Weeks 1 & 2 (Luke Jones) Flashcards
Why is the perception of form and organisation important?
- The environment contains hundreds of overlapping objects
- Yet the perceptual experience is of structured, coherent objects which we can recognise, use and instantly name
- The starting point is that light, reflected from objects reaches the eyes
- How do we then go from this perceiving to a coherent, stable, 3D object?
What are the two most common fallacys of the visual system?
The camera analogy
The homunculus (little man)
What is the distribution of receptors in the retina?
- Highly concentrated and small RF’s in the centre (fovea)
- More sparse and larger RF’s in the periphery
- There are uneven volumes of the visual cortex devoted to different visual arrays
Which side of the brain processes information from the left eye?
The contralateral- right side.
What shape is the retina and what does this mean for vision?
- The retina is curved
- Means there has to be some processing to decode the distorted image reflected here
How many times a second is the retinal information updated?
50 times
What elements does perception have to account for when visualising something?
- Uneven retinal distribution
- Curved image on the retina
- Eye movement, body movement and object movement
- Uneven cortical devotion to different parts of the visual field
- No explicit visual information from the cortex
What is Distal stimulus
The item in the world
(for example a tree)
What is the proximal stimulus?
The image on the back of the retina
What does the proximal stimulus lead to?
Perceptual experience
Why is perception important? (2)
- Is our only source of information about the world
- All other cognitive systems rely on perception
What are 4 main theories of visual perception?
- Gestalt approach
- Gibson’s ecological theory
- Marr’s Information processing theory
- The constructavist approach
How are the different theories of perception approached in different ways?
- Emphasis on Bottom-up vs Top-down processing
- The Goal of perception
- The methods to study perception
What is the primary maxim behind Gestalt psychology?
The whole is greater than the sum of its parts.
What approach does the Gestalt theory base its argument around?
- Top- down approach
- However, it acknowledges there is an aspect of bottom-up but there are elements missing in this approach when describing the visual system
What are Gestalt psychologists interested in?
How we group parts of a stimulus together and the way we separate figure from ground
- Segregation
and - Grouping
What is a necker cube and how does it explain the Gestalt approach?
- A 2D image of a cube which can be perceived in different ways by the same viewer
-This shows that sensation is not the same as perception. The fact that we can flip to different hypotheses of the cube indicates top-down information, separate from simple retinal vision
What is an issue with the Necker cube as an explanation of the Gestalt approach?
- Is an unfair representation
- This is because it is a 2D image of a 3D structure in which we can normally actively engage with in the environment.
Who are three key members of the Gestalt school?
Max Wertheimer
Kurt Koffka
Wolfgang Kohler
How does Gestalt argue against structuralism?
Argues for Reaction in the environment
Structualism ignores the relationship between stimuli and interpretation
How do we differentiate ambiguity in the environment according to Gestalt? (3)
- Through perceptual organisation
- Through Innate rules which determine ways in which objects are perceived
- Through experience and knowledge
What is an Issue with Gestalt theory claiming the ‘whole is different from the sum of its parts’?
- Suggests that when we look at a bike, we percieve the whole bike and not its elements
- Unconscious processing, however may infact take into account these elements
- We may then see objects as there elements in the unconscious
- We are only aware of the answer however
What are Gestalts laws of perceptual organisation?
- Similarity
- Good continuation
- Proximity
- Connectedness
- Closure
- Common Fate
- Familiarity
- Invariance
- Pragnanz (good figure)
What is Gestalts perceptual rule of similarity?
Similar things appear to be grouped together
What is Gestalts perceptual rule of Good continuation?
Points that, when connected, result in straight or smoohtly curving lines, are seen as belonging together, and the lines tend to be seen in such a way as to follow the smoothest path
What is Reifiction?
When there is more spatial information than what is actually present.
(Brain fills in missing information - think about knowing there is a chair under the desk although it is occluded)
What is Gestalts perceptual rule of proximity?
Things that are near to one another appear to be grouped together- Colour and form
What is Gestalts perceptual rule of connectedness?
Things that are physically connected are perceived as a unit
What is Gestalts perceptual rule of Closure?
Of several geometrically possible perceptual organisations, a closed fgure will be preffered to an open figure
What is Gestalts perceptual rule of common fate?
Things that are moving together in the same direction are grouped together (also orientated in the same direction)
What is Gestalts perceptual rule of familiarity?
Things are more likely to form groups if the groups appear familiar or meaningful
What is Gestalts perceptual rule of invariance?
We can recognise different orientations and distortions of the same object
What is Figure-Ground segregation and what is an illusion involved?
- Humans are good at determining what is the background and what is the foreground
- Rubin cube however provides an example where we are not so good at this
Why is Gestalt interested in figure-ground segregation
The ability to easily determine the foreground from the background indicated top-down processing
What are the 5 ways in which properties are segregated as figure or ground?
- Symmetrical areas are usually figure
- Convex shapes are usually figure
- Stimuli with comparatively smaller area are usually figure
- Vertical and horizontal orientations are usually figure
- meaningful objects are more likely to be seen as figure
What are the problems with the Gestalt approach?
- Underplay the parralel processing and unconscious processing that the brain does
- Explanation of how some of their laws worked was wrong
- Their laws provide a description of how things work rather than an explanation
- Thier laws are ill defined- Pragnanz- what is the simplest and most stable shape?
- Is he stating the obvious?
What approach is Gibson’s ecological theory based around?
Bottom-up approach
What does Gibson’s ecological theory of perception state?
- Perception is direct
- Rich information received from the environment is sufficient for perception
- Complex, cognitive processes are unnecessary
Why does Gibson not support lab studies of perception?
- A major component of vision is movement
- When studying 2D displays we remove this major component
- Therefore is not representative of the truth
What is an ambient optic array?
The information coming into the eye and reflecting on the retina, this can change due to observer movement
From one viewpoint, you have a fixed image of the world, as soon as you move, the array of which light reflects on the back of the eye changes
What is the most foundational observation of Gibson?
Perception is ACTIVE, movement of the observer provides additional stream information
What are invariants in Gibson’s ecological theory of perception?
- Unambiguous information about the environment
- Can be directly perceived
- For example:
Texture gradients
Horizon ratio
What is Horizon Ratio?
The proportion of the object above and below the horizon line is constant for objects of the same size standing on the same ground
What are texture gradients?
Changes in texture in the optic array tells us about distance, orientation and curvature of surfaces
This is the consequence of the fact that when objects recede away, their image becomes smaller on the retina
How has texture gradients been used to help pilots?
- Looking out of a plane onto a series of regular rectangluar fields
- When whe orientation changes, the array will change in a consistent way
- This gives a strong direct cue to the orientation to the ground
What is empirical support for Gibson’s ecological theory?
Participants could correctly identify objects, state their colour, identify the lighting conditions and the objecys spatial orientations just from black and white photos of object surfaces 2/3 correctly.
How does motion highlight invariance?
In a static scence, everything is invariant, so introduce motion to highlight these invariant properties
What are two types of motion?
- Object movement
- Observer movement (focus of Gibson)
What is the Motion Parallax?
Things far away move more slowly than things nearby
Speed of movement tells us about distance to an object
This is a monocular cue to depth
How do squirrels and pidgeons use motion parallax?
- Use because they don’t have much binocular overlap
- Use head bobbing to increase the motion parallax
- Will run orthogonally/ along the side to increase the motion cue
What is Optic Flow?
A combination of parallax and retinal size
For example
When a ball comes towards us the motion parallax means it gets larger on the back of the retina until it hits our face
How do self-driving cars function?
Use optic flow to detect the changing environment
How are road lines designed to use optic flow?
- Horizontal lines are painted on the road when coming to a junction
- The distance between the lines becomes progressively narrower
- This uses optic flow to fool the brain that the lines are approaching faster
- Causes us to slow down
What are Affordance’s in Gibson’s theory?
(think chair)
- There are certain properties of objects that are innately grasped as opportunities for action so the nature of the object affords the action.
- Memory and experience are unecessary
- For example:
A chair tells us there is opportunity to sit.
What discipline uses the theory of affordance? (Think industry)
- Object design
- Try to design objects by emphasising certain uses of the object
What are criticism’s of Gibson’s ecological theory?
- Vague- how is this information picked up?
- Ignores top-down experience (falls into a camera eye fallacy)
- Ignores neuroscience
What approach does Marr surround their theory of perception around?
- Bottom-up
BUT
- Emphasises computational nature of perception which does involve top-down information also
What are Marr’s four stages of image analysis?
- Grey level description
- Primal sketch
- 2 1/2D sketch
- 3D object-centred description
What is the basic overview process of Marr’s computational approach?
- Information comes into the eye
- Image is projected onto the retina
- A computer would process the image by breaking the problem down into stages
- Each stage results in an output which is the input for the next stage
What is Marr’s grey level description stage?
1st stage
Measuring the intensity of light at each point in a visual array/image
Activation of retinal photoreceptors
What is Marr’s 2 1/2D sketch stage?
3rd stage
Representation of orientation, depth, colour relative to the observer (only reports what we can see)
What is Marr’s Primal sketch stage?
2nd stage
Representation of contrast change (blobs, edges, bars etc) over a range of spatial frequencies using Gaussian blurring, identifying intensity changes and assigning primitives.
Primitives are then grouped together and assigned a place token which can be grouped to form higher-order tokens
Grouping is based on clustering and curvilinear aggregation
What is Marr’s 3D representation stage?
Representation of the objects independent of the observer by analysing 2 1/2D sketch for 3D volume primitives
Is a conscious experience of vision
What is Gaussian blurring?
the blurring of images to different degrees
What are primitives?
4 types of intensity change:
- Edge-segment
- Bar
- Termination
- Blob
How does primal sketch become 2 1/2 D sketch?
Primal sketch is combined wth depth cues, colour and motion
It is not 3D because it is observer-orientated
What are examples of 3D volume primitives?
Cylinders
Cones
cubes
Why is a computational approach important?
An algorithm is more likely to be understood by understanding the problem that has to be solved, rather than examining the mechanism in which it is embodied.
To understand perception by studing neurons is like trying to understand bird flight by studying only feathers.
What are some criticisms of Marr’s computational approach?
- Retinal imge is not always sufficient to allow reconstruction
- What is the role of memory and experience?
What is the constructivist approach to perception?
Combines both top-down and bottom-up approaches
The retinal image does not provide sufficient information
AND
Perception depends upon stored knowledge
What is unconscious influence as proposed by Helmholtz?
- Involuntary, pre-rational and reflex-like mechanism which is part of the formation of visual impressions
What are two examples of unconscious inference in neural processing?
- Illusions: impervious to experience
- Naive optics: movement of the sun in the sky
What is the likelihood principle as proposed by Helmholtz?
When we look at the world, the brain wants to know what the most likely outcome is
What is the Perceptual hypothesis suggested by Gregory?
- The brain generates its best guess of what is out there by reverse engineering information
- Many illusions are explained by stored knowledge leading to inaccurate perceptual hypotheses
- For example
The Hollow mask illusion is explained by our stored knowledge that faces are convex
Who is Richard Gregory?
1923-2010
Founder member of Experimental Psychology Society
He resurrected some of Helmoholtz’ ideas and made them more current in research
What is a criticism of the Constructivist approach?
Its too vague, how does the brain decide on the different hypotheses (the mechanics)
How are some theories described as ‘false debates’?
- There are different interests and focuses
- Tries to point out things that are missing
- How far can theories go with certain elements?
- Poor neuroscience until recently
What is colour?
Visable light forms a narrow band of frequencies on the electromagnetic spectrum
What are wavelengths of light?
Different frequencies which have different hues, ranging from red (long) to violet (short)
How many nm is the visual spectrum?
400
What gives objects their colour?
- Don’t have colour themselves
- Have different intrinsic properties that filter out some wavelengths and absorb others
- The colour also depends on the light source
What does the wavelenght of the light reflected determine?
The hue
What are 3 things that perceived colour is determined by (psychological attribute)?
- Hue
- Intensity of the refelcted light (how bright)
- The saturation of the colur (how much white light is mixed in with the pure hue)
What are 3 properties of light that we can measure?
- Wavelength
- Intensity
- Spectral Purity
What the luminance?
The measurable light reflecting off an object
What is Helmholtz’ trichromatic theory of vision centered around?
There being three types of cone which are senstive to different wavelengths of light:
- Short
- Medium
- Long
What is the bases of Hering’s Opponent process theory?
The presence of three comparative processes that measure the relative availability of the following colours:
- Red-Green
- Blue-Yellow
- Black-White
What does the opponent process theory (Hering) attempt to explain?
- Subjective experience of 4 primary colours
- Colour after effects
What is the dual-process theory?
- The trichromatic stage explains the retinal level of colour vision
- The opponent process stage explains the LGN and gangion level of colour vision
What is the light constancy theory?
Lightness remains constant despite changes in light levels.
What is an example of Light constancy?
- You have a black dog
- Regardless of where you are, the dog is perceived black
- Even when the light coming off the dog has changed by a certain amount of illumination
What is an example of light constancy being tricked/opposite occuring?
- Things of the same illumination are perceived as different
- The brain takes into account the scene to figure out what we are looking at
- But the actual information is that there is infact the same intensity
What type of quality is lightness?
- Perceptual quality (white/grey/black)
- The perceived shade of the surface
What type of quality is reflectance?
- Physical quality (% photons reflected)
- Proportion of light reflected from the surface
What is luminance?
Amount of light reflected from a surface (can measure the light coming from the surface)
What is the luminance, illumination and reflectance equation?
Luminance = illumination x reflectance
What is illumination?
The amount of light emitted from a source (light hitting the object)
What is reflectance?
Proportion of light reflected from the surface (the filtering of the illuminance and reflected proportion)
What is the issue of lightness perception and reflectance?
- Our eyes only receive information about luminance
- We don’t receive information about how much light is bouncing off the object
- So we do not know about reflectance
What is the inverse problem of lightness perception?
A particular luminance could have been produced by infinite combinations of illumination and reflectance
What is Adaptation as a theory of lightness constancy?
The visual system becomes less/ more sensitive in bright/ dull conditions
What is a problem with adaptation as a theory of lightness constancy?
Adaptation is slow and cannot account for fast changes in lighting
What is unconscious inference as a theory for the lightness constancy?
Prior experience allows us to estimate illumination
What is an issue with unconscious inference as a theory of lightness constancy?
We are not sensitive to absolute levels of illumination
What are relational theories as an explanation for lightness constancy?
- Luminance ratios determine lightness perception
- The relatioships between objects and the different rules the brain uses
- The relative reflectance of two objects remains the same and this is what the brain uses to maintian constancy
What is back-up evidence of relational theories to explain lightness constancy?
- When participants were asked to match luminance, they matched the luminance rations instead of the absolute luminance
Why are edges important in relational theories of lightness constancy?
The light coming off the object is constant but dips and increases before each edge (lightness perceived)
However the physical illuminance expressed all one shade of grey
What is the retinex theory of relational theories of lighness constancy?
- We calculate the luminance ratios at edges
- We ignore any gradual changes (usually due to shadows) in luminance
- Tells you relative reflectance of surfaces
SO
- Luminance ratio remains constant and reflectance is always reflecting the same proportion of light regardless but the illumination level is changing
What is the scaling problem as an issue for relational theories of ligthness constancy?
We can have a ratio of reflectance between two objects but there are different greys and blacks for example that produce the same ratios of reflectance
What can the scaling problem be solved by?
- Anchoring heuristic
- Assume the highest luminiance is white then scale all other regions relative to this (white balance)
What is illumination edges as a problem for relational theories of lightness constancy
- Retinex theory assumes all changes in illumination are gradual (flat objects)
- This cannot account for sudden changes in illumination (curved object)
What are reflectance edges?
Neighbouring regions have different relfectance- different shades are reflecting different proportions of light
What are illumination edges?
Neighbouring regions receive different amount of light- no difference in reflectance but is the difference in light hitting the object
How do you determine between illumination edges and reflectance edges (3 ways)?
- Illumination edges are fuzzy and reflectance edges are sharper
- Planarity heuristic
- Ratio magnitude heuristic
What is the planarity heuristic?
Determine the difference between coplanar and not coplanar regions (whether things are on the same plane)
- Coplanar: same plane, is more likely to be a difference in reflectance
- Not coplanar: different plane, is more likely to be a difference in illumination
(that is causing us to see a difference in the perceived light)
What is the ratio magnitude heurisitc?
- Determines the ratio of light hitting the eye of two objects
- Back to white ratio is around 1:9
- Differences in illumination can be much higher >1:1000
- Therefore is luminance ratio is very high, likey to be an illumination edge
What is colour constancy?
Perception of constant surface colour despite changes in illumination and viewing conditions.
There are different wavelengths coming off an object, but regardless we don’t perceive the object to be changing colour
How good is human colour constancy?
- Not perfect, but remarkably good
Can our colour constancy be tricked?
- No, the brain is figuring out the truth of reality
- But we can see different wavelengths when they are infact the same
What is the illumination spectrum?
Amount of incident illumination at each wavelength (nature of the light source)
What is Reflectance spectrum?
Proportion of light reflected at each wavelength (material characteristic of the object)
What is the luminance spectrum?
The amount of light reflected at each wavelength (amount hitting the eye)
What is the colour constancy equation?
Illumination spectrum x reflectance spectrum = luminance spectrum
How do we recover the reflectance spectrum from the luminance spectrum?
Have to do a backward analysis of the reflectance spectrum which will be different depending on the light conditions.
Have to determine from the illumination spectrum.
Does the surrounding of an image influence how we perceive a colour?
No, If present a patchwork of different colours and change the illumination, the colours remain the same
However, when the surroundings are masked there is a significant change in the colour
The brain takes into accound the ambient light and the nature of the illumination
How did Hansen et al study colour memory?
METHOD
- Presented iages of grey fruit against grey background
RESULTS
- When they were set to be identical shades of grey, the observer still perceived the fruit to be slightly coloured
- Infact, when the participant ad to set the colour of a banana to be grey, they actually set the colour to blueish (opposing colour)
CONCLUSION
- Colour memory helps us to accurately perceive colour under different illuminations
What is the disease called when one can no longer visualise colour?
Cerebral achromatopsia
What does cerebral achromatopsia suggest about colour perception in the brain?
- Suggests that there is a colour centre
- Because their visual system is still intact, still have access to wavelength information
How does the V1 visualise colour?
- ‘blobs’, small patches of cells that respond to colour of stimulus (physical wavelength)
How does the V4 visualise colour?
- Cells respond to perceived colour
What areas of the primary cortices visualise colour?
V1, V2, V3, V4, V8
What does V8 visualise in colour?
- Colour after-effect
- Suggests V8 might be the site of colour consciousness
