VISUAL PERCEPTION Flashcards
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.)
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
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
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
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)
General
Direct processing of environmental stimuli; data driven
Stimulus > Attention > Perception
General
Processing of environmental stimuli is influenced by expectations (past experience, prior knowledge)
Stimulus > Attention + Prior knowledge > Perception
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
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
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
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)
Perceptual organisation - Gestalt Principles - Proximity
items close together are grouped
Perceptual organisation - Gestalt Principles - Similarity
items that look ‘similar’ are grouped
Perceptual organisation - Gestalt Principles - Continuation
preservation of continuity over abrupt change
Perceptual organisation - Gestalt Principles - Closure
preferencefor‘closed’over‘open’figures
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
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 - Perceptual Hypotheses - 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 - Perceptual Hypotheses - Other theories: Ecological theory
we can receive information directly from environment without
any higher order cognitive processing/internal representation required
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
Fixations
eye ‘rests’ on a fixed location
Saccades
rapid eye movement between two fixations
Smooth pursuit
eye tracks a moving object
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.
