PSGY1010 Cognitive Psychology 1 Perception 2 Brightness & Colour PDF

PSGY1010 Cognitive Psychology 1 Perception II: Brightness and Colour Dr Chung Kai Li [email protected]  Today’s lecture Learning objectives: ▪ Understand the stimulus for vision and basic structure of the human visual system ▪ Describe the factors that shape the perception of br...

PSGY1010 Cognitive Psychology 1 Perception II: Brightness and Colour Dr Chung Kai Li [email protected]  Today’s lecture Learning objectives: ▪ Understand the stimulus for vision and basic structure of the human visual system ▪ Describe the factors that shape the perception of brightness ▪ Understand the principles of normal and abnormal human colour perception 2  Light: the stimulus for vision ▪ Visible light is a band of energy within the electromagnetic spectrum (wavelengths from 400-700nm) ▪ Different wavelengths of light are associated with different colour perceptions ▪ Light can also be described as consisting of small packets of energy called photons ▪ Light intensity or luminance (number of photons per unit space) is associated with perception of Note: brightness and colour brightness are perceptual properties, NOT physical properties 3  Light: the stimulus for vision ▪ Light interacts with objects and surfaces in the environment, e.g.: ▪ Absorption as photons collide with particles of matter ▪ Reflection as light strikes opaque surfaces ▪ Transmission as light passes through transparent matter ▪ Different material properties are associated with different perceptions. e.g. ▪ ‘bright’ objects reflect more light than ‘dark’ objects ▪ ‘red’ objects selectively absorb short wavelength light and reflect longer wavelengths 4  The human eye ▪ Single-chambered eye uses convex cornea and lens to project an image onto the retina ▪ Enables directional sensitivity – can represent the spatial structure rather than sum total of light ▪ Photoreceptors transduce light into an electrical potential ▪ These signals then flow through a network of neurons to retinal ganglion cells and then out the back of the eye via the optic nerve 5  Two types of photoreceptor: rods and cones Rods ▪ Located primarily in peripheral retina ▪ Capable of operating in low light levels (can detect single photon) Cones ▪ Concentrated in centre of retina (fovea) ▪ Require higher light levels (daylight, 100s of photons) to respond ▪ 3 different photopigments, sensitive to short, medium and long wavelengths of light  The visual pathways ▪ Visual information is transmitted from the retina to the brain ▪ The main pathway consists of: Retina Optic nerve Optic chiasm LGN Primary visual cortex (V1) 7  Brightness perception ▪ Light intensity is related to perceived brightness (higher intensity regions tend to be perceived as brighter) ▪ However, this relationship is not straightforward – brightness perception is influenced by both bottom-up and top-down processes. Bottom-up Top-down ▪ The retina does not simply record ▪ The brain also uses knowledge light intensities about how light interacts with objects ▪ Responses are shaped by processes when determining perceived occurring within the retina, most brightness (e.g. shadows) notably light/dark adaptation and lateral inhibition 8  Brightness perception – light/dark adaptation ▪ Consider a piece of paper with black text viewed either under indoor lighting or outside on a bright sunny day ▪ We perceive the text as black and paper as white in both situations (we call this brightness constancy) ▪ Why is this? The white paper will actually reflect less light indoors than the black paper will reflect outdoors The luminance of a retinal image depends on both the amount of light falling onto an object and the relative reflectance of the surface(units: cd/m2) 9  Brightness perception – light/dark adaptation Light / dark adaptation Example of a retinal ganglion cell ▪ The sensitivity of the retina is constantly response under different lighting conditions adjusted to compensate for changes in mean 100 luminance 80 Percentage response ▪ Sensitivity is reduced when the mean intensity of the image is high and increased 60 when it is low 40 ▪ This process, termed light/dark adaptation dictates that the retina encodes contrast 20 (the ratio of an object’s luminance relative to 0 the mean or background luminance) and 10000 100000 1 10 100 1000 Luminance plays a critical role in achieving brightness (cd/m2) constancy 10  Brightness perception – negative afterimages ▪ While light/dark adaptation is essential for our visual systems to function in different lighting conditions, it can produce illusions under some circumstances + 11  Brightness perception – lateral inhibition Lateral inhibition ▪ early form of information processing in retina ▪ retinal ganglion cells receive both excitatory (+) and inhibitory (-) input from neighbouring photoreceptors ▪ arranged in a centre-surround configuration across the retinal image - - - - - ++ Example receptive field of an ON-centre OFF- - + surround ganglion cell - - - - 12  Brightness perception – influence of lateral inhibition ▪ Lateral inhibition makes the visual system sensitive to changes in luminance (important for detecting edges and borders of objects) ▪ Can have dramatic effects on perceived brightness Hermann grid illusion - - - - - - - - - - ++ ++ - + - + - - - + - - - - - The intersections are surrounded by more high intensity (white). This results Notice the illusory grey spots at the in more inhibition from the surround in 13 intersections on-center, off-surround receptive fields  Brightness perception – top-down influences ▪ Our visual systems also uses knowledge of how light interacts with 3D objects in the world when determining brightness ▪ for example, it tries to maintain brightness constancy when the amount of light falling on a surface is affected by shadows ▪ this can result in ‘errors’ in 2D images portraying 3D scenes Checker-shadow illusion 14 A & B have identical luminance, but different brightness  Colour perception Why does the world appear devoid of colour under low-light conditions? ▪ only rod photoreceptors are sensitive enough to operate ▪ rods contain a single type of photopigment (rhodopsin) ▪ light of different wavelengths and intensities can elicit identical responses ▪ this makes it impossible to accurately signal different wavelengths 15  Colour perception - trichromacy Cone photoreceptors contain one of three different photopigments, each with different wavelength sensitivities ▪ S-cones: Cones that are preferentially sensitive to short wavelengths (blue cones) ▪ M-cones: Cones that are preferentially sensitive to middle wavelengths (green cones) ▪ L-cones: Cones that are preferentially sensitive to long wavelengths (red cones) 16  Colour perception - trichromacy The relative outputs of the three cone types allows unambiguous signalling of wavelength 17  Colour perception – variations from trichromacy Monochromacy ▪ individuals have either 0 or 1 functioning cone type, resulting in complete colour blindness ▪ extremely rare (approx. 1 person in 100,000) Dichromacy ▪ only 2 functioning cone types ▪ Protanopia (1% males, 0.02% females) = missing L-cones ▪ Deuteranopia (1% males, 0.01% females)= missing M-cones ▪ Tritanopia (0.002% males, 0.001% females) = missing S-cones Bunch of flowers Bunch of flowers as as seen by a seen by a typical trichromat protanope 18  Colour perception – variations from trichromacy Anomalous trichromacy ▪ more common form of colour perception deficiency ▪ characterised by defect in one of the cone types ▪ Protanomaly (1.3% males, 0.02% females) = L-cone defect ▪ Deuteranomaly (5% males, 0.35% females)= M-cone defect ▪ Tritanomaly (0.01% males, 0.01% females) = S-cone defect ▪ Commonly assessed using the Ishihara Colour Test Example test plate (warning – uncalibrated) - those with normal trichromacy typically see an 8 - deuteranomalous individuals typically see a 3 - A monochromat would be unable to discern any number 19  Colour perception – opponency Like brightness, perception of colour is also shaped by bottom-up processing of visual information in the retina and beyond Colour opponency ▪ retinal ganglion cells receive excitatory (+) and inhibitory (-) input from different cone types ▪ results in distinct Red/Green and Blue-Yellow pathways 20 Colour perception – negative afterimages ▪ colour opponency can be demonstrated using negative afterimages ▪ e.g., staring at a red object will result in a green afterimage ▪ adaptation to red causes a reduction in the sensitivity of long wavelength cones, creating an imbalance in the inputs to red/green opponent retinal ganglion cells 21 The ‘Lilac chaser’ revisited Can we now explain why this illusion occurs? 22  Colour perception – top-down influences ▪ Our visual systems also try to achieve colour constancy by accounting for the intensity and composition of light hitting different surfaces ▪ Colour constancy – the tendency for the perceived colour of objects to remain the same, even if the lighting changes ▪ This can give rise to illusions in which the same wavelength of light is perceived as different colours 23  #thedress revisited Why do people perceive the dress differently? 24  Summary Learning objective: Understand the stimulus for vision and basic structure of the human visual system ▪ Visible light is a band of energy within the electromagnetic spectrum, and can be described by ▪ its wavelength (difference in peaks of the electromagnetic waves) ▪ its intensity/luminance (amount of photons) ▪ The optics of the eye project light onto the retina, which converts the energy into electrical signals ▪ transduction achieved by different types of photoreceptors (rods & cones) ▪ information processing begins through circuitry of wiring between photoreceptors and retinal ganglion cells ▪ signals leave the eye via the optic nerve and are relayed to visual cortex at the back of the brain 25  Summary Learning objective: Describe the factors that shape the perception of brightness ▪ Brightness is associated with the intensity of light reflected by an object ▪ However, our perception of brightness is heavily influenced by: ▪ Bottom-up factors (how the visual system processes input to the eyes) ▪ Light/dark adaptation ▪ Lateral inhibition ▪ Top-down factors (knowledge about the environment) ▪ Impact of shadows ▪ These factors help vision function in normal environments (e.g. achieve brightness constancy) but can result in illusions under some circumstances 26  Summary Learning objective: Understand the principles of normal and abnormal colour perception ▪ Colour is a perceptual property associated with the wavelength of light ▪ Normal human colour perception relies on the relative output of three types of cones (trichromacy) ▪ Abnormal colour perception occurs ▪ in very rare instances where individuals have less than 3 functioning cone types (monochromacy, dichromacy) ▪ in rare instances where defects exist in one of the cone types (anomalous trichromacy) ▪ Colour perception is also shaped by both bottom-up and top-down factors ▪ Colour opponent processing ▪ Colour constancy 27 Thank you Any questions?