Visual Perception

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Visual Perception
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Perceptual process

• Environmental stimulus
• anything in our environment that we can perceive
• Attended stimulus
• focus of attention

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Visual perception

• Perception can be defined as a conscious sensory
  experience
• light is emitted or reflected from an object
• image is formed on the receptors of the retina of
  the eye
• stimulus on the receptors are transformed into
  electrical signals
• electrical signals produced by the neurones are
  processed by the brain and converted to the
  experience of seeing
• high level processes (recognition, action,)

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Visual system

• Eyes

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Eyes

• cornea approx. 2/3 of eyes focussing power
• lens can change shape
• eye can focus on object at various distances
  (accommodation)
• focus at one depth at a time
• limited depth of field
• pupil
• diameter varies from 3mm to 7mm
• change in retinal illuminance

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Eyes

• Fovea
• less than 1 of retinal size
• Most densely populated with photoreceptors
• Image fixated on the retina
• Saccades
• quick, simultaneous movements of both eyes
• Blind Spot (optical nerve exit)

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Blindspot
R L
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Receptors

• Rods
• 120 million
• Mainly in periphery
• Monochromatic, low light
• Cones
• 6 million
• Concentrated in fovea
• Colour, bright light
• 3 types
• peak at different wave lengths

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Colour vision

• humans can perceive wavelengths between 400 to
  700 nanometer
• Just noticeable difference
• 1 nm in green - yellow wavelengths(human
  colour perception is most sensitive here)
• 10 nm and more in the red and blue

 Source Hunt, 2004 (via http//www.handprint.co
m/HP/WCL/color1.html)Note "c" means "complement
of" wavelength for extraspectral hues
(mixtures of "red" and "violet" light).
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Colour vision
Colour blindness 6-8 European4-6
Asian 2-4 African
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Visual System

• 126 mil. receptors converge to 1 mil. ganglion
  cells
• 120 rods, 6 cones
• Connect to nervoussystem
• Visual cortex
• Cells respond to different aspects of stimuli
  (feature detectors)
• e.g. orientation, motion, edge

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Visual perception

• the world or the image is not 'given', but
  constructed.
• we are not like passive cameras
• we interpret the world actively
• Knowledge / Experience
• Affects processing, perception, recognition
• Top down

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Environmental factors
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Environmental factors

• Experience, education, culture, values, etc.
  influence perception
• Carpentered world
• Experiement (Segall, Campbell and Herskovitz
  (1966) - Zulu people are less affected by this
  illusion

Müller-Lyer Illusion
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Environmental factors

• people who lived in very open rural environments
  tended to be more subject to the
  horizontal-vertical illusion than others
• visual perception is in part at least learned

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Environmental factors
the two-pronged trident
impossible object by L S and R Penrose
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Environmental factors

• Gender differences
• Coren, Porac and Ward (1978)
• brush vs. comb
• target vs. dinner plate
• head vs. cup

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Gestalt psychology

• Model of Perceptual Organization
• perceiving individual sensory stimuli as a
  meaningful whole
• whole is different from the sum of its parts
• Overarching principle pragnänz
• the simplest and most stable interpretations are
  favoured
• Gestalt principles / laws
• proximity, similarity, good continuation,
  closure, symmetry, common fate

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Proximity

• Elements in spatial or temporal proximity appear
  to be grouped together

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Similarity

• Tendency to group similar elements (form, colour,
  size, brightness)

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Good continuation

• The mind continues visual, auditory, and kinetic
  patterns

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Closure

• Interpretations which produce 'closed' rather
  than 'open' figures are favoured

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Symmetry

• Symmetrical areas tend to be seen as figures
  against asymmetrical backgrounds

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Common fate

• Elements moving in the same direction appear to
  be grouped

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Attention

• Inattentional blindness
• video
• Attentional Blink
• inability to detect a new target for a short time
  after the first target is presented (200-500
  ms)http//psych.hanover.edu/JavaTest/Cognition/Co
  gnition/AttentionalBlink.html
• Change blindness
• inability to detect unattended changes
• example

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Visual search
One stimulus dimension (colour)
Parallel searchpre attentive
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Visual search

• Other pre-attentive stimuli
• shape, depth, motion (direction), orientation,
  curvature, size,

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Visual search
Conjunction searchcolour shape
serial search
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Visual search

• Nakayama Silverman
• stereoscopic depth different in conjunction
  searches
• depth colour depth motion different to
  colour motion
• reaction times constant over set sizes
• segregation of visual array into planes
• parallel searches in one plane without
  interference from distracters in other plane

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Multi Layer Display
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Conditions - single
shape
colour
depth
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Conditions - conjunction
colour-shape
colour-depth
colour-depth (tf)
shape-depth
shape-depth (tf)
colour - shape-depth
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Task
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Results
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Stroop Effect
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Green
Brown
Purple
Red
Blue
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Stroop Effect

• interference
• mind automatically determines semantic meaning -
  then must override this first impression with the
  identification of the coulor
• http//cognitivefun.net/test/2

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Colour

• Colour effect (colour psychology)
• varies in different cultures
• passion, strength, energy, fire, love, sex,
• peace, unity, harmony, calmness, trust,
• life, nature, bad spirits, spring, fertility,
  youth,...
• joy, happiness, earth, optimism, intelligence,
• modernity, power, sophistication, formality,
• boldness, depth, natural organisms, nature,
  richness,
• http//en.wikipedia.org/wiki/Color_symbolism

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Colour

• Warm vs. cold colours
• seems related to the observed contrast in
  landscape light

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Same colour illusion

• Local contrast B is lighter than its neighbours
• Visual system tends to ignore gradual changes
• Chromatic adaptation (colour constancy with
  different types of light)

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Object consistency

• perception of objects is more constant than our
  retinal images

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Depth perception

• Binocular depth cues
• Stereopsis or retinal disparity
• Accommodation
• Convergence
• Monocular depth cues
• Motion parallax
• Depth from motion
• Colour vision
• Perspective
• Relative size
• Occlusion
• Texture gradient

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Depth perception
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Afterimage
receptors adapt from the over stimulation and
lose sensitivity (colours are muted)
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Afterimage
Afterimages of complementary colours create
apparent movement in our peripheral vision as
our eyes shift across the page
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Movement
Phi phenomenon
Beta movement