Perception

Perception

• seems easy - when we open our eyes, we see

• large portions of cerebral cortex are devoted to perception

• 20% cerebral cortex devoted to processing vision

• hard to get computers to do what we find easy

Recognising the Perceptual World

Recognition → involves both bottom-up and top-down processing

Bottom-up → information from the sensory receptors (edges, colours, tones), assembled into a whole. Data-driven.

Top-down → information from higher levels of brain (knowledge, expectations.) Concept driven.

Bottom-up Processing

Retina → LGN → primary visual cortex → areas beyond primary visual cortex

Early visual system sensitive to basic features (spots light) but as move up the visual system increasing sophistication (orientation, motion, colour, form, faces)

The perception of a whole object is built up from smaller parts of vision

• Marr → computational model

• Treisman → Feature Intergration Theory

• Biederman → Recognition-by-components

Marr: Computational Model

Three stages:

1. Primal Sketch = Local features stage

2. 2D Sketch = Shapes stage

3. 3D Model representation = Objects stage

Triesman: Feature Integration Theory

2 stages

1. Preattentive stage: stimulus decomposed into basic properties (primitives.) Automatic, rapid, unconscious, parallel (independent of number distractors), bottom-up Visual search: by measuring time taken to find targets can see which primitives “pop-out”

2. Intergration stage - once primitives are extracted they are combined (conjunction.) Not automatic, slow, conscious, effort, serial (depends number of distractors, top-down

Biederman: Recognition-by-components

Theory how recognise 3D objects based on building complex objects from set primitives. 36 basic geons - the ‘alphabet’ of objects

Each geon has unique set features that allow it to be easily identified from different points of view

Destroying geons by obscuring their distinguishing features makes it hard to recognise objects

Direct Perception - Ecological

J.J. Gibson (mid 20th century) - rejected need for internal representation

External world (optic flow fields) so rich that perception is ‘direct and immediate’

Subsequent work with computers showed not true - vision is hard.

Ambiguities of Retinal Projections:

Retina image could be due to any of the lines A to E

Top-Down processing

Top-down processing influences how interpret bottom-up information

Experience creates perceptual sets - readiness to perceive a stimulus in certain way

Constructivist

Sensory systems construct internal representation of world (Helmholtz, 1856)

• internal representation mediates perception (Necker cube) - top-down. strong role of past experiences/knowledge

Heavily influenced modern computational theories which stress computation upon representation (A.I.) - Richard Gregrory (1923-2010)

Basic processes in perceptual organisation

The perceptual system needs work out what stimuli go together to form objects

Basic processes in perceptual organisation

• figure-ground

• grouping (gestalt laws)

Given unconnected elements (unrelated arrows and Ys) perceptual system forms 2 possible objects even creating subjective contours

Figure-ground

Complex stimulus automatically broken down by perceptual system into figure (emphasized foreground) and ground (background)

• figure - has meaning, stands in front, has edges that seperate it from background

Certain properties stimulus influence what area seen as figure and what seen as background

• Area size – stimuli with smaller area more likely to be seen as figure

• Symmetry – stimuli possessing symmetry more likely to be seen as figure

• Meaning - stimuli posessing meaning moree likely to be seen as figure

Grouping

Gestalt psychologists came up with a set of ‘laws’ that predict what tends to group with what tends to be percieved as figure or ground

Gestalt Laws:

1. Proximity

Objects that are closer to me tend to be grouped together

1. Similarity

Objects that are more similar to one and other tend to be grouped together. Similarity can be in terms of any diension: shape, size, colour or luminance

1. Continuity

Sensations that appear to create a continuous form are percieved as belonging together

1. Closure

People tend to fill in missing contours to form complete objects

1. Common fate

Objects that move in same direction and speed are perceived as a group.

• Camouflage - situations where difficult separate figure from ground

• Woodland Encounter by Bev Doolittle (1985)

Summary

In recognising the perceptual world we use both bottom-up and top-down processing

Basic processes in perceptual organisation were covered – how components of scene are grouped together (Figure-Ground and Grouping using Gestalt Laws)