Chapter 5: Top-down perception

Question 1

What were some of the issues that arose with computer vision?

Answer 1

• Could only identify faces head-on
• Very poor with image cluster, unable to identify objects when partially occluded
• The variable views that are associated with many objects

Question 2

What’s the inverse projection problem?

Answer 2

• The retinal stimulus is ambiguous because it’s perceiving firing rates from 3D objects on a 2D screen, that being the retina
• This means that many objects in our visual field can create the same pattern of firing on the receptors on the retina
• It would be impossible to differentiate the objects using this information alone, that’s why bottom-up perception does not work in this situation

Question 3

What was Stephen Palmer’s 1975 experiment? What did it demonstrate?

Answer 3

• First asked participants to name line-drawn objects
• Before seeing the object, participant was shown a scene to provide context. The scene shown was either appropriate or inappropriate for the line-drawn object that would be shown.
• There were two types of inappropriate objects: either looked similar to an appropriate object or it looked different from an appropriate object
• Results: performance was better when object was presented after seeing the appropriate context. There was more accurate and faster identification of the object
• If object presented briefly, people often mistake the incorrect, but similar object for the context.

Question 4

What are the three major components that are emphasized in top-down perception?

Answer 4

1) Context
2) Attention
3) Organization/structure

Question 5

How does context impact top-down presentation?

Answer 5

• Context helps us form expectations on how we think things should be (ex. Palmer’s study)
• Also can be seen in the Jumbled Word Effect
• Seen in Harold Kelly’s ‘Guest Lecturer’ (students had preconceived notions of how the lecturer would act based on pamphlets they were given) - demonstrate perceptual sets

Question 6

What are perceptual constancies?

Answer 6

• The tendency to experience certain things the same way, even if there are changes in the sensory input
• If this didn’t happen, we would have to rediscover what something is everytime it changes

Question 7

Why is size constancy important in top-down perception?

Answer 7

• States that object size is not determined solely by the size of the image on the receptors
• The size of an object is perceived as constant, even across different distances, despite the fact that farther objects stimulate less space on the retina
• Still able to perceive bigger objects that are farther away as being larger compared to those which are small but up close

Question 8

Why is colour constancy important in top-down perception?

Answer 8

• States that object colour is not solely determined by the wavelengths of light reflected from its surface
• The colour of an object is generally perceived as constant, even across different lighting conditions, even though the wavelengths reflected back are physically different
• Helps us perceive different colours in different lightings

Question 9

What’s inattentional blindness and how is it related to top-down perception?

Answer 9

• The failure to register unattended stimuli in consciousness
• Think the gorilla video
• People can’t really multi-task
• Want to expect what we should pay attention to, don’t notice the unexpected
• This is also important for evolutionary importance, as we don’t want to be distracted by things that aren’t really important
• Want our attention to be grabbed by sexual stimuli or threatening stimuli.

Question 10

What are the different components of perceptual organization?

Answer 10

• Representing edges and regions
• Figure vs. ground
• Perceptual grouping (gestalt laws)
• Perceptual interpolation

Question 11

What’s edge extraction?

Answer 11

• Done during object recognition
• The process by which the visual system determines the location, orientation, and curvature of edges in the retinal image

Question 12

What specific regions of the brain correspond to orientation and curvature?

Answer 12

• V1 - orientation
• V4 - curvature

Question 13

What assumptions do we make when determining the figure-ground organization?

Answer 13

• We assume objects that are found on top of a continuous background
• The edges and contours we perceive belong to the object that is found in front.
• The assumption that the background objects/patterns continue behind the foreground is critical to perception

Question 14

What factors help with figure-ground determination?

Answer 14

• Depth
• Surroundedness
• Symmetry
• Convexity (usually perceive objects in front as having convex borders)
• Meaningfulness (the visual system can immediately recognize familiar objects)

Question 15

Why is perceptual grouping crucial when observing a visual scene?

Answer 15

• Single objects can be broken into many different sections within the retinal image
• Perceptual grouping combines and groups these processes together so that we perceive the whole image, even if portions are occluded
• “Perception of the whole” Gestalt psychologists

Question 16

Why is it important to consider Gestalt psychologists when discussing top-down processing?

Answer 16

• Because their laws reflect top-down processing
• Their understanding was that the world tends to be relatively predictable, and our perceptions tend to match this idea as well.

Question 17

What’s the law of continuity?

Answer 17

• Points that when linked together, would make a straight or smoothly curved, continuous line are seen as belonging together
• Ex. Reading roads on a map.

Question 18

What’s the law of proximity?

Answer 18

• Elements that are near one another tend to be perceived as going together
• Ex. Groups of friends at lunch

Question 19

What’s the law of common rule?

Answer 19

• Elements that move in unison are likely to be perceptually grouped
• Ex. synchronized swimmers

Question 20

What’s the law of symmetry and parallelism?

Answer 20

• Elements that are symmetrical/parallel tend to be grouped together
• Ex. Yellow lines along a highway

Question 21

What’s perceptual interpolation?

Answer 21

• Process of filling in edges and surfaces that aren’t visible
• Often objects are occluded by other objects or blend into the background

Question 22

What two processes assist during perceptual interpolation?

Answer 22

1) Edge completion - the perception of a partially hidden edge as being complete, even though the edge is not physically present. Sometimes referred to as illusory contours
2) Surface completion - the perception of a partially hidden surface as being complete (but hidden/occluded)

Question 23

What’s the law of meaningfulness?

Answer 23

• Tend to perceive things in terms of meaningful or familiar groupings

Question 24

What’s the principle of Pragnanz (parsimony)?

Answer 24

• Tend to perceive things such that the resulting structure is as simple as possible
• Resort to the simplest interpretation of our surroundings
• Sometimes called the law of simplicity

Question 25

What has modern research shown us regarding perceptual organization?

Answer 25

• Perception is affected by both the physical and semantic properties of our world
  *Semantic - the overall ‘meaning’ of a visual scene. Think of the Palmer experiment

Question 26

What are two very common physical regularities?

Answer 26

1) Oblique effect - people are more accurate in estimating and perceiving horizontal and vertical lines, compared to angled lines. Horizontal and vertical lines are much more common in the physical world
2) The Light-From-Above assumption - perceive indentations and protrusions by assuming that light is coming from above (like the sun)

Question 27

What are semantic regularities?

Answer 27

• The meaning of a scene or object and how it relates to what it does or how it’s used, or what happens in that place.

Question 28

Why was there such difficulty with computer vision?

Answer 28

• Image clutter
• Object variety
• Variable views
• Retinal stimulus is ambiguous