Perception - Brightness and Colour Flashcards
What is light?
Stimulus for vision
What is visible light?
Band of energy within electromagnetic spectrum (wavelengths from 400-700nm)
Consisting of small packets of energy called photons
What is absorption?
As photons collide with particles of matter
What is reflection?
As light strikes opaque surfaces
Do bright or dark objects reflect more light?
Bright
What is transmission?
As light passes through transparent matter
How many chambers does a human eyes have?
Single-chambered eye
What does the human eye enable?
Directional sensitivity (represent spatial structure rather than sum total of light)
What do photoreceptors do?
See light and process it
Transduce light into electrical potential
How do signals move through the eye?
Flow through network of neurons to retinal ganglion cells then out back of eye via optic nerve
What are the types of photoreceptor in the retina?
Rods
Cones
Where rods primarily located?
Peripheral retina
What is the function of rods?
Capable of operating in low light levels (can detect single photon)
Where are cones primarily located?
Concentrated in centre of retina (fovea)
What is the function of cones?
Require higher light levels (daylight) to respond
2 different photopigments sensitive to different wavelengths of light
What is the visual pathway?
Retina -> optic nerve -> optic chiasm -> LGN -> primary visual cortex (V1)
What is intensity/luminance related to?
Brightness
What is luminance?
Numbers of photons per unit space
When are there more photons is it generally darker or brighter?
Brighter
How does bottom up processing affect brightness perception?
Retina doesn’t simply record light intensities
Responses are shaped by processes occurring within the retina
What are the examples of bottom-up processing for brightness perception?
Light/dark adaption
Lateral inhibition
What is light/dark adaption?
Luminance of retinal image depends on amount of light falling onto object and relative reflectance of surface
Sensitivity low when mean intensity high and sensitivity high when mean intensity low
What is brightness constancy?
Not having perception massively change when brightness changes
In light/dark adaption, what does the retina encode?
Contrast (ratio of object’s luminance relative to the mean or background luminance)
Plays critical role in achieving brightness constancy
What illusions can light/dark adaption produce?
Negative afterimages
What is lateral inhibition?
Early form of information processing in retina
Retinal ganglion cells receive both inhibitory (-) and excitatory (+) input from neighbouring photoreceptors arranged in central configuration across retinal image
Makes visual system sensitive to changes in luminance
Can have dramatic effects on perceived brightness
Why is it important that the visual system is sensitive to changes in luminance?
Detecting edges and borders of objects
What illusion can have lateral inhibition produce?
Herman grid illusion
What are the top-down influences on brightness perception?
Brain uses knowledge about how light interacts with 3D objects in the world when determining brightness
Tries to maintain brightness constancy when amount of light falling on surface is affected in shadows (can result in errors in 2D images portraying 3D scenes)
What does wavelength affect the perception of?
Colour
In low light conditions, what is colour perception like?
Often world devoid of colour
Why is the world often devoid of colour in low light conditions?
Only rod photoreceptors sensitive to operate and they only contain single type of photopigment (rhodopsin)
Lights of different wavelengths can elicit identical responses so it’s difficult to accurately signal different wavelengths
What is trichromacy?
When all the cone photoreceptors function correctly
Relative outputs of 3 cone types allows unambiguous signalling of wavelengths
What are the three cone photoreceptors?
S-cones
M-cones
L-cones
What are S-cones sensitive to?
Short wavelengths
Blue
What are M-cones sensitive to?
Medium wavelengths
Green
What are L-cones sensitive to?
Long wavelengths
Red
What are variations from trichromacy?
Colour blindness
Monochromacy
Dichromacy
Anomalous trichromacy
What is monochromacy?
Most severe
0 or 1 functioning cone type
Complete colour blindness
Extremely rare (~1 in 100,000)
What is dichromacy?
2 functioning cone types
Protanopia = missing L-cones (1% males, 0.02% females)
Deuteranopia = missing M-cones (1% males, 0.01% females)
Tritanopia = missing S-cones (0.002% males, 0.001% females)
What is anomalous trichromacy?
Defect in 1 of cone types
More common form
Commonly assessed using Ishihara Colour Test
Protanomaly = L-cone defect (1.3% males, 0.02% females)
Deuteranomaly = M-cone defect (5% males, 0.35% females)
Tritanomaly = S-cone defect (0.01% males, 0.01% females
Is opponency a top-down or bottom-up process of colour perception?
Bottom-up
What is opponency?
Retinal ganglion cells received excitatory (+) and inhibitory (-) input from different cone types
Distinct Red/Green and Blue-Yellow pathways
What illusions can opponency cause in colour perception?
Negative afterimages
Staring at red object will result in green afterimage - adaption to red causes reduction in sensitivity to long wavelength cones, creating imbalance in inputs to red/green opponent retinal ganglion cells
What are the top-down influences to colour perception?
Colour constancy by accounting for intensity and composition of light hitting surfaces
Gives rise to illusions where some wavelength of light perceived as different colours
What is colour constancy?
Tendency for perceived colour of objects to remain the same even if lighting changes
