Perception 2- brightness & colour Flashcards
Stimulus for vision overview
Light
Different wavelengths are associated with different colour perceptions
Light is small packets of energy called photons
What type of properties are brightness and colour?
Perceptual properties
Ways that light interacts with objects and surfaces
Absorption as photons collide with particles of matter
Reflection as light strikes opaque surfaces
Transmission as light passes through transparent matter
Human eye overview
Single-chambered- convex cornea projects image onto retina
Enables directional sensitivity- represents spatial structure rather than total of light
Photoreceptors trance light into an electric potential
Signals go through neurone to retinal ganglion cells then out via optic nerve
Two types of photoereceptors
Rods and cones
Rods
Located primarily in peripheral retina
operate in low light levels
Cones
Concentrated in centre of retina
Require higher light levels
3 different photopigments sensitive to different wavelengths
Main visual pathway (eye to brain)
Retina, optic nerve, optic chasm, LGN, Primary visual cortex
Brightness perception- bottom-up
The retina doesn’t record light intensities
Responses shaped by process in the retina
Brightness perception- top-down
Brain uses knowledge about how light interacts with objects
Can result in errors
Brightness perception- light/dark adaptation
Perceived in the same way due to brightness constancy
sensitivity of the retina is constantly adjusted to compensate for changes in mean luminance
Sensitivity: reduced when the mean intensity is high and increased when it’s low
Brightness perception- negative afterimages
Brought on by prolonged viewing of stimulus
Vital for function in different light conditions
Can produce illusions
Brightness perception- lateral inhibition
makes visual system sensitive to changes in luminance
Early for of information processing in retina
Retinal ganglion cells receive excitatory and inhibitory input from photoreceptors
Arranged in centre- surrounded configuration
Colour perception in low light
Only rod photoreceptors operate
Only contain single type of photopigment
Different wavelengths can elicit same response
Impossible to accurately signal different wavelengths
Colour perception- trichromacy
Cone photoreceptors contain one of three different photopigments
Relative output of different cones allows unambiguous signalling of wavelength
S-cones
Sensitive to short wavelengths
(blue cones)
M-cones
Sensitive to middle wavelengths
(green cones)
L-cones
Sensitive to long wavelengths
(red cones)
Monochromacy
Individuals have 0 or 1 functioning cone type
Dichromacy
Only 2 function cone types
Protanopia- missing L-cones
Deutreranopia- missing M-cones
Tritanopia- missing S-cones
Anomalous trichromacy
Defect in one of the cone types
Most common form of colour perception deficiency
Protanomaly- L-cone defect
Deuteranololy- M-cone defect
Tritanomoly- S-cone defect
Types of colour perception
Opponency- shaped by bottom-up processing
Negative afterimages- reduction in sensitivity of cone type, creates in imbalance in the inputs to opponent ganglion cells
Herman grid illusion
Grey spots appear in white areas between black squares
Because retinal ganglion cells responding to this location are inhibited more than cells responding at other positions along the line
