VISUAL PERCEPTION Flashcards by Katie Read

perception definition

Our experience of the outside world.

• What we see appears ‘complete’, but our mental re-creation of the world is not completely accurate.
• Although seems effortless, perception is hugely complex!
– Derived from array of external information (e.g., light)
– Influenced by internal states/experiences (e.g., arousal, past experience etc.)

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Why study visual perception?

• Visiontypicallyconsideredthemost important/useful of the human senses
> 80% of information received through visual modality
• Substantialamountofourbraindevotedto processing visual information
~50% of cortex involved in visual processing

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The action and structure of the eye

Muscles in iris adjust size of pupil to let in more/less light
Image focussed on to the retina by the lens
Retina contains photoreceptor cells (rods & cones) that convert light into electrochemical signals that can be interpreted by the visual cortex

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Primary Visual Pathway

Retinal information carried to visual cortex, in occipital lobe, via optic nerve
Axons partially cross over at optical chiasm and passes through lateral geniculate nucleus (LGN)
Information then projected to primary visual cortex

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Two-streams hypothesis

Ventral and dorsal streams

Processing of visual information takes place in different visual pathways

Ventral stream: ‘what’ pathway (object recognition)
Dorsal stream: ‘where’ pathway (spatial location) or ‘how’ pathway (guides movement)

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General

Direct processing of environmental stimuli; data driven

Stimulus > Attention > Perception

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General

Processing of environmental stimuli is influenced by expectations (past experience, prior knowledge)

Stimulus > Attention + Prior knowledge > Perception

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Marr’s (1982): how you get from a 2D retinal image to a 3D
‘perception’

– Primal sketch:
• Raw primal sketch is detection of gradient, edges & boundaries (i.e., sudden changes in light intensity)
• Full primal image is group of boundaries to give outlines/features which are then compared to original retinal image

– 2.5D sketch: adds depth, motion & shading from perspective of the viewer
– 3D sketch: adds object recognition to provide full 3D representation that is stable independent of the viewer angle & comparable to stored visual information

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Gregory’s (1970; 1997) constructivist approach

We apply our stored knowledge of the world to interpret visual information.

Constructivist theories see perception as:
– an active process rather than simply sensation
– influenced by experience and individual differences

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Perceptual organisation 
-
Gestalt Theory 
-
Outlines general principles of perceptual organisation

What is experienced is much more than incoming information
– The whole is other than the sum of its parts (‘gestalt’ is German for ‘form’ or ‘shape’)

It is not the individual elements but the relation of each with the other that is important

• We can fill in gaps in information to create a whole, ‘unified’ perception independent of its component features

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Perceptual organisation 
-
Gestalt Theory 
-
Figure ground relations/ segregation

world is made up of objects against background
need to identify figure from its background
it is easiest to distinguish boundaries of objects where colour/ light changes
ambiguous figures exploit this distinction
Rubin’s vase (1915)

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Perceptual organisation 
-
Gestalt Principles 
-
Proximity

items close together are grouped

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Perceptual organisation 
-
Gestalt Principles 
-
Similarity

items that look ‘similar’ are grouped

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Perceptual organisation 
-
Gestalt Principles 
-
Continuation

preservation of continuity over abrupt change

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Perceptual organisation 
-
Gestalt Principles 
-
Closure

preferencefor‘closed’over‘open’figures

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Perceptual organisation 
-
Gestalt Principles 
-
Evaluations

Mainly based on theory rather than experimental evidence
Descriptions of processes rather than explanations
Identifies many but not all perceptual laws

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Evidence for top-down processing

Perceptual constancies highlight our cognitive assumptions about the world
Visual illusions help us understand how we translate the external world to the internal world

Perceptual Hypotheses

Perception is an active, empirical process, not passive receipt of information
– Perceptual system processes visual information & looks for interpretation that best fits the sensory data → hypotheses
– Experience strengthens hypotheses
– Strong hypotheses win against weak hypotheses

Perceptual Constancy

Perception (hypothesis) of object properties remains constant even if retinal image (sensation) changes.

Size constancy
and example

Object size perceived as constant regardless of viewing distance
Size perception largely dependent upon perceived distance
Ames room

Shape Constancy
and example

Object shape remains constant, despite changes in viewing angle
Retinal image of door changes, but perception of physical shape of door does not change

Light Constancy
and example

Relative lightness of objects constant in different ambient light brightness
colour constancy on checkerboard

Perceptual organisation 
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Perceptual Hypotheses
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Issues

– Where do first set of hypotheses come from?
– How often do we actually test our hypotheses?
– Why do individuals develop similar hypotheses?
– Whataboutnon-humans–doratshavehypotheses?

Perceptual organisation 
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Perceptual Hypotheses
-
Other theories: Ecological theory

we can receive information directly from environment without

any higher order cognitive processing/internal representation required

Developing depth perception - The visual cliff Gibson and Walk, 1960

- innate ability - young children fearful of crossing the fake drop - maybe not innate but learnt thorough interaction with the 3D world (development of crawling) - variations in animals- turtles crossed because they did not perceive depth asa danger (because its not in water)

Two broad categories of depth cue: Monocular cues

Requires the use of only 1 eye (although are still used | when both eyes are open)

Two broad categories of depth cue: Binocular cues (also Oculomotor cues)

Involve both eyes being used together

``` Depth Cues - Monocular - Linear Perspective ```

We are used to the tendency for parallel lines to converge as they head toward the horizon.

``` Depth Cues - Monocular - Texture ```

- texture elements become dense with distance - texture elements become smaller with distance - foreshortening (circles become ovals) when the surface is tilted away

``` Depth Cues - Monocular - Elevation ```

More distance objects are imaged higher in the scene AND relative size is influential, with larger objects perceived as closer

``` Depth Cues - Monocular - Interposition ```

A more distant object is partially blocked by a more proximal object

``` Depth Cues - Monocular - Shading ```

Flat 2D shapes cast no shadow, shading replicates “shadow” and so suggests 3D object. We view pictures and assume light source is from the top.

``` Depth Cues - Binocular Disparity - Stereopsis ```

Slight difference in image of scene by the two eyes. The combination of the images can help to provide depth info. *only useful at short distances so limited uses

``` Depth Cues - Oculomotor Cues: based upon contraction of the eye muscles - Convergence ```

The larger the visual angle, the closer the object. *only useful at short distances so limitations on use

``` Depth Cues - Oculomotor Cues: based upon contraction of the eye muscles - Accommodation ```

The brain determines how tense the eye muscles are to focus on an object, and uses this to determine how far away an object is-feel less strain when something moves further away. *only useful at shorter distances so limitations on use

Eye movements - Fixations

eye 'rests' on a fixed location

Eye movements - Saccades

rapid eye movement between two fixations

Eye movements - Smooth pursuit

eye tracks a moving object

Eye movements - Gaze patterns

scan paths change as function of the task

Change blindness

- failure to notice substantial changes to objects of scenery - studies on change blindness provide info on how we allocate out limited attentional capacity.