Optical Illusions: Why the eye sees what it

sees?Arjun Gandhi (1001070119)

Alejandra Fuentes Valdez (1002335706)

Content• Types of illusions• Static illusion – Explained examples• Motion illusions – Explained examples

Type of Illusions• Physical illusionsCaused by distortion of the light stimuli before reaching the

eyes by phenomena such as reflection, refraction, dispersion, etc. e.g. Mirage

• Cognitive illusions Caused by the way the light stimuli is perceived.

Physiological: Physical characteristics of the visual system. E.g. Equiluminance illusion

Psychological: The way brain perceives the received stimuli.

E.g. Müller-Lyer size illusion

Static Illusions

• Equiluminance• Müller-Lyer illusion• Blind spots

Equiluminance Illusion

• An object defined by equiluminant colors can be seen by the “What” system of the visual system depending on the LGN Parvocellular cells but is invisible (or poorly seen) by the “Where” system depending upon the LGN Magnocellular cells.

• Such an image may seem flat, it may seem to shift position or it may seem to float ambiguously because there is too little luminance contrast to provide adequate information about its three-dimensional shape, its location in space or its motion (or lack of motion).

Plus Reversed Illusion

Müller-Lyer illusion

• This is a psychological illusion.• Researchers found varying results among people as per

demographics.• People in Africa vs people in America• Males vs females.• People’s perception shaped by their surroundings• Skin colour effects the pigmentation in eye which causes

varying effects of this illusion among people

Which one is longer ?

How about now ?

Working mechanics

• We see the image of the object rendered by the brain.• Brain adjust according to the perception of edges in a 3d

scenes, applies concept to the 2d image and hence the illusion.• Inward edges vs outward edges.• When an object is away, brain expects it to appear shorter than

it is and our perception is moulded accordingly while creating the image

Other examples ?

• People moving away do not seem short.• Hands do not seem of different size.• The stimuli coming from Moon does not become small when

near the horizon so brain thinks that its larger in size.• Hence moon seems larger in horizon than when it is overhead.

Blind spots

• There are no photoreceptors in the eye where the optic nerve meets the eye.

• When using both eyes, their visual fields overlap and overcome this fact.

• When using one eye, the brain fills in the void with the most obvious option. E.g. The background colour

Illustration of Blind spot

Motion illusions• Perception of motion• Motion illusions:

• Reverse phi• Waterfall effect (motion after-effect)• Rotating snakes (Peripheral drift)• Flash-lag

Perception of motion• Motion is a primitive mechanism

of vision• Motion sensitive neurons in V1

send information to higher areas• V5/MT area: cells sensitive to

speed and direction.

Motion aftereffect (1/2)• Viewing directional motion for a period of time and subsequently

stationary objects will seem to move in the opposite effect• Known as “waterfall effect”• Mechanism: Adaptation of directionally selective neurons.

• Demo: https://www.youtube.com/watch?v=GkRHN0rnfME

Motion after effect (2/2)• Similar to ON/OFF centre

ganglion cells • After adapting to a

unidirectional movement, the baseline of what is static is changed

Reverse phi• The reversal of perceived motion

direction when the contrast is reversed.• Direction-selective cells in cortex

reversed excitatory and inhibitory regions when two different contrast bars were flashed sequentially during a two-bar interaction analysis.

• [DEMO]

Flash-lag effect (1/2)• Two aligned objects, one moving

object and one flashed, appear to be misaligned

• Flashed object seems to be behind

Flash lag effect (2/2)• Possible explanations:

• Visual system is slow so it has to ‘predict’ the position of a moving object

• Visual system processes moving object faster than flashing objects

• Due to integration time, the visual system perceives what happened slightly after the flash (similar to backward masking)

Peripheral drifting (1/2)• A static object appears to be

moving • Factors: eye movement,

luminance, and contrast • Higher luminance/contrast

points are processed faster than low luminance/contrast points

Questions?