Dr Kevin Paterson

1

• Topics
• Introducing the eye the visual system
• Theories of visual perception
• Using visual illusions to study vision.

• Dr Kevin Paterson
• k.b.paterson_at_derby.ac.uk
• Essential reading
• Eysenck, M. W., Keane M. T. (2001). Cognitive
  psychology. Psychology Press Hove, UK.

2

• FUNCTION OF EYE
• Light is reflected from surface of objects. It
  passes through pupil lens before landing on
  retina at the back of the eye.
• The optic nerve passes neural data to the
  mid-brain. This passes information onto the
  visual cortex.
• Visual data is processed at the retina, by areas
  of the mid-brain, and regions of the visual
  cortex.

3

• BOTTOM-UP TOP-DOWN PROCESSES
• Major debate is vision predominantly a bottom-up
  or top-down process?
• Bottom-up perception (e.g. Marr, 1980) sensory
  information is processed in terms of basic
  features, features are combined to form objects,
  and objects are recognised. Perceiver
  reconstructs objects in world.
• Top-down accounts (e.g. Gregory, 1970) perceiver
  uses knowledge and expectations to impose
  interpretation on incoming sensory data.
  Perceiver hypothesises about objects in world.

4

• Visual illusions provide evidence for top-down
  processes in vision
• In order to recognise the image, viewers do not
  just process feature information, but must
  interpret it.
• What do you see when you look at the image on the
  right a young woman or an old one?

5

• Visual illusions provide evidence for top-down
  processes in vision
• Do you see top image as glass, or as two faces?
• Note you can only see one version at a time.
  Reversals follow shift in attention.
• Look at version below. Depending on what you read
  it is possible to bias your interpretation
  towards one of the possibilities.

6

• Knowledge-based visual illusions
• In top image girls appear to be different height
  - however, they are in fact the same height.
• Ames room is constructed so that it looks like
  real room, but has different dimensions and
  shape.
• From viewing point, distant people appear nearby.
  Knowledge about normal rooms influences
  perception.

7

• Muller-Lyer illusion
• Are lines in top image of same or different
  lengths? Most people say that right-hand line is
  longer.
• Illusion may occur because of familiarity with
  vertices in buildings and other environments.
• Knowledge that ? angle usually indicate distant
  edge and ? angle usually indicates nearby edge
  may influence our perception of length.
• Again, evidence that knowledge influences
  perception?

8

• Cues to distance
• Ames room - dimensions shape of room provides
  cue to distance.
• Muller-Lyer - knowledge about vertices provides
  cue to distance.
• Interposition / occlusion - object in front is
  taken to be nearer.
• Relative size - large objects are taken to be
  nearer.

• Location in visual scene - objects high in scene
  are more distant.

9

• Cues to distance
• Texture gradients - Close objects have a coarse
  texture distant objects have a finer gradient.
  See how the shape of the cobbles disappears fade
  into the distance in this painting.
• Convergence of parallel lines indicates
  increasing distance.

10

• Cues to distance
• Convergence in binocular vision (two-eye)
• Eyes converge at same point of the visual scene.
• Eyes turn in for close objects.
• Eyes look straight ahead for distant objects.
• Eyes serve as range finders.
• Most important for perceiving distance of objects
  located near us

11

• Can you identify the following cues to depth?

12

• Constructivist theories of visual perception
  (e.g. Gregory, 1980) assumes that perception is
• Active constructive meaning that perceiver
  determines nature of the visual image and is not
  a passive recipient of information.
• Perception is influenced by top-down factors
• expectations knowledge motivation factors
  emotional factors
• However, demonstrations rely on errors (when most
  perception is correct?) and on use of unnatural
  stimuli such as illusions.

13
Marrs (1982) computational approach to vision

• Vision is an information processing task in which
  retinal information is processed as low-level
  feature that are combined bottom-up to produce a
  representation of objects and scene.
• First extract basic features from visual image
  and use these to determine edges of surfaces.
  Having identified edges, it is possible to
  separate objects and identify them.

14
Marrs (1982) computational approach to vision

• Stages in processing of visual information
• Processing is bottom-up

15

• Direct perception (Gibson, 1979)
• Ecological approach emphasises the importance of
  interaction of people and their environment.
• Dynamic visual perception rather than perception
  of static images. Dismissed illusions as
  tricks.
• Emphasises use of optic array, which is the flow
  of environment during movement.
• Argues that objects provide affordances about
  meaning.

16

• Direct perception of distance is afforded by
  motion parallax.
• Motion parallax occurs during movement. More
  distant objects appear to be passed or move more
  slowly than near objects.

Compare near and distant objects next time you
travel by car or train.
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• Affordance refers to the actionable properties
  of objects.
• According to Gibson, objects are directly
  perceived in terms of the actions they afford.
• For instance, a door handle affords the action of
  opening.
• Objects can be throwable, hittable, can be sat on.

• Gibson argued that mailboxes directly afford the
  act of posting mail. Do you think this is true?

18
Comparing approaches to vision

• Constructive approach
• Emphasis on top-down processing, but does not
  demonstrate how these effects occur.
• Emphasis on processing of static images.
• Gibsons approach
• Emphasis on processing of dynamic visual
  information.
• Concepts vague at time.

• Marrs approach
• Detailed testable.
• Consistent with physiological knowledge.
• Emphasis on bottom-up processes.
• However, emphasis on processing of static images.
• What happens when we view dynamic visual scenes?