Vision Flashcards
Etymology of psychology
Psychology = psyche + logos
Psyche - soul or mind
Logos - word or study
Therefore it is the study of the mind
Three truths of psychology
- ‘It depends’ - Hardly anything is true about the behaviour of all people all the time
- ‘Process depends on good measurement’ - New discoveries and ideas depend and advance on good measurements
- ‘Confidence in the conclusions should depend on the strength of the evidence’
Etymology of cognition
Means thinking and knowledge
From latin conogoscere - get to know
Psychopathology definition
An abnormal pattern of behaviour that is unusual, distressing, dysfunctional, and may cause the sufferer to be dangerous to self or others
Structuralism
The analysis of mental structures
Introduced by E.B. Tichener
Functionalism
Studying how the mind works to enable an organism to adapt to and function in its environment.
Introduced by William James
Gestalt Psychology
Perceptual experiences depend on the patterns formed by the stimuli and on the organisation of experience
Introduced by Max Wertheimer, Kurt Koffka and Wolfgang Köhler
Psychoanalytic perspective
An orientation toward understanding behaviour in terms of unconscious motives stemming from sexual and aggressive impulses.
Subjectivist perspective
Human behaviour is a function of the perceived world, not the objective world.
To understand human social behaviour, we must grasp the person’s own ‘definition of the situation’, which is expected to vary by culture, personal history, and current motivational state.
What is the distal and proximal stimulus
Distal stimulus - any object in the world
Proximal stimulus - Image of an object in our eyes
Iris
The coloured structure surrounding the pupil
Pupil
Aperture to allow light into the eye
Cornea
Transparent structure on the outer surface of the eyeball. It yellows with age and gets scratches on it
Lens
Structure that focuses light into and onto the back of the eye. Have scratches on them and lens muscles weaken with age.
Retina
Contains light sensitive cells called photo-receptors
Fovea
Small part of the retina that contains the majority of photoreceptors - allows detailed and coloured vision
Cells in the retina
Cones and rods: receive light
Horizontal cells: connectivity with the retina
Bipolar cells: connectivity with the retina
Amacrine cells: connectivity with the retina
Ganglion cells: receive input from cones and rods and carry information out of the eye
How many cones and rods are there in the retina
6 million cones and 120 million rods
The highest concentration of cones is towards the centre of the retina.
The fovea only consists of cone cells - vision becomes weaker towards our peripheries
Cones
Photopic visual system Found mostly in and near the fovea Works best in intense light Detects high wavelengths - bright blues, reds and greens. Detects high frequencies - fine detail
Rods
Scotopic visual system Found mostly in the retinal periphery Works best in low light conditions Detect low wavelengths - greys Detect low frequencies - coarse details
Dark adaptation
Gradual improvement in ability to see in the dark
Takes approx 20 mins
Only rod mediated vision is sensitive enough to detect low levels of light
Light adaptation
Gradual improvement in ability to see in bright light
Takes approx 5 mins
Only cone-mediated vision is possible - rods are bleached in bright light
Why is it hard to read at night
- Retina is not uniform
- Retina contains mostly rods
- Rods only work in dim light, colour and detailed vision are not possible so we can’t read
- Fovea is almost blind in dim light
Direction of impulses in the retina
Photoreceptors send impulses via bipolar cells to the ganglion cells (neurons).
Axons from all the ganglion cells form the optic nerve
The optic nerve is the only output from each eye to the brain.
It leaves each eye with a small hole in the retina
The blind spot
Hole in the retina where all the axons leave is called the blind spot - no photoreceptors here
Same are that blood vessels enter the eye - causing many small blind spots where there are shadows of the blood vessels in the retina
We aren’t aware of our blind spots with what is most likely to be there
The optic chasm
- Half of the optic nerve from each eye crosses to the opposite side of the brain at the optic chasm
- Stimuli on the left visual field project mostly to the right visual cortex
- Stimuli on the right visual field project mostly to the left visual cortex
Lateral Geniculate Nucleus (LGN)
- Most axons of the optic nerve send information to the LGN before reaching the primary visual cortex
- Some axons send information to a smaller structure in the thalamus called the superior colliculus
Primary Visual Cortex
- At the back of the occipital lobe in the cortex
- Primary because it is the first place in the cortex where the visual information lands
- Also known as the striate cortex or V1
- Visual info is processed further to allow us to perceive the visual scene and interpret what we see.
Cortical magnification of the fovea
- Majority of the optic nerve carries info from the fovea to the brain
- Info from the fovea is richer than from other retinal areas (high acuity, colour vision) so it requires a larger cortical area.
- Fovea is over-represented in the cortex relative to the retina (0.01% of retina, 8% of cortex) - this is the cortical magnification of the fovea.
Path of visual image
- Info is received at our eyes
- Travels down the optic nerve
- Crosses at the optic chasm
- Some stops at the LGN and some stops at the superior colliculus
- Is received by the primary visual cortex
What is the dorsal and ventral visual stream
Dorsal visual stream - Allows us to perceive movement
Ventral visual stream - Allows us to perceive patterns and objects
What arrives from the eyes to the visual cortex
- A flipped image
- Colour info comes mostly from the fovea
- Info from the fovea is magnified by 50x approx
- Takes approx 100-150 milliseconds to get to V1 so is considered ‘late’
3 properties of colour
Hue - colour quality of the light and corresponds to the colour names we typically use
Saturation - purity of the light
Brightness - amount of light present
Hue
Our perception of hue is not directly related to the nature of wavelengths hitting the retina, it is created by the workings of our nervous system
Brightness
Brightness is a psychological concept. The perceived brightness of a surface does not necessarily correspond to the actual intensity of the wavelengths detected by our retina.
Saturation
Vividness of a colour diminishes the longer you stare at it although the colour is still the same.
Colour mixing
Make any colour in the spectrum by mixing and adjusting 3 different colurs.
Can’t be done by less than 3 - red, yellow, blue
The trichromatic theory
- Refers to the 3 types of cones in the retina that allow us to see colour
- Cones that respond to short wavelengths enable us to see blue
- Cones that respond to medium wavelengths enable us to see green
- Cones that respond to long wavelengths enable us to see red
- Wavelengths coming in that maximally stimulate the red and green cones are perceived as yellow
Red-green colour deficiency
- Lost or limited functions of the red cone pigment (protan) or the green cone pigment (deuteran)
- Caused by a recessive gene on the X chromosome
- Majority of people with a colour deficiency can still see colour
Blue-yellow colour deficiency
- Rarer
- Caused by missing or limited colour functions of the blue cone photopigment (tritan)
Rod monochromatism
- Very rare hereditary condition
- No functioning cones
- Ability to perceive only in white, grey and black tones
- True colour blindness
- Poor visual acuity
- Sensitive to bright light
Opponent process theory
- Trichromatic theory can’t account for all aspects of colour perception
- Wavelength information is passed from cones to specific ganglion cells (G cells)
- Some G cones process differences between L and M cones which are responsible for our perception of red or green.
- Other G cones process differences between S cones and a combined signal from both L and M cones - giving us yellow or blue
Negative after images
Ganglion cells receive input from cones and opponent cells respond to two types of wavelengths, but in an opposing manner.
- Some respond to yellow or blue, but not both
- Some respond to red or blue, but not both
Recognition of an object (tennis ball example)
Super-ordinate level - tennis ball is recognised as ‘an inanimate object’
Basic level - tennis ball is recognised as ‘a ball’
Sub-ordinate level - tennis ball is recognised as a specific token e.g. ‘tennis ball’ or ‘Barney’s ball’
Stages in object recognition
- Perception of features - visual system detects features e.g. colours, edges, lines
- Perception of groups - individual features are grouped into simple figures that are distinct from the background of other features
- Recognition - Matching the visual percept to memory. The groups of features are matched to existing representation in long-term memory
What are the first available features in vision?
Edges
This is due to the difference in light being the first and simplest feature in vision - known as a luminance edge
How do we detect edges?
- Ganglion cells in the retina
- 1 mill ganglion cells per retina, G cell axons are the only output from the eye
- Axons form the optic nerve
- Place where axons leave the retina is the blind spot
What is a magno ganglion cells? (magnocellular)
Receive input from many rods and cones, which means that they have large receptive fields (RFs)
What are parvo (parvocellular) ganglion cells?
Receive input from very few cones which means that have very small receptive fields (RFs)
Ganglion cells’ receptive fields
- Each has their own receptive field
- Whole area of the world that we can see at any one time is our visual field
- Have a left and a right visual field
- Part of the visual field to which any single neuron responds to is that neuron’s receptive field
What are on-centre ganglion cells
When the central region of the ganglion cell is excited and the surrounding region is inhibited by light.
Strongest response happens when light falls directly onto the central region.
When light falls to the surrounding regions the cell doesn’t fire as much due to receiving both excitatory signals and inhibitory signals which means there is no activation
When light covers the entire RF, the cell is barely active
How do ganglion cells detect edges?
- G cells far from the edges have low or no activity
- At and around the edges the activity is either increased or decreased
- When an increase or decrease is detected the cell is active. This activation is then transmitted down the visual system
What evidence do we have for the existence of specialised cells?
- Bains of other species contain cells with the properties of feature detectors
- After staring at certain patterns, we see after effects that imply fatigue of feature detector cells in the human brain
Hubel & Wiesel 1968
- Inserted thin electrodes into the occipital cortex of cats and monkey
- Recorded activity of the cells when various light patterns struck their retinas
- Used points of light which produced little response and then used lines
- Some cells became active only when a vertical bar of light strikes a given portion of the retina, others only for a horizontal bar
- Discovered different layers in V1, each containing different types of cells, and each type of cell was sensitive to different features
What are simple cells?
- Have elongated receptive fields
- Makes them maximally sensitive to a line or edge of a particular orientation at a particular location of the retina.
What are complex cells
Respond strongly to lines of a particular orientation moving in a particular direction
What are hyper-complex/end-stopped cells?
- Respond best to lines of a particular length and moving in a particular direction
- Also responds to moving corners or angles
- Some hyper-complex cells fire when a line ends in their receptive fields - good size detectors
What are blobs?
- Discovered by Margaret Wong-Riley in 1979 by using a cytochrome oxidase stain
- Groups of neurons found in the visual cortex
- Neurons in the blob are sensitive to colour and have no orientation preference or ocular dominance
- Receive input from parvocellular cells in layer 4CB of the primary visual cortex and output to the thin stripes of area V2
Motion aftereffects
- ‘Waterfall illusion’ was first described by Aristotle
- Still lacks a satisfactory explanation
- Suggests that we have cells that specialise in detecting the motion direction of lines
- Some versions can survive 24 hours between adapt and test. You can see the after-effects on any stationary surface
Colour aftereffects
Suggest that we have cells that specialise in detecting different colours
Summary of feature detection
Orientation - cells in V1
Size - cells in V1
Colours - cones in the retina, then V1, then V4
Motion and direction - rods in the retina, then V1, then V5
Gestalt psychology in perception
- Our ability to perceive something in more than one way - our ability to perceive overall patterns
- Main premise was that perception cannot be broken down into its component parts. A melody broken up into individual notes is no longer a melody.
- Feature detectors are not enough to explain perception
What are bottom-up processes
Tiny elements (features of the visual world) are detected and combine to produce larger items e.g. feature detection
What are top-down processes
Where we apply our experiences and expectations to interpret what each item must be in the current context
What are the Gestalt principles of perceptual organisation
Several principles of how we organise perceptions into meaningful wholes
Proximity - tendency to perceive objects that are close together as belonging to a group
Similarity - tendency to perceive objects that resemble each other as a group
Good continuation - when lines are interrupted we perceive continuation, a filling in of the gaps
Closure - when a familiar figure is interrupted we perceive a closure of the figure; we imagine the rest of the figure to see something that is simple, symmetrical, or consistant with our past experience
Common fate - we perceive objects as part of the same group if they change or move in similar ways at the same time
Good figure of Pragnanz
What happens when grouping processes don’t work?
- Ability to perceive objects breaks down causing visual agnosia (inability to perceive patterns)
- Visual agnosia is not blindness, only the inability to group simple visual features together
What is the lateral occipital cortex?
- Located in the temporal lobe somewhere between V4 and IT
- Very important for object recognition
- Neurons in LOC process simple groups of features - squares, triangles, circles that correspond to object parts.
What is visual apperceptive agnosia?
- Damage to the LOC following stroke or carbon monoxide poisoning leads to this
- It is the inability to perceive any object through visions
- Patients with damage to or around the area of the LOC only can detect simple features e.g. lines but can’t
perceive simple forms - Tested using copy drawing and perceptual matching tasks
- Patient’s visual system has lost the ability to group different features into a single form or figure
What are our perceptual responses to a stimulus?
Detecting - Becoming aware of a barely detectable aspect of a stimulus
Perceiving magnitude - being aware of the size or intensity of a stimulus
Recognising - placing a stimulus in a specific category
Describing - indicating characteristics of a stimulus
Searching - looking for a specific stimulus among a number of other stimuli
What is psychophysics?
The study of the relationship between physical stimuli detected by our senses, and our psychological responses to them
Typical psychophysics experiment
- Participant is presented with stimuli at varying intensities
- Would report whether they can detect the stimulus or not
- Typical graph is S curve
Signal detection theory
The study of people’s tendencies to make certain responses when asked to detect the presence of a physical stimulus
Responses are known as:
- Hits (stimulus is present and person correctly reports it)
- Correct rejections (stimulus is absent and person correctly reports it as such)
- Misses (target was present but person missed it)
- False alarms (target was absent but person said it was present)
What determines our answers in signal detection?
Our answers and perceptions are determined not only on what is detected by our senses, but also on the task instructions and our strategies
What is visual associative agnosia?
- The inability to recognise an object by sight - damage to the memory stage (area IT) of object recognition
- Tested by asking patients to name objects or categorise objects
What are the stages in object recognition
Feature detection - visual system detects features e.g. colours, edges, lines
Grouping - individual features are grouped into simple figures that are distinct from the background of other figures
Recognition - matching the percept to memory
What is subliminal perception?
- Limen is latin for ‘threshold’, therefore, subliminal means ‘below the threshold’
- Idea that stimuli can influence our behaviour even when they are presented so faintly or briefly that e do not perceive them consciously
What can’t subliminal perception do?
Control people’s buying habits
What can subliminal perception do?
- Dimberg, Thunberg, & Elmehed (2000)
- Happy or sad face flashes on screen for less than one thirtieth of a second
- No one reported seeing a happy or angry face
- When happy face flashed, participants slightly and briefly moved their facial muscles in the direction of a smile
- After the angry face flashed, participants tensed their muscles slightly and briefly in the direction of a frown
- So it can influence our physiology
Stage 1 of object recognition: Feature detection
Recognition of objects begins with the detection of simple features:
Colour - cones in the retina, then V1, then V4
Motion - rods in the retina, then V1, then V5
Lines - ganglion cells in the retina, then V1
Orientation - cells in V1
Stage 2 of object recognition: Grouping of features into forms
- Features extracted from the retinal image need to be grouped in a way that will allow us to know which lines, colours, and motions belong to which object
- Grouping is mainly done in structure further down the ‘what’ processing stream e.g. Lateral Occipital Cortex
- Neurons in LOC are best activated by simple groups of features
Stage 3 of object recognition: Recognition
- Visual system has extracted simple features in the image and has used innate principles to organise the features into separate groups
- At stage 3 you now only need to recognise the object by matching the percept to the representation of the object we hold in long-term memory
What is the IT in the temporal pathway (what pathway)
- Neurons in this are have 10x larger receptive fields than neurons in V1
- Cells are activated by combinations of complex forms with colours and textures
- Often respond to a specific category e.g. objects, places, faces, hands etc
- Cells exhibit perceptual constancy
Response is the same independent of:
- The location of the object’s image on the retina due to large bilateral receptive fields
- The size of the image
- The cue that defines the objects shape
What do we know from brain imaging
- Which parts of the brain detect the features important for object perception and recognition
- The parts of the brain that are important in grouping these features and for object recognition
What do we know from behavioural studies
How the visual system organises the percepts into different objects using Gestalt laws of perceptual organisation
What don’t we know from object recognition
How we can recognise objects despite drastic changes in shape, size or more importantly viewpoint
How do we recognise objects, regardless of changes in viewpoint?
2 theories:
View-independent theories e.g. recognition by components
View-dependent theories e.g. view-based theories
What is recognition by components?
- One of the most influential theories of object recognition
- Biederman, 1987
- Every object can be described in terms of simpler forms called ‘geons’, and their configuration
- 36 geons are sufficient to describe over 1 million objects
- Objects are perceived and stored in memory as a collection of distinct geons in specific spatial configurations
- Recognition will be accurate regardless of viewpoint as long as one can see the objects’ components and their spatial configuration
What are non-accidental properties (NAPs)
- Geons are defined in terms of their non-accidental properties
- The 36 geons are different from each other in terms of their NAPs
- NAPs are very simple shape features that couldn’t have occurred in the image by accident
NAPs include:
- Curvature: points on a curve
- Co-termination: edges terminating at a common point
- Parallelism: sets of points in parallel
- Co-linearity - points along a straight line
How to identify the geons of an object
- The NAPs that define geons are visible from most viewpoints
- Geons can be identified when viewed from most viewpoints, unless viewed from a rarely occurring viewpoint
View-based theories
- Each object is represented in terms of a few learned viewpoints
- Recognition of any object is the result of a match between the current view and the views of the same object already stored in memory
- Recognition speed and accuracy will depend on the deviation of the perceived view from the view(s) of the object stored in memory
Evaluation of RBC
- Other features in addition to geons can help us identify objects e.g two birds of the same shape can have different feathers or markings
- Some objects, such as faces and shoes, are difficult to describe in terms of geons
- Evidence that object recognition is not completely viewpoint invariant
Which object recognition theory is correct?
- Depends on the task and stimuli
- When we need to make categorical discriminations (cars vs. bicycles) then it seems to be view-independent
- When the task requires difficult discriminations within a category (e.g. between different types of car or different cups) than we use view-dependent theories
What is face pareidolia?
We tend to ‘see’ face, in chance arrangements of objects and parts
What is the face inversion effect?
We are much poorer in accurately perceiving faces that are upside-down to when they are upright
Test to see whether inversion effects works with other objects
- In the study phase, patients studied pictures of faces or other objects, e.g. houses or vehicles, one at a time
- In the test phase, participants saw 2 pictures simultaneously, one that was studied and a new one
- Task was to select the one they had previously studied
- Results were that inversion disrupted face recognition, but not object recognition
Why are we poor at accurately perceiving inverted faces?
- Objects are perceived and recognised on the basis of the shape of their component parts and their spatial configuration
- Faces are perceived on the basis of the overall configuration of features, but the features themselves are not accurately perceived
- When faces are inverted, the configuration of features breaks down
- We now switch to noticing individual features to make perceptual decisions
- Relying on individual face features is problematic because we aren’t very good at perceiving them
What does the ‘Thatcher Illusion’ tell us about how faces are perceived?
- Thatcher illusion, amongst other inversion effects, tells us that parts of the face are not processed independently but in the context of the face
- These effects show that we process faces holistically or configurally and not in terms of their individual features alone
What is prosopagnosia
- Inability to identify faces by vision
- Type of associative agnosia
- Sufferers can see and describe a face, and accurately make same/different judgements and categorise faces according to gender
Preserved ability to identify most other objects well - Often patients have an emotional response to familiar faces
- Faces have a lot of similarity between them, making them difficult stimuli to recognise, causing a ‘specialised’ breakdown in recognition
Are faces special: Evidence from fMRI studies
- Area in the brain for face processing known as fusiform face area (FFA)
- Amplitude of brain activity in response to photos of faces is greater than for places
- FFA is located in the inferior-temporal lobe (area IT)
Is FFA unique to faces?
- According to some psychologists the FFA may be the expertise as opposed to the face area of the brain
- It is involved every time we recognise objects we have a lot of experience with
- fMRI findings have shown that thee FFA responds to faces but also objects we see regularly
Is FFA unique to faces experiment
- Greebles used by Gauthier et al. (1999)
- Participants trained over a few weeks to name each Greeble
- Greebles are face-like stimuli which all have the same parts but are arranged in slightly different configurations, just like our faces
- Before the training FFA was only activated for faces
- After training FFA was activated for both faces and Greebles
What is visual imagery?
The act of generating mental images in the absence of environmental stimulus
Imagery vs. hallucinations:
- Images are actively generated and not confused with reality
- Charles Bonnet syndrome patients are unable to see or form mental images, but have vivid hallucinations which are indistinguishable from perception
The imagery debate
- Visual imagery is described as ‘seeing with the mind’s eye’ suggesting important links between visual imagery and perception
Rene Descartes (17th C): - We have a great experience of the world and that is where the thinking was done; our experience is a faithful, exact copy of what is out there
Imagery debate in late 1990s and early 2000s revived this old idea
What is the medium of thought?
What sorts of representations do we use when we think about things?
- Analog or picture-based accounts: we form pictorial representations and use them during imagery and thinking in general
- Propositional accounts: abstract, knowledge-based representations are used during imagery
Medium of thought: analog representations
- Kosslyn (1994)
- Representation that is stored in memory is a one-to-one representation of the world
- Mental images are like pictures in the brain
- Information within mental images is spatially organised the same way as information in the visual percept
- When performing a visual imagery task e.g. counting how many windows your house has, e use visual images generated in the same parts of the brain as the visual perception
Evidence for analog representations
- Shared brain activity with visual perception (e.g. Kosslyn et al., 1999)
- Mental rotation times are similar to actual rotation times
Is V1 involved for visual imagery: Study 1 (Kosslyn et al., 1999)
- 8 Participants memorised the stimuli in 4 quadrants
- Scanned as they closed their eyes and visualised the display
- Experimenter gave them two numbers followed by the name of a dimension e.g. length
- Task was to decide whether the set of stripes in the first named quadrant was longer than than the set of striped in the second named quadrant
- Resulting brain stimuli was compared with a control condition in which the same type of instructions were given but no imagery was used
- Key result was the activation in Area 17 e.g. V1
Mental scanning experiments
- Kosslyn (1973)
- Ps memorise a picture of a boat
- Create an image in their mind and focus on one part e.g. anchor
- Asked to look for another part of the boat e.g. motor
- If imagery is spatial it should take longer for Ps to find parts that are far from the initial point of focus cause . they would be scanning across the image of the object
- This is what happened, suggesting that visual imagery is spatial in nature like perception
Findings for analog representations of perception
- Images during visual imagery are pictoral in nature
- They have spatial layout like real images
- Mental rotation times are proportional to the actual difference between two viewpoints
- V1 is activated for both perception and imagery
- V1 seems necessary for visual imagery, just like visual perception
Evidence for propositional representations . of perception
- Existence of dissociations between perception and imagery in neuropsychological patients poses problems for Kosslyn’s theory and support the propositional theory
- Anton’s syndrome patients with damaged primary visual cortex are blind but have vivid imaginations
- They don’t know they are blind but what they see is their imagination
- Closed-head injury patients can have good visual perception but inability to create mental images
Pylyshyn’s main argument of peerception
- Scientists fall prey to their own intuitions
- ‘If it feels like an image in the head, it must be an image in the head’
- Just because we experience visual images doesn’t mean that the underlying representation is pictorial in nature
What is a propositional representation
One in which relationships can be represented by abstract symbols, e.g. a statement such as ‘The cat is under the table’
How does propositional representation work?
- What is consulted during mental imagery is the tacit knowledge that we have about the world
- This knowledge is used to create the image
- Images are not the mechanism of visual imagery but the result
Evidence for propositional representations
- Mental rotation of familiar objects is less accurate
- Naive physics shows that a lack of accurate tacit knowledge leads to erroneous images
What is naive physics in relation to mental imagery
- We can have mistaken beliefs about the behaviour of moving objects
- These lead to mistaken predictions about the behaviour of objects
Supports Pylyshyn’s theory:
- Without the correct knowledge there is no correct imagination even though we may have repeated visual images of the same events in the past
Perception of Depth
- Distance needs to be inferred from available information on the retina as we have no cells in the visual system that tell us whether something is near or far
- Perception of depth depends on cues
2 types of cues
Binocular cues - visual cues that depend on both eyes
Monocular cues - visual cues for distance which are just as effective with one eye as with both
What is retinal disparity
The difference in the apparent position of an object as seen by the left and right retinas
What is convergence
Degree to which the eyes turn in to focus on a close object
Binocular depth cues: Retinal disparity
- The amount of discrepancy between two eyes is one way to gauge distance
- The greater the disparity, the closer the object must be
Binocular depth cues: Convergence
- The more the muscles pull, the closer the object must be
- When you focus on a distant object, your eyes look in almost parallel directions
- When you focus on something close, your eyes turn in, and you sense the tension of your eye muscles
Monocular depth cues: Linear perspective
- As parallel lines stretch out towards the horizon, the come closer together
- This is the linear perspective cue to depth
Monocular depth cues: Texture gradient
- At greater distances, elements of the scene come closer and closer together
- The ‘packed together’ appearance of objects gives us another cue to their approximate distance
Monocular depth cues: Clarity or bluishness
- Distant objects are more blue and less clear due to the refraction of light by oxygen in the atmosphere
- The further the light has travelled, the more refracted into the bluish wavelengths it is
Monocular depth cues: Object size
- A nearby object produces a larger image on the retina than a distant object does
- This cue only helps for objects of a familiar size
Monocular depth cues: Relative height
- For objects that appear below the horizon line, those further . away have higher bases
- But for objects that appear above the horizon line, those closer have higher bases
Monocular depth cues: Occlusion
- An object which is partially hidden is perceived to be further away
Monocular depth cues: Shadows and shading
- The more separated the object is from its cast shadow the closer it appears to us
Monocular depth cues: Accommodation
- Lens of the eye changes shape to focus on nearby objects
- Brain detects the change and infers the distance to an object
- If the lens is flat (and thinner) then the object must be far
- If the lens is rounder (thick) the object must be near
- Works with real objects as opposed to objects in photographs or paintings
Monocular depth cues: Motion Parallax
- If you are in a car and fixating on the horizon, nearby objects move quickly across the retina, whilst farther objects move slower
- The difference in speed of movement of images across the retina as you travel is motion parallax
- Helps us to perceive depth whilst moving
- Used mostly by animals who lack binocular vision e.g. pigeons
Size constancy
Even though the retinal projection becomes smaller when an item moves away, we do not perceive the object as smaller, we correctly perceive the item’s correct physical size
What is no depth cues are available?
- If we know from experience either size or the distance, we can estimate the other one
- If we misperceive either one, we will be mistaken about the other also
What happens when we misjudge distance?
- When people see an unfamiliar object in the sky, they often misjudge its distance
- If we overestimate the distance, we also overestimate an object’s size and speed e.g. UFOs
Optical illusions
- When we are misled by cues we experience an optical illusion
- It is a misinterpretation of a visual stimulus
What we learn from optical illusions
- Perception is a constructive process
- Perception is not just adding up all the events that strike the retina
- We impose order on haphazard patterns (Gestalt psychology)
- We see 3 dimensions in 2 dimensional drawings
- We see optical illusions, often as a result of misinterpretation of the image cues, or as a result of unusual circumstances
- Brain does not compute what light is striking the retina but it tries to learn what objects are preesent and what they’re doing
What is motion perception good for?
- Critical for sight
- Critical for survival
- Allows interaction with the environment
- Allows us to perceive the results of our actions e.g. filling a glass of water up
What us Akinetopsia
- Motion blindness
- Damage to medial temporal area (MT) causes inability to perceive motion
Processing motion info down the temporal and parietal pathways
- Ganglion cells from the periphery of the retina (M ganglion cells) process motion info better than those from the fovea (P Ganglion cells
- Most cells are sensitive to light in V1, they send their output to area MT in the parietal lobe
- In MT motion info from V1 becomes integrated into useful info about motion of objects
The aperture problem
- Most cells in V1 are sensitive to motion
- V1 cells respond to input from within a very small area so cells respond as though they are viewing a small portion of the visual fields through an aperture (tiny window)
- Viewing only a small portion of a larger stimulus can result in misleading info about direction in which a whole object is moving
- Perception of the correct direction of a moving object requires integration across many V1 neurons
- This integration happens in MT, where individual neurons receive input from many V1 neuons
- Mt neurons cover as much as 30 degrees of visual angle up to seconds long delay
How to explain motion perception
- Motion on the retina is neither sufficient or necessary for us to perceive motion of objects in the world
- One approach to explain the 3 motion perception situation is the corollary discharge theory
What is retinal slip and corollary discharge
Retinal slip: an image slips across the retina. 2 situations may have caused this
A) Movement of the eye
B) Object’s image moves
Helmholtz suggested that retinal slip could be combined with an internal sense of our own eye movements to improve our perception of motion
This internal sense of eye movement was a copy of the eye movement command and is called corollary discharge
Corollary Discharge Theory
A) If the image moves to the right on the retina (retinal slip) whilst the eye is moving left (corollary discharge is negative) then image motion can be readily attributed to eye motion, not object motion
B) If the image moves on the retina (retinal slip) whilst the eye is still (corollary discharge is zero) then motion on the retina must be due to the object
Brain areas in biological motion perception
- Activates neurons in STS (superior temporal sulcus) and in the FFA (fusiform face area)
- Patients with motion blindness can still perceive biological motion
What is the function of biological motion perception
- Related to social behaviour
- STS is connected to the orbito-frontal cortex and amygdala
- Necessary for recognition of social and biological events such as recognition of emotional expression and personal intent
