Pulfich Illusion Flashcards
The Eye – Blind Spot
Each eye has a blind spot but there is no black hole in vision
VISION IS CONSTRUCTED!!
Completion
Receptors around the blind spot provide information to fill in the gaps
Edges are continued
Surfaces are interpolated
Best guess at what is in the blind spot based on what is around it
Three components of the Pulfrich Illusion
Three components combine to give rise to the Pulfrich Illusion
- Stereoscopic Vision
- Simple Harmonic Motion of the Pendulum
- The Filter / Lens
Stereoscopic Vision
The two eyes get slightly different images of the world
The brain combines the two images to get a single percept
This is important fordepth perception
Fixation
When you fixate on something, your eyes focus and align so that its images fall on the foveas.
But what about things you’re not fixating on?
Uncrossed disparity
when object is further away from the fixation point
Crossed disparity
when object is closer than the fixation point
Uncrossed & Crossed DisparitySummary
Images of objects located
Farther than the fixation point
Have uncrossed disparity
Image APPEARS
further to the left in space to the left eye
and
further to the right in space to the right eye.
Images of objects located
nearer than the fixation point
have crossed disparity.
Image APPEARS
further to the right in space to the left eye
and
further to the left in space to the right eye
Crossed & Uncrossed Disparity (cont)
The brain will interpret Objects with uncrossed disparity as further away in depth and Objects with crossed disparity as nearer in depth
- Simple Harmonic Motion
- Pendulum stationary until gravity overcomes ball’s inertia (the resistance of any physical object to a change in its state of motion)
- Accelerates (speeds up) until the bottom
- Reaches peak speed at the bottom
- Gravity slows down (decelerates) the pendulum
- Pendulum momentarily stationary at other side
- Dimming Filter / Lens
Signal transduction from retina to binocular regions of the brain is faster for brighter (more luminous) images.
The filters you put over your eyes dim the image that falls on your retina.
Putting the filter on only one eye means that signals from that eye to the visual cortex are very slightly delayed.
For your information (not quizzed) – ideally we’d use a neutral density filter which reduces luminance from all wavelengths of light and all orientations equally.
Left eye filtered – summary.
Left-to-right motion -> uncrossed disparity -> illusory backward arc.
Right-to-left motion -> crossed disparity -> illusory forward arc.
Combined = illusory motion in a clockwise ellipse
Both directions combined.Left eye filtered.This slide is for revision at home.
During left-to-right swing.
Acceleration of the pendulum combined with the filter dimming light to the left eye causes increasing uncrossed disparity as the ball moves from the left apex toward the bottom.
Thus the ball appears to get further away.
Deceleration on the upswing causes uncrossed disparity to decrease until the ball reaches the right apex.
Thus the ball appears to get closer.
Combined, this creates a backward arc.
During the right-to-left swing:
During the downswing crossed disparity increases as the ball accelerates toward the bottom.
Thus the ball appears to get closer.
During the upswing, crossed disparity decreases as the ball decelerates.
Thus the ball appears to get further away.
Combined, this creates a forward arc.
All together, it creates an illusion of motion in a clockwise ellipse.
Right eye filtered – summary.
Left-to-right motion -> crossed disparity -> illusory forward arc.
Right-to-left motion -> uncrossed disparity -> illusory backward arc.
Combined = illusory motion in an anticlockwise ellipse.
Both directions combined.Right eye filtered.This slide is for revision at home.
During left-to-right swing.
Acceleration of the pendulum combined with the filter dimming light to the right eye causes increasing crossed disparity as the ball moves from the left apex toward the bottom.
Thus the ball appears to get closer.
Deceleration on the upswing causes crossed disparity to decrease until the ball reaches the right apex.
Thus the ball appears to get further away.
Combined, this creates a forward arc.
During the right-to-left swing:
During the downswing uncrossed disparity increases as the ball accelerates toward the bottom.
Thus the ball appears to get further away.
During the upswing, uncrossed disparity decreases as the ball decelerates.
Thus the ball appears to get closer.
Combined, this creates a backward arc.
Together, it creates an illusion of motion in an anticlockwise ellipse.
What if you don’t have stereo vision?
10% of general population – deficient stereo vision.
Can’t see Magic Eye pictures
Can’t see the Pulfrich Illusion
But these people are still able to appreciate depth to some extent and don’t crash into things all the time. How?
Monocular depth cues
Parts of the visual scene that allow you to appreciate things in depth with only one eye.
Mono- = ‘one’
-ocular = ‘of the eye’
Can you think of any?
Occlusion
When things cover up a part of something else, we know that object must be closer than the one its occluding.
Retinal size
More distant objects project a smaller image to the retina.
Our brain knows that the ruler isn’t actually changing size, thus the changing retinal size is due to a change in distance of the ruler.
Linear perspective
Parallel lines appear to converge with distance
Texture Gradient
What is Texture Gradient?
It’s a gradual change in the appearance of objects.
Close objects- Rough and more distinct
Far objects- Smooth and less distinct.
Shape from Shading
Shading can give rise to depth perception.
Light typically comes from above – the protruding bits should get light and things under them should be shaded.
Light Scatter
Distant objects appear hazier & bluer.
Motion parallax
Perspective changes due to head movements similar to getting input from multiple eyes.
Sometimes you can see an animal waggle its head to get depth info via motion parallax.
Monocular Cues Summary
Linear perspective
The way parallel lines converge.
Occlusion
Objects further away blocked ‘occluded’ by objects in front.
Retinal size
Nearer objects cover more of the retina (looks bigger).
Texture gradient
Similar to retinal size (texture elements decrease in size).
Light scatter
Further objects seem more hazy & blue (due to dust/moisture).
Motion parallax
Perspectives change due to head movements.
Compare monocular perspectives.
Shape from Shading
Shading can cue depth perception based on assumptions about direction of light.
