Illusions Flashcards
What is an optical illusion?
When a percept differs from the generating stimulus (image) in a meaningful yet misleading way.
What are some examples of different types of visual illusions?
- Illusions of size
- Moon illusion
- Figure-ground illusions
- Ambiguous figures
- Pulfrich effect
- Hybrid images
- Orientation illusions
- Impossible figures
Why do illusions of size occur?
They occur due to misapplied size constancy
What is size constancy?
Size Constancy = ability of visual system to perceive the actual physical size of objects despite differences in their distance (and consequent differences in the visual angle of the image on the retina)
[For example, in the photo we perceive the person viewed far away as bigger than the drink viewed close up, even though the angular size of the drink is bigger on the retina.}
What does size constancy rely on?
Size constancy relies on depth perception – depth cues allow us to determine distance of object and to scale our perception of size accordingly.
We have a size-distance scaling system.
[E.g. in the photo the man in the white shirt appears a normal size but if we placed him next to the woman in the pink shirt we can see he is actually of much smaller retinal size]
What is the moon illusion and how does it work?
Photographically we can prove that angular size of the moon is the same, regardless of its position in the sky. However…we perceive it as being BIGGER on the horizon than when it is higher in the sky.
Why do some people disagree with the moon illusion?
People will usually say that the moon on the horizon looks closer or the same distance as the moon high in the sky! Contradiction between observer’s reports and apparent distance theory due to angular size contrast theory.
We compare the angular size of the moon to the angular size of other objects on the horizon with a smaller visual angle than the moon,
Makes moon appear big by comparison.
What is an example of an illusion that demonstrates angular size contrast theory?
What are the Gestault laws for the interpretation of retinal images?
- Law of simplicity – every stimulus pattern perceived in way that makes resulting structure as simple as possible.
- Law of similarity – group together objects which are similar in shape, lightness, size etc.
- Law of good continuation – lines always seen as following the smoothest path.
- Law of Proximity – objects close together appear to be grouped together.
- Law of common fate – things moving in the same direction appear to be grouped.
- Law of familiarity – objects will be grouped if groups are familiar and meaningful
Why are we able to see the cube in this image?
and even faintly fill in ‘illusory contours’ between.
It is the configuration of the overall stimulus that determines our perception. Not the individual contours.– i.e. higher processing.
What are ambiguous figures illusions?
Sometimes information provided via our eyes can be ambiguous and incomplete.
So our perceptual system needs to disambiguate and interpret it.
An example of this is ambiguous figures where there are two (or even more) likely perceptual interpretations of the same image, but never seen both at once.
Describe figure ground illusions
What you see depends on what you interpret to be the background or figure in question.
What is the pulrich effect and why does it occur?
Google the video- and wear sunglasses or a neutral density filter
Without the filter, it looks like the pendulum is moving in a linear motion…but when you put a neutral density filter/sunglass lens over one eye, there is a perceived elliptical rotation.
This is because the filter reduces the retinal illuminance in one eye, this causes a delay in the retinal processing of the image in that eye compared to the other eye.
There is a discrepancy in perceived position of the object between the two eyes. The filtered eye perceives the object as lagging behind position seen by unfiltered eye.
The brain fuses the delayed image with the non-delayed image causing this apparent elliptical movement of the object.
How can unwanted pulrich effect be managed?
Can be managed with the use of lenses with different tints in both eyes, media opacities and contact lenses (Rodriguez-Lopez et al., 2020).
Whom may an unwanted pulrich effect (i.e. seeing an eliptical movement when there is a linear one) be experienced by?
- May be experienced by patient with optic neuritis (demyelination of nerve fibres causing slowed neural transmission).
- A px who has one eye dilated (this causes problems with driving)
- Other conditions which have previously been found to elicit these symptoms include cataract and anisocoria
When do impossible figure illusions occur?
When the brain tries to form a 3D perception from a 2D image, but can’t form a stable ‘possible’ solution.
