Topic 3 - Visual Perception Flashcards
Challenges of object perception
- Stimulus on the receptors is ambiguous
- objects can be hidden or blurred
- Objects look different from different viewpoints
Inverse projection problem
an image on the retina can be caused by an infinite number of objects - the task of determining the object responsible for a particular image on the retina
Viewpoint invariance
the ability to recognise an object seen from different viewpoints, regardless of the viewpoint
Structuralism (Wundt)
perceptions are created by combining elements called sensations
- can no explain apparent movement or illusory contours
Illusory contours
illusion of physical edges where there are none
Gestalt Approach
The whole is different from the sum of its parts - perception is a result of perceptual organisation
(Principles of perceptual organisation)
Good continuation
connected points resulting in straight or smooth curves belong together
(Principles of perceptual organisation)
Pragnanz (principle of simplicity)
every stimulus is seen as simply as possible
(Principles of perceptual organisation)
Similarity
similar things appear to be grouped together
(Principles of perceptual organisation)
Proximity
things that are near each other are grouped together
(Principles of perceptual organisation)
Common fate
things that are moving in the same direction appear to be grouped together
(Principles of perceptual organisation)
Common region
elements that are within the same region of space appear to be grouped together
(Principles of perceptual organisation)
Uniform connectedness
a connected region of the same visual properties is perceived as a single unit
(Principles of perceptual organisation)
Synchrony
elements occurring at the same time are seen as belonging together
Perceptual segretation
the perceptual segregation of one object from another
Figure-ground segregation
determining which part of the environment is the figure so that it stands out from the background
- Figure is more “thing-like”, more memorable, closer
- Ground is more unformed, extends behind figure
- border separating the two belongs to the figure
Reversible figure-ground
Figure and ground that can be perceived either way
Image based factors for figure
- Elements are located in lower part of displays
- On the convex side of borders
- Units are symmetrical
- Elements are small
- Oriented vertically
- Have meaning
Global image features
perceived rapidly and are associated with specific types of scenes
- Degree of naturalness
- Degree of openness
- Degree of roughness
- Degree of expansion
- Colour
Physical regularities in the environment
- Light from above assumption
- Oblique effect - people perceive horizontals and verticals more easily than other orientations
- Uniform connectedness - elements are defined by areas of the same texture and colour
Semantic regularities in the environment
Characteristics associated with the functions of scenes
- Scene schema - knowledge of what scene typically contains
Hollingworth - observers were presented with a scene and then an object, and asked where the object would be placed in the scene
Palmer - Observers saw a context scene, followed by a target picture, flashed briefly, and are asked to identify the target picture
Theory of unconscious inference (Helmholtz)
made to explain why stimuli can be interpreted in more than one way, but we prefer one
Likelihood principle
We perceive the object that is most likely to have caused the pattern of stimuli we have received
Bayesian inference
our estimate of the probability is determined by
- the prior probability (initial estimate)
- the likelihood (extent to which available evidence is consistent with outcome)
Contextual modulation
stimuli outside of neuron’s receptive field can affect neural firing
- when they follow good continuation and are perceived as part of a figure
Binding
Binding Problem
the process by which features are combined to create perception of coherent objects
- only occurs when we pay attention
Binding problem - features of objects are processed separately in different areas of the brain
Treisman and Gelade
- Preattentive stage - features of objects are separated
- Focused attention stage - features are bound into a coherent perception
Illusory Conjunctions
features that should be associated with an object become incorrectly associated with another
Triesman and Schmidt
- Stimulus was four shapes flanked by two numbers, flashed briefly, followed by a mask
- Task was to report numbers first followed by shapes at four locations
- Incorrect associations of features occurred 18% of the time
Balint’s syndrome
patients with parietal lobe damage show lack of focused attention results in incorrect combinations of features
Conjunction search
finding target with two or more features
- patients with parietal lobe damage cannot perform conjunction searches well
Functions of colour perception
- helps classify and identify objects
- Facilitates perceptual organisation of elements into objects
- May provide an evolutionary advantage when foraging for food
Colours can be changed by:
- Intensity which changes perceived brightness
- Saturation
- Wavelength
Reflectance curves
plots of percentage of light reflected for specific wavelengths
Chromatic colours
eg. blue, green, red - occur when some wavelengths are reflected more than others (called selective reflection)
Achromatic colours
eg. white, grey, black
Selective transmission
only some wavelengths pass through the object or substance
Transmission curves
plots of the percentage of light transmitted at each wavelength
Making green
- Blue absorbs long-wavelength light and reflects some short and medium wavelength light
- Yellow absorbs some short-wavelength light and reflects some medium and long wavelength light
- The only wavelengths reflected when mixed are those that are reflected by both paints in common - aka some medium-wavelength light
Mixing paint and light
Mixing paints is called a subtractive colour mixture
Mixing lights is called an additive colour mixture
- All light reflected from two colours is reflected
Hues, Saturation, Value
Hues - another word for chromatic colours - differ because of saturation and value
Saturation - how much white is added
Value - the light-dark dimension
Trichromatic Theory of colour vision
- Proposed by Young and Helmholtz
Three different receptor mechanisms are responsible for colour vision
Colour matching (experiment)
- Observers adjusted three wavelengths in a comparison field to match a test field to one wavelength
- Needed three wavelengths to match colour
- Those with colour deficiencies could do it with less wavelengths, but only matched in their eyes
Physiological evidence for trichromatic theory
- Researchers measured absorption spectra of visual pigment in receptors, found pigments that responded maximally to short, medium and long wavelengths
- Genetic differences for coding proteins of three pigments
- Colour perception is based on the response of the three different types of cones
- Colour matching experiments show that colours that are perceptually similar (metamers) can be caused by different physical wavelengths
Are three receptors necessary?
Vision with one receptor (monochromats) - absorption of a photon causes the same effect, no matter what the wavelength is - any two wavelengths can cause the same response by changing the intensity
Two receptors (dichromats), three receptor types (trichromats)
Principle of univariance
once a photon is absorbed by a visual pigment molecule, the identity of the light’s wavelength is lost
Opponent-Process theory of colour vision
Colour vision is caused by opposing responses generated by blue and yellow, and by green and red
Behavioural evidence - types of colour blindness are red/green, blue/yellow - colour afterimages (complementary afterimages) and simultaneous colour contrast show opposite pairings
Opponent-process mechanisms
Three mechanisms: red/green, blue/yellow, white/black
- The pairs respond in an opposing fashion to the absorbance of light, such as positively to red and negatively to green
- Believed to be the result of chemical reactions in the retina
- Researchers found opponent neurons located in the retina and LGN
- Respond in an excitatory manner to one end of the spectrum and an inhibitory manner to the other
Trichromatic and Opponent-process theories combined (“zone” theories)
Each theory describes physiological mechanisms in the visual system
- Trichromatic theory explains the responses of the cones in the retina (long, medium and short)
- Opponent-process theory explains neural response for cells connected to the cones further in the brain, colour blindness and afterimages
Opponent neurons
single-opponent (centre surround)
double-opponent (side by side) neurons
Monochromatism
- usually hereditary, occurs in 10 out 1 million
- No functioning cones, only need one primary to match colour
- Can be called colour blind
- Only see in white, grey and black
- Poor visual activity and very sensitive to bright light
Dichromatism
Missing one of the three cone pigments, experience some colours, need two primary colours to match
Unilateral dichromat - trichromatic vision in one eye and dichromatic in the other
Forms of Dichromatism
Protanopia - 1 percent of males, 0.02 percent of females, missing long wavelength
Deuteranopia - 1 percent of males, 0.01 percent of females, missing medium wavelength
Tritanopia - very rare, missing short wavelength
Anomalous trichromatism
needs three primaries in different proportions to normal trichromat
Colour constancy
perception of colours as relatively constant in spite of changing light sources
- Sunlight has approximately equal amounts of energy at all wavelengths
- Tungsten lighting has more energy in long-wavelengths
- LED bulbs emit light at substantially shorter wavelengths
Chromatic adaptation
prolonged exposure to chromatic colours
- Receptors “adapt” when the stimulus colour selectively bleaches a specific cone pigment
- Receptors decrease in sensitivity to colour
- Occurs to light sources, leading to colour constancy
- The eye reduces sensitivity to the light shining on the eye, therefore making colours seem constant
Chromatic adaptation Uchikawa experiment
- Observers shown sheets of coloured paper in three conditions:
- Baseline - paper and observer in white light
- Observer not adapted - paper illuminated by red light; observer by white
- Observer adapted - paper and observer in red light - partial colour constancy is shown
Possible causes of colour constancy
Effect of surroundings
Memory and Colour - past knowledge of an object’s colour can impact perception
Colour constancy Hansen experiment
- presented observers with pictures of fruits with characteristic colours against a grey background
- Adjusted the colour of fruit and spot of light to match the background
- Spot of light was perceived as grey but fruit still perceived as coloured
