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Lecture 5


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University of Toronto Scarborough
Matthias Niemeier

Lec 5 SAQ: Describe one experiment that would suggest that faces aren‟t special. What were participants asked to do? Which method was used? What was found? - Greebles experiment - Researchers created artificial things called greebles made of geons and there were entire families - Participants were asked to train to recognize them - Those trained to recognize the greebles, had brain areas that were stimulated by looking at the greebles - FFA was activated - Not the greebles themselves, but someone with expertise with the greebles caused these neurons to fire - Point: FFA is not really a face area, but is an area of visual expertise o Whatever you are expert to will cause this area to light up - LOC  any lesions here cause problems with RECOGNITION Color and Motion - Colour is in our minds - The „univariance‟ problem Colour: not a physical property, but rather a psychophysical property - We mostly use light that is reflected  bounces off object - We have a single light source - We only see between 400-700 nm The problem of univariance - Rods used in dark conditions, cones in the light - During night, we don‟t perceive colours Ex. picture of clown - Considering the green layer o Looking through a green filter o Would see that the nose has about the same luminance as the stars on the cheek of the clown  In actuality, nose is red, stars are blue o Can‟t distinguish between any colours Problem of univariance: some combination of wavelength and intensity can elicit the same set of response in one set of cones - One type of photoreceptor can‟t tell differences in colour based on wavelengths (graph) - Shining different colours of light onto the cone, causes different responses  somewhat bell shaped curved - Peaks in the middle  responds best to the colour green - Blue light creates the same response as orange on the same photoreceptor Scotopic: referring to dim light levels - Rods sensitive to this type of light - All rods have same photopigment molecule: rhodopsin Types of cones: S, M, L - Newton found that shining light through a prism causing a rainbow o found that there are spectral components o colour is something the brain is creating  sensation not physical Young-Helmholtz theory - states that there is trichromatic colour vision - colour vision is based on 4 photoreceptors sensitive to certain ranges of wavelengths - can recreate any coloured light by using blue, green and red light and mixing them Cone photoreceptors: - s-cones: sensitive to short wavelengths - M-cones: sensitive to middle wavelengths - L-cones: sensitive to long wavelengths - Blue light has a certain set of responses in cones, while orange will also have a different set of responses - We are much less sensitive to blue light We seldom see one wavelength at a time - We normally have an entire range - Entire spectrum of light is represented as it goes from raw to cooked meat Looking at 2 wavelengths, red and green cones: - If you mix red and green light together, result is yellow light o See as yellow because of our inability to see mixes composed of different wavelengths  METAMER o Looks yellow because we sample the wavelengths o M cone responds with same amount of response as if there was a combination of green and red in the yellow light Metamer: any pair of stimuli that appear identical even under different physical differences. In terms of light, it means different types of wavelengths that look identical Additive colour mixture: a mixture of lights. If light A and light B are both reflected from a surface to the eye, in the perception of colour, the effects of those two lights add together - What happens when mixing light with different colours - Different from mixing different coloured paints Paints work in terms of subtractive colour mixture Subtractive colour mixture: mixture of pigments if pigment A and B mixed, some of the light shining on the surface will be subtracted by A and some by C. only the remainder contributes to the perception of colour (fig. 5.7) - See white light on a coloured patch - If light is shined on broadband range of wavelengths - If blue; blue is being bounced off and red, yellow, etc are being absorbed and absorb other wavelengths  subtracting - Yellow patch o Blue is absorbed and yellow is reflected o Subtracting blue - Mixing these two colours together - Get a narrow range of wavelengths  becomes darker - Mixing more colour in, less colour escapes and eventually turns black Colour space is 3D because we have 3 cones - Hue: chromatic aspects of colour - Saturation: chromatic strength of hue - Brightness: distance from black in colour space - Can see colours not expressed by wavelengths o Can use 3D cones to represent single wavelengths Non- spectral hues: hues that don‟t exist as pure forms of light but only as mixtures of different wavelengths - Single wavelength cannot make magenta Trichromacy Opponent colour theory - Has a lot to do with aftereffects Hering said 3 dimensions of colour are like dimensions of space - If colours are 3D, every combination of red green and blue should be possible - Can have things like boundaries with language - No such thing as reddish-green or yellowish- blue - Certain colour combinations don‟t exist Opponent colour theory - Perception of colour is based on 3 mechanisms, 2 colour mechanisms - Each mechanism is based on red-green, yellow-blue, white-black o Opponent organizations o Red and green are close together - Helmholtz and Hering were enemies o Both were right in their statements - Black and white in normal daylight are still seen with cones; can pull over the response of the other cones - Get black and white through the addition of all S,M,L cones - Get red and green from M and L cones subtracting - Get blue-yellow from M and L adding and S subtracting - At some point, colours work on the theory of opponency Neurophysiological support for the Opponent Colour theory - In LGN find that there are colour-opponent cells: neurons whose output is based on a difference between sets of cones - Have opponency in center-surround - In V1, have blobs - In V2 have thin stripes containing regions part of the colour system - Area V4 = human colour area o Lesion here causes an achromatopsia  Colour blindness  Inability to view colours due to damage to the central nervous system - If lesion is in right hemifield, can‟t see in the left eye Does everyone see colours the same way? Yes! - General agreement on colours - Same metamers - Some variation due to age – lens start to turn yellow after a while o Visual system is not meant to last 80 years Does everyone see colours the same way? No! - 8% of males have some form of colour vision deficiency - This is because they have reduced colour vision - Even normal colour vision in males is not as good as females Two types of colour blind people: - Cone monochromat: only one type of cone, truly colour-blind o Difficulties with night vision - Rod monochromat: only rods in their retinas o Can‟t see colour at all o Truly colour-blind o Visually impaired in bright light  need to wear sunglasses all the time 3 types of colour-anomalous people: 1. Deuteranope: no M-cones 2. Protanope: no L-cones 3. Tritanope: no S-cones o Have difficulties with telling apart yellow and blue Does everyone see colours the same way? Maybe - Cultural experiences - Cultural relativism - Some colour experiences are dependent on culture o Has an impact o
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