L5: Perceptual Organisation, Gestalt Psychology, and Face Perception

Low-level vision (early vision)

Extracts local information about lines, bars and edges 

Mid-level vision

Joins isolated features (from early vision)  into larger more meaningful groups, forming the basis for object recognition in high-level vision

• How is local information that is processed in V1 grouped into larger, meaningful units

What are the two extrastriate visual pathways

Ventral (‘what’) and dorsal (‘where’) streams (two extrastriate pathways)

What is the role of the ventral stream and what happens in animals when it is damaged?

• Object identification

• If lesion this area, animals have problem with object identification tasks BUT no problems telling you where in space the object is located

What is the role of the dorsal stream

Visuo-spatial information processing (‘where’ stream)

How do the two visual streams interact?

No strict seperation

• Complex connectivity between the two pathways 

• No strict anatomical or functional separation of ‘what’ and ‘where’ streams

Receptive fields and tuning in the ventral stream

Receptive field size increases as does tuning complexity

What is key in overcoming ambiguity (how to group line and edges together in a meaningful way?)

• In order to resolve this ambiguity we need constraining principles

• Objected projected as 2D image onto the retina

• Overcome ambiguity as out visual system make assumptions or has prior knowledge on how the image should be interpreted

  • Key in Gestalt psychology school

What is the main principle (motto?) of Gestalt psychology

The whole is greater (different) than the sum of its parts

Gestalt Principles of Perceptual Organisation

1. Principle of Proximity

2. Principle of similarity

3. Principle of common fate

4. Principle of good continuation

Gestalt principle of Proximity

If you change the spatial proximity of these elements, it will influence how you see them (see more as rows or columns)

Gestalt principle of Similarity

Things that are somehow similar are bound together in perception

Gestalt principle of Common Fate

• Things that move together are grouped together e.g. flock of birds

Gestalt principle of Good Continuation

Visual system assumes (prefers) smooth continuity

What does it mean when principles are concerned with Figure-Ground Assignment

How visual system decides what is in the foreground and background

Important general notes on Gestalt psychology

• Insight concerning the role of relationships was important but:

  • Original Gestalt principles were largely descriptive

  • Little or no experimental evidence 

• Recent work in visual neuroscience and psychophysics has established the why and how of some of these principles 

• Principles can work together or against each other

Who was the first person to push the idea that there are areas in the brain specialised for face procesing

Nancy Kanwisher

What are two accounts for the question: Is face processing special?

• Domain specificity

  • Mechanisms operate independently of general object perception

• Expertise 

  • Mechanism derive from general object perception but become finely tuned due to extensive experience (with faces)

• Both accounts agree face processing is special, disagree on why

Holistic Processing: Part-whole effect

Features are easier to identify when presented as part of a face

Holistic Processing: Face Inversion Effect (‘Thatcher Illusion’)

• Inversion disrupts processing of fine details and relationship between features 

• Less good at recognising faces upside down

• Parts of the brain specialised to process faces not triggered when inverted therefore makes holistic processing much harder

Prosopagnosia

Failure to identify or distinguish between faces, despite (otherwise) normal visual and cognitive ability

Delusional Identification

Part of prosopagnosia (not everyone has this though)

• Able to recognise faces but not the person (yes that face is identical to my wife but I don’t get the familiarity feeling that is my wife)

What is norm-based code in face perception

• Facial features are represented as deviations from the average face

  • Brain has stored representation of the ‘average’ face, all other faces are compared to that

  • In theory, the further the face deviates from the norm, the easier it is to remember

  • Useful as makes explicit what is distinctive about a face

Summary: Is Face Processing Special (don’t need to memorize just to help revision)

• Evidence suggests

  • Special neurons in inferotemporal cortex

  • Relies on holistic processing

  • Prosopagnosia can disrupt face recognition selectively

  • Norm-based code

• On-going debate about whether face processing is special due to domain specificity or expertise 

  • None of the above points explain why face processing is special (ongoing debate)