optical illusions

Question 1

mach band (luminance based)

Answer 1

description: look across space and see luminance level, starts at light grey moves to dark grey. see a light line on left and dark line on right.
explanation: elongated receptive fields at V1, slide receptive field
(see notes for diagram)

Question 2

scintillating grid and hermann grid (luminance based)

Answer 2

description: white dots where you’re looking, dark ones where you aren’t looking. intersections are white/bright where you look, dark/black away from where you look
explanation: viewpoint dependent;, basic receptive field processing, center surround RF in LGN; smaller RF in the fovea, larger RF in periphery, in the fovea, RF is small enough that you activate diff ones at each place you focus on, periphery RF large enough that they overlap and perceive luminance incorrectly

Question 3

check shadow illusion (contrast effects)

Answer 3

describe: A looks darker than B, even though they are the same
explanation: local contrast difference, A is darker because of the light surround (contrast enhancement), B is brighter because of the darker surround and has a shadow over it, retina wants to reinterpret lighting

Question 4

ponzo illusion (contrast effects)

Answer 4

describe: railroad tracks, top one looks larger
explanation: linear persepective and depth ordering, line has to be bigger in real world to be represented same on retina; local size contrast

Question 5

cow head grouping (object based illusions)

Answer 5

describe: black and white spots-look like cow face
explanation: bayesian framework

Question 6

lion face (object based illusions)

Answer 6

describe: see a face in the trees
explanation: prior exposure to faces encourages you to identify it as a face

Question 7

moving hallway (motion based illusions)

Answer 7

describe: if you block off surround, it goes slower, if you block off middle it goes faster; altered optic flow; designed to have 2 diff speeds represented
explanation: cue combo; averaging/combining information

Question 8

wagon wheel effect (motion based illusions)

Answer 8

describe: always going counterclockwise, appears to go clockwise
explanation: aliasing; physical reality only captures particular direction, perceptual-discrete motion detectors

Question 9

letter shifting (motion contrast illusions)

Answer 9

letters seem like they’re moving locations, not moving around, changing luminance

Question 10

pigeon neck (motion contrast illusions)

Answer 10

looks like the bird is moving nek back and forth

local motion perception speed is dependent on the contrast with the background

Question 11

watermelons and rotating snake (static movements)

Answer 11

physically constant image that looks like its moving as you move your eyes
medium luminance in the middle, bright luminance on one side, dark luminance on the other
line motion illusion; perceive motion with abrupt changes in local luminance

Question 12

high/low spatial frequency images

Answer 12

woman looking out at winder, blue image-looks like abe Lincoln
move far away pixilates the picture
edges and details are high freq info, light levels in each square are low freq (easier to see things in low freq)

Question 13

distance/size face switching

Answer 13

when you squint, they seem to switch; angry face is in med high freq, neutral face is in low freq; change in perception when you shrink it (angry face is still very high frequency, neutral face goes from low to med)
spatial contrast sensitivity function; most sensitive/perception is based on med freq