistakes as you know whatyou'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 ito Skipping over data We have coherent visual percepts even if the exact sensory stimuli are:• Missing/wrong/ambiguous• Different in different situationo 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 humano Shape constancy Turn an object to profile – know that whenthings 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
  47. 47Object perception Inferenceo 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
  48. 48Object perception AIo Robots lack this part of perceptiono Top down processing is hard to build How to give them knowledge base Need every single object in every shape and size and with obscuritieso Let them learn these things Play with things, pull them apart, move them around Integrate with motor robotics to learn perceptual tacticso Merging motor and AI Have to learn all these things throughout lifetime Build a baby robot and teach from infancy about perceptual world
  49. 49Motion perception- How do we add motion? Especially complex motionoSuperior colliculus-dorsal visual stream gives some infoo V1 feature detection gives some info o V1-V2-V5/MT V5 is needed to bind feature motion into global motion STS – biological motion
  50. 50Damage to V5o Lack fluid motion perceptiono Can't see movement in environment See frames – brain updates position of things but can't see the movement
  51. 51Motion + Biological motion1. Motiono Is there something moving in our visual scene o The pattern of motion helps with recognition o V5/MT2. Biological motiono Is there something animate moving in our visual scene o Is it alive vs a treeo The pattern of motion helps with recognitiono What is the critical components of the pattern: limb movement o Superior temporal sulcus STS
  52. 52Biological motion- Recognising movement as animate/alive vs inanimate- Point light motion experimentso Stationary frames difficult to comprehend, but joint based motion almost immediately enhances our perceptual understanding of biological motion STS activationo Can even discriminate identity, gender, emotion in dancing point lightparticipantso Attention to biological motion develops ~4months, and does not decline with age Inter-species characteristic- Applicationso 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
  53. 53Eyes in constant motion- Yet we don’t perceive constant motion- Our eyes are movingo When we move, our eyes move with our body Visual scene stays still VORo When we turn our head fast our eyes move fast We don’t see a blur in visual field VORo When we dart our eyes around to read/drive/watch TV We don’t see motion Saccades, not VOR
  54. 54Saccadeso Rapid eye movements that occur between visual fixation pointso 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 thingso Maybe brain suppresses visual shift when due to a saccade Motion sensitive cells in MT are silenced during saccades suggestsbrain monitors self-induced shifts in visual field
  55. 55Saccadic suppressiono Gives you perceptual stabilityo You don’t see the motion, sudden changes in stimuli are not perceived due to V5/MT suppressiono Motion perception suppressed
  56. 56Examples of perceptual stability- VOR- Blind spot filling in- Blink suppression- Saccadic suppression
  57. 57Driving and visual perception- Driving itself is a massive feat of visual and sensori-motor integration- Self-driving carso Can’t build a car that has the function of human visual perceptiono Can't tell the difference between a cat and a leaf- Driving involves a lot of visual motion processingo Constant saccades to scan sceneo Tracking to track cars/pedestrianso Calculating distances, speeds, ect.o Predicting changes in distance, speed, etc.- Alcoholo Slows initiation and velocity of saccadeso Impedes trackingo The brain tries to catch up by initiating fast saccades = jerky eye movements Perceptual instabilityo Not only is your motor response time impaired but your ability to visually detect events in the first place is impaired
  58. 58Case Studies Oneo 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 amusingo Evaluative tests Visual acuity – vision 20/400 and 20/200 Macular degeneration CT scan fine MMSE 23/29 No drugs other than prescriptiono 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 – justnot CBS Specific type of hallucination Visual hallucination in established scotoma A release hallucination – a response to sensory deprivation Lose input – those neurons become unmasked, releasedo 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
  59. 59Case Studies Twoo 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 beenhappening 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
  60. 60Case Studies Two #2• Transient alteration in visual object perception due to transientchanges 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, epilepsyo Hyperactivity in brain regions• Neuronal injuryo Lose perceptual capacity• Neurdegeneration• Electrical stimulationo Make neurons active to cause perception• Drugso Heaps of drugs can cause alteration in S&P particularly at high doses, but only one drugs class does this intentionally – psychedelics
  61. 61Week 9 The Visual System

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Flashcards on Week 9 The Visual System, created by Imogen Taylor on 10/06/2022.
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Imogen Taylor · Jun 2022
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Jun 10, 2022
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