Colour Perception and Spatial Vision Flashcards
The dress explanations
- different angles
- different naming of colours / fixed answers
- colour blindness
- individual differences in colour processing
- individual differences in experience
-failure in colour constancy
consequence of overexposed images
- measured colours being different from the real image
how can viewing conditions cause variations in perceived colour?
- when theres a change in image size and viewing angle
- under lab settings (neutral light source) it was found that the same answers were given , so viewing conditions is insig
Problems naming colours
- Subjective experience of colour may be the same , when forced to choose= different names
- however when given a flexible choice (colour swatches) there was still two distinct answers
- so experience of colours is different
Visible spectrum
- Our conscious experience is purely dependant on cells that are sensitive to particular wavelengths of light
- This is called the visible spectrum
how many cones are involved in daylight vision?
- three types of cone receptors sensitive to and respond to red green and blue
- these pick up different frequencies of light then pass info into the LGN
which respond to a wide range of colours
-but respond maximally to their own, firing more strongly
how many rods are involved in night vision?
only one rod photoreceptor, explaining why we are colourblind at night and cannot differentiate between colours
Animals and cone types
- most mammals have only two cones
- other animals have multiple cone types
- snakes can see infra red (we dont because we are cold blooded)
- birds and insects see ultra violet (hurts eyes over long period, so we screen these out)
one cone type (monochromat)
can see a range of colours but not discriminate between them
two cone types (dichromats)
can discriminate these colours but no others
three cone types (trichromats)
can discriminate all possible colours in the visible spectrum
Colour blindness in gender
- red-green cone is carried on the X chromosone
- so this colour blindness is more common in men than women
- some women have 4 chone types (tetrachromats)
Rod cmonochromats
- truly colorblind
- cannot see in daylight
what can colour blindness be caused by?
mistuned cone types
cerebral achromatopsia
what is the result of mistuned cone types?
- inability to discriminate between colours, most likely mistuned green cones
- this means cones for e.g. red dont fire maximally at a certain wavelength
how can cerebral achromatopsia result in colour cortical blindness?
- damage to V8 (via e.g. stroke)
- we still have the ability to process colours at the level of the retina +LGN but cannot consciously perceive or differentiate between colours because of damage
- can still detect brightness
what is colour-opponent processing?
- the visual system processes complementary colours in pairs
- we have two colour systems
1. the luminance + Blue/Yellow system
2. recent adaptation of a newer Red Green system (adaptation of the yellow part of the older system)
How does colour opponency work?
-The visual system construct red receptive fields from the output of red AND green photoreceptors
- We then compare the ratio between red and green
for yellow, we combine the output of red and green to creates luminance info
- This also gives us info about yellow which is then fed into the output from the blue to create a receptive field comparing yellow and blue
what do the centre-surround properties of retinal ganglion cells (RGC) provide?
the mechanism for colour constancy, by calculating the ratio of colours to provide edge detection
How can our opponent processing affect colour perception
- what people see as neutral varies between colours
- individual variations in peoples colour-opponency systems explain differences in colour perception
what does inhibition over space and time suggest for colour perceptions?
-spatial context and lighting can affect colour perceptions, due to lateral inhibition compressing information about colour
- simultaneous contrast illusions
colour after-effects
what does inhibition over time and time suggest for colour perceptions?
- temporal context can affect colour perception
- e.g. you look at a colour for a long period of time (to save energy) active cells are turned off/inhibited
- so we see opposite colours
what do colour centre-surround cells in the retina transmit?
- only the colour edges, which allows the cortex to reconstruct the colour body of objects
- means surrounding colours might facilitate “filling-in” due to excitation in the cortex, altering our perception of colours
why is individual differences in experience not a valid explanation for colour perception
- Whilst we there may be individual difference in how we experience colour, these aren’t long lasting
- Asking people under laboratory settings to name the colour we still get a 50/50 split of colour
what is colour constancy?
using a white-balance correction to discount the light source colour which may influence perceived colour of an object
occurring in V8 (subsection of area V4)
how are V1, V2, V4, and V8 involved in colour?
colour-sensitive cells in V1 and V2 pass signals onto V4 and V8 (responsible for conscious perception of colour, and colour constancy)
What cell activity can we expect between V1 and V8 in terms of colour constancy
- when we show a the same colour (e.g. white ) under neutral and ambient light there should be no activity in area V8 as it discounts the ambient light, instead there will be activity change as it processes it as a new colour
classical view of lines (hubel and weisel, 1950s)
spatial relationships between parts of objects are processed through their oriented edges and bars
- simple cells
- complex cells
- hypercomplex cells
properties of simple cells
- receptive fields
- well-defined on/off regions
- different oreintations and sizes
- respond to edges
Why are receptive fields crucial in simple cells
because of this rectangular shape of receptive field that enables v1 cells to detect orientation position and size of stimuli which a concentric receptive field (in the retina/LGN) cannot do
well-defined on/off regions
presenting a stimulus in the on-region excites the cell to generate an AP, but the reverse happens with light in the off-region
selectivity within simple cells
- orientation selectivity
- position sensitivity
- sensitivity to size
how does orientation selectivity occur in simple cells?
- only fires maximally when the stimuli touches the on-part of the receptive field, this detects the specific orientation of the stimuli
how does position selectivity occur in simple cells?
they generate a higher AP when the object moves onto part of the receptive field, providing us with info about the position of objects
how does size selectivity occur in simple cells?
able to respond to the width of stimuli, e.g., thinner light
maximally excites the on-region to produce more APs, wider overlaps with on and off regions, cancelling out activity = less APs
how can V1 receptive fields be assembled?
can be assembled from the output of concentric/receptive fields at the LGN, as off-parts construct their respective parts in simple cells to allow for contrast , vice versa
properties of complex cells
- high resting output (no stimulus = more activity)
- unstructured,overlapping, with no defined on-off receptive field
- respond to orientation and size selectivity, but more for complex lines/shapes
why do complex cells have no defined on-off receptive field?
they are constructed from the output of simple cells with various types of receptive fields , resulting in an overlapping on-off receptive fields that have the ability to differentiate between complex cells
properties of hypercomplex cells
- no serpate on/off regions
- peak sensitivity for line length and orientation
- some size slective
- not position selective
how are hypercomplex cells constructed?
- by complex cells with two inhibitory receptive fields at either end and an excitatory center
what does the construction of hypercomplex cells allow them to do?
- turn off when the stimulus gets greater than their peak sensitivity size/length
what do inhibitory regions of hypercomplex cells provide?
an end-stop signal to cells upon excitation from the ends of the stimulus, enabling the visual system to work out the length of stimuli
What is the difference in length sensitivity between a hyper complex cell and a simple complex cell
Simple cell: no inhibitory stop mechanism so as stimulus gets longer receptive field will excite but will not turn off
Hyper complex cell: Increase in firing rate as the stimulus gets longer BUT when it starts to project onto the inhibitory ends the cell gets turned off with a decrease in APs
what does texture segmentation believe texture is?
spatial detail at a scale which is finer than the object scale
defines texture by statistical properties to work out boundaries, borders, and locations
same statistical properties= difficult with differentiation
texton theory (julesz, 1984)
- textures will segment if they differ in the density of their textons (simplest/most basic defining properties of textures, based on V1 properties)
- e.g. how many endings do the lines have
- same number of textons= difficulty to differentiate
orientation contrast model (nothdurft, 1990)
believes the brain can differentiate texture due to sensitivity to changes in local contrast and local orientation, rather than using statistical textons
when does segmentation by orientation develop?
around 3 months of age
what does segmentation orientation contrast involve?
segmenting on the basis of local differences, this makes an imaginary boarder between both elements allowing for differentiation
it is also possible to segment objects based on local size,colour. shape differences
types of centre-surround cells with receptive fields specialised for orientation
- single opponent cells
- double opponent cells
single opponent cells (orientation sensitive)
- area V1
- receptive field sensitive to only one orientation and local contrast (so when theres a different orientation)
- cell stops working in the introduction of a second (different orientational) stimuli
double opponent cells (orientation insensitive)
- area V1
- cells with sensitivity for both orientations – vertical and horizontal – and also local changes in vertical and horizontal stimuli in the periphery
what does the simultaneous tilt illusion show?
- how orientation can be misinterpreted due to the inhibitory influence of surrounding orientations
- centre lines are perfectly verticle
-as a result of data compression on centre-surround cells, centre looks slanted - Overall, Spatial inhibition influences our neural code for orientation
evidence of simultaneous contrast for size
- ebbinghaus illusion presents inhibiton over space
- overall illusion shows how shape sizes around our periphery can influence our perception of target object
for spatial vision, what can inhibition over time result in?
colour and tilt after-effects, as long-time exposure inhibits orientation-sensitive cells
this increases the resting level of the opposite direction and results in a greater output, tricking us into misperceiving colour and orientation by changing the neural code
perveived oreintation is determined by…
- a population of cells sensitive to different orientations (the neural code)
- new orientation= alters neural code
