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5PAHPSIP: Distorted Worlds revision notes
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Distorted worlds lecture notes- good summary of each lecture and information is organised based on learning outcomes
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LECTURE 1: CAN YOU BELIEVE WHAT YOU SEE?1.1: Understand the reasons • Charles Bonnet Syndrome—occurs when central visual field degenerates—can
behind our perception of visual sometimes result in rich visual hallucinations
illusions - Evidence shows spontaneous nervous activity in brain associated w/facial
processing which causes perception/hallucination
• Common misconceptions about vision:
- Automatic + effortless process
- Vision sends exact copy of what is in front of you to brain
- We perceive a rich + continuous visual environment—we only see things in
detail and in colour that we fixate upon→ evidence from this comes from retina
(photoreceptors: rods and cones)—
◼ RODS: Work in low light conditions—vision in dark is B+W + unable to
process colour info
◼ CONES (found primarily in fovea): Sensitive to colour—enable powerful
acuity and only work when we have lots of light
- Graph shows acuity across the visual field—there is no acuity at blind spot
bcos there are no rods and cones (light receptor cells)—i.e. we only get rich
colour info based on what we fixate upon
- Evidence saccades (quick simultaneous movement of both eyes between two or
more phases of fixations in same direction) help build up a picture
- Transsaccadic memory buffer enables us to take forward visual info from one
saccade to the next saccade→ however would take up large cognitive demand
as we would be building up picture through many individual fixations
- Evidence for this comes from change blindness— eyes fixate parts of
interest (people, sphinx)→ if distracted by flicker miss change then evident
that eyes are not building up a veridical (i.e. true representation) image of a
scene through saccades
How does vision work
,• Vision= hierarchical (i.e. low lvls/aspects of visual input is dealt with separately to
high lvls/aspects of visual input) and modular (i.e. different modular structures of
brain deal w/different aspects of visual processing)
• But processing is in parallel + projections feedback as well as feedforward
• V1/Primary visual cortex (striate cortex) = cortical area of brain where most input
is directed
- Vision= retinatiopically mapped—i.e. damage to V1 associated with damage to
sight in associated field
- Loss of V1= cortical blindness
- Damage to part of V1= blindness for corresponding part of visual field
(hemianopia)
• V4= Evidence suggests area is involved in colour perception
- Damage to both V4s (rare) can lead to loss of colour vision + inability to
identify/discriminate colour—however still preserved processing of form +
motion
- Rich et al (2006)—
- Group of synaestheses perceived a colour when saw particular letter
e.g. saw blue letter even if it wasn’t blue – there was cross between
colour + form info
- Investigating whether perception of seeing colour implicated neural
processing for both coloured and greyscale letters
- Findings showed synaestheses and non synaestheses activated V4
(blue) for coloured letters and (red) grey letters—shows ppts perceive
colour THEN brain area activated
• V5= Deals w/motion info
- Damage to V5s→ motion blindness (perceive things as static images)
- TMS studies on V5 also reveal its role (e.g. Beckers and Zeki, 1995)
Visual pathways beyond early visual cortex:
• VENTRAL (‘What) STREAM—
- Where object info occurs→ human face recognition (fusiform) + colour
recognition
- Damage to ventral stream can result in visual agnosia
• DORSAL (‘Where) STREAM—
- Process spatial info (i.e. where are things in relation to each other)
- Damage to dorsal stream can lead to visual spatial neglect or optic ataxia= can
be seen when ppl attempt to reach for object but cannot even though they are
aware of object
,1.2: Describe and define visual • VISUAL AGNOSIA= ‘Failure to know’ visual info—i.e. can no longer identify by
agnosia sight
- Visual agnosia can only be detected if patient has no problem w/memory or
lang skills needed to produce words but unable to detect info purely through
sight
1.3: Understand evidence Challenges for visual perception:
suggesting that ‘normal’ visual - Input often insufficient (i.e. things can often be excluded from vision),
perception is not an accurate ambiguous + overwhelming (i.e. vast amount of visual input)
representation of the world - ‘The Blind Spot’—demonstrates brains ability to ‘fill in’ which leads to over
around us interpretation of incoming stimuli using laws of occlusion even if this is not
true representation of what we actually see
- Illusory contours—
- Automatic computation of occluders—Sometimes occluders are added to
create meaningful shape→ occuluders show how visual formation forms
conscious perception
1.4: Describe the progression of • Ambiguous input: Binding the correct parts together—visual system binds
visual input from the eye to together different aspects of objects to help differentiate things→ ambiguous
complete conscious perception input demonstrate that this is v. cognitively demanding task
and understand how this process - Cannot see both percepts at same time— visual system designed to only see
might differ from common one at a time→ e.g.
misconceptions of vision
- Bind visual input together to form meaningful object
• Gestalt principles:
- Wertheimer (1923) wanted to look @ rules of vision that help us bind
elements of visual input together→ e.g. using rules of similarity, proximity,
continuity and closure to bind visual elements to understandable whole
, - However, we now use fMRI to look @ visual processing within brain
• Ambiguous input: The importance of context
- We have evolved mechanism to use context to help resolve ambiguities
associated w/ visual input
- Context can help resolve illusions such as ebbinghaus illusion—i.e. where
both orange circles are same size but harder to identify with surrounding
blue circles (dorsally we know size of circles but ventrally w/context we don’t
know)
- e.g. Ponzo illusion—knowing that the lines are the same size does not
change how we perceive them visually (top line looks longer)
• Overwhelming input:
- Sometimes vast amount of visual info may not allow us to process all visual
input—therefore important selection mechanism= attention
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