1. Introduction to Visual Perception
Visual Illusions

2. What is Visual Perception?
The active process of organising and interpreting sensory stimuli sent from the senses
It is used to create a meaningful mental representation of the world
It forms the basis of our behaviour.
It includes the structures involved in vision, including the eyes, receptors, nerve pathways and the visual cortex

4. So how do we see?

5. The Process of seeing

6. How do Visual Illusions work?
Visual illusions are mistaken perceptions of visual stimuli.
We misjudge the length, position, size or direction in a visual illusion.
The brain is tricked – which means that what we perceive is different to reality.

7. So how do organise information?
Visual perception relies on three key principles to organise information
Gestalt principles
Depth perception
Perceptual constancies

8. Gestalt Principles Figure –ground Closure Similarity Proximity
It is believed that we organise stimuli into categories or groups to understand them
Figure –ground
Closure
Similarity
Proximity

9. Figure-ground
A stimulus appears to stand out (figure) against the background (ground)

10. Closure
We complete the gaps in a figure to view the stimulus as a whole

11. Similarity
We perceive objects that have similar features (size, shape, colour) as belonging to a group

12. Proximity
We perceive objects that are physically close together as belonging to a group

13. Depth perception The ability to see in 3D and to judge distance
Binocular cues – use both eyes to judge depth
Convergence - inward turning of the eyes to focus on nearby objects try looking at the end of your nose
Retinal disparity – the difference between the images of both eyes....line an object up in the distance with your finger. Try looking at this with the left eye and then the right eye separately. What happens
Monocular cues – use one eye to judge depth
Accomodation – the bulging and flattening of the eye to focus
Pictorial depth cues – found in pictures, drawings and photos.

14. Pictorial depth cues Linear perspective Relative size Interposition
There are many cues that we use in pictures to understand depth
Linear perspective
Relative size
Interposition
Height in the visual field
Texture gradient

15. Linear perspective
Two lines that converge (come together) as they go into the distance

16. Relative size
The object that has a smaller retinal image is seen as being further away

17. Interposition
An object that sits in front of / obscures another object is perceived as being closer.

18. Texture gradient
An object that is further away from the person will have less detail

19. Height in the visual field
An object that is closer to the horizon is seen as being more distant

23. Perceptual constancies
Our ability to understand that an object’s size, shape, brightness or orientation will stay the same even though it has a different retinal image.
Size constancy
Shape constancy
Brightness constancy
Orientation constancy

24. The Ames Room A man-made illusion that affects size constancy......
It appears as if people are growing and shrinking as they cross the room.

25. The Ames Room Why does it work?
The room is trapezoid in shape with a sloping floor. The people look larger because they are closer.
We cannot judge distance as we can only use monocular cues (peephole)

26. The Ames Room

27. The Muller-Lyer illusion
Two lines of equal length appear different in size.
One has arrows facing in (which makes the line look longer) and the other has arrows facing out

28. The Muller-Lyer illusion
How does it work? We see the line with arrows on the end as the outer, vertical corner of the building which is closer than the inner, vertical corner

29. The Moon illusion
The moon close to the horizon looks bigger than the moon high up in the sky, although retinal image is the same.

30. The Moon illusion
How does it work? The moon’s perceived distance is greater at the horizon due to depth cues being present.

31. The Ponzo illusion
Two horizontal lines are the same length. When placed within two converging lines, it looks like the line further away is longer than the closer line.

32. The Ponzo illusion
How does it work? The top line looks to be further away because we use our pictorial depth cues (linear perspective, Height in the visual field) Also due to size constancy the distant top line appears to be enlarged.