Colour Perception and Spatial Vision Flashcards
consequence of overexposed images
measured colours being different from the real image
how can viewing conditions cause variations in perceived colour?
change in image size and viewing angle
what did livingston and hubel (1988) observe about colour processing?
V1, V2, and V3 are involved in colour processing
how many cones are involved in daylight vision?
three types of cone receptors, which respond to a wide range of colours but respond maximally to their own
how many rods are involved in night vision?
only one rod photoreceptor, explaining why we are colourblind at night
what type of vision is more sensitive to light?
peripheral vision
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
what is the visible spectrum?
the part of the colour spectrum that human eyes are sensitive to
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
what are rod monochromats?
have no cones at all, meaning they cannot see at all in daylight/truly colourblind
how can cerebral achromatopsia result in colour cortical blindness?
damage to V8, leads to inability to differentiate colours
what is colour-opponent processing?
the visual system processes complementary colours in pairs
- the luminance + BY system
- recent adaptation of a newer RG system
what do the centre-surround properties of RGC provide?
the mechanism for colour constancy, by calculating the ratio of colours to provide edge detection
individual variations in 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 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
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)
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
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?
due to their oblong-shaped receptive fields, which allow increased AP when light aligns with the on-region
how does position selectivity occur in simple cells?
they generate a higher AP when the object moves onto part of the receptive field
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
how can V1 receptive fields be assembled?
can be assembled from the output of receptive fields at the LGN, as off-parts construct their respective parts in simple cells to allow for contrast
properties of complex cells
- high resting output
- unstructured, with no defined on-off receptive field
- respond to orientation and size selectivity, but more for complex lines
why do complex cells have no defined on-off receptive field?
they are constructed from the output of simple cells, resulting in an overlapping on-off receptive field for complex cells
properties of hypercomplex cells
- similar to complex cells, but have a peak sensitivity for line length and orientation
how are hypercomplex cells constructed?
by complex cells with two inhibitory receptive fields at either end
what does the construction of hypercomplex cells allow them to do?
turn off when the stimulus gets greater than their peak sensitivity size
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 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
texton theory (julesz, 1984)
textures will segment if they differ in the density of their textons (simplest defining properties of textures, based on V1 properties
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 3m of age
what does segmentation orientation contrast involve?
segmenting on the basis of local differences
it is also possible to segment objects based on local size differences
types of centre-surround cells with receptive fields specialised for orientation
- single opponent cells
- double opponent cells
single opponent cells (orientation sensitive)
sensitive to one orientation and local contrast, specifically for one orientation
double opponent cells (orientation insensitive)
cells with sensitivity for both orientations – vertical and horizontal – and also local changes in vertical and horizontal stimuli
what does the simultaneous tilt illusion show?
how orientation can be misinterpreted due to the inhibitory influence of surrounding orientations, as a result of data compression on centre-surround cells
evidence of simultaneous contrast for size
ebbinghaus illusion presents inhibiton over space
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
