Lecture 5

Question 1

The binding problem

Answer 1

How perceptual organization works. How does the brain combine distant element and features into a coherent whole object and background.

Question 2

Effect of experience

Answer 2

Once you get an organized percept of an image that is difficult to parse, the percept stays and is hard to unsee.

Question 3

Mechanisms of perceptual organizaton

Answer 3

Psychological mechanisms:
- Gestalt laws
- Texture segregation

Neural mechanisms:
- Selective deficits
- Grandmother cells vs assembly coding
- Anatomical connections
- Contextual modulation

Question 4

Gestalt laws

Answer 4

• Proximity
• Similarity
• Connectedness
• Common fate
• Closure: convex shapes over concave
• Good continuation/collinearity
• Symmetry
• Pregnanz/good form: reduction to simplest forms
• Figure-ground: what is background and foreground

Question 5

When does segregation take place with different features

Answer 5

Orientation matters but relative phase does not. Phase is the relative position of the segments.
When there is equal vertical and horizontal energy the segregation does not take place.

Question 6

Apperceptive agnosia

Answer 6

• Selective deficit in perceptual organization.
• Usually diffuse lesions in right hemisphere occipital/temporal/parietal cortex
• Patients see individual features but cannot integrate them into a whole
• Perform badly on degraded letter task, Gollin picture task and are bad at copying drawings

Question 7

Integrative agnosia

Answer 7

• Similar to Apperceptive agnosia.
• Can copy, but piece by piece.
• Unusual views vary.
• Overlapping objects are not seen individually.
• Elements are not grouped into objects

Question 8

Fast Feedforward Sweep

Answer 8

• Speeds through visual system within 80ms by means of feedforward connections
• Provides the neurons with their RF tuning properties
• Provides fast detection of hardwired features and feature constellations
• In that sense provides a set of grouping operations
• Enables intelligent sensorimotor reflexes

Question 9

Feedforward convergence

Answer 9

Cells get increasingly larger receptive fields and get progressively more complex tuning properties

Question 10

Horizontal connections in visual cortex

Answer 10

Long range horizontal fibers in V1 selectively interconnect cells with the same orientation preference. This is the neural substrate for the Gestalt law of grouping by similarity. By way of horizontal connections, elements of the same orientation tend to group together.

Question 11

Horizontal connections and grouping by line

Answer 11

The horizontal connections in the visual cortex tend to spread along a line. This connects cells with the same orientation and lying along a line in visual space. This explains the Gestalt law of good continuation.

Question 12

Gestalt law of common fate neural substrate

Answer 12

Horizontal connections in the MT, where neurons are direction selective, cause objects moving in a similar direction to be grouped together.