Visual Perception
Simon Bartlett
This area of study focuses on
perception and the general
characteristics of the visual
sensory system.
What are the stages?
• Reception – eye senses a stimulus
• Transduction – changes it so brain can
understand it
• Transmission – sends it to the visual cortex
• Selection – aspects selected of stimulus
• Organisation – grouping of elements to form a
whole
• Interpretation – given meaning with the aid of
psychological factors
Reception
• Electromagnetic energy (light) sensed from the
environment
• RODS (black and white) and CONES (C for
colour) are photoreceptors that pick up the
electromagnetic energy
• This energy must be CHANGED for the brain to
be able to interpret it…
Transduction
• The electromagnetic energy is converted,
changed or ‘transduced’ into electrochemical
energy.
• This is the energy all neurons use.
• This means that information sensed by the rods
and cones can be sent as neural impulses to
the visual cortex.
Transmission
• Neural impulses once triggered are sent to
visual cortex of the occipital lobe
• Like any neuron, the “all or none” law applies –
there is no such thing as a big or small impulse.
The stimulus must be ENOUGH for the
threshold to be reached to make the neuron fire.
• They are sent via the optic nerve.
Selection
• This is where discrimination and identification of
the FEATURES of the stimulus takes place.
• Feature detectors are cells that are in the
retina, optic nerve and visual cortex, that
respond to patterns, lines, edges and angles
• Selection is also happening at a basic level in
reception with rods and cones responding to
different electromagnetic energy wavelengths.
Organisation
• This can only happen once the brain has
received the neural impulses.
• Single elements are grouped to form a
whole, using perceptual principles that
work like ‘rules’ of organisation.
• These principles are called Gestalt
principles.
Interpretation
• The ‘whole’ that has been created is given
meaning.
• Memory is used to compare incoming
information with what is already known.
• This creates a perceptual hypothesis.
• Interpretation involves the brain using
psychological factors in order to make sense
of what it is considering.
Absolute Threshold
• The minimum amount of energy needed
to detect a stimulus under ideal conditions
50% of the time.
• VISION: A candle flame 50km away on a
clear, cloudless night with NO light
pollution (moon, other lights, cities)
Differential Threshold or JND
• The smallest difference in the intensity of a
given stimulus that a specific sense can detect
50 % of the time.
• This is about DIFFERENCE in the intensity of a
stimulus; there must be comparison.
• If there is enough change, there will be enough
stimulus for receptor cells to fire and send a
message to the brain to be interpreted.
• VISION: a dimmer switch does not turn a light off
but changes the intensity of the light.
Physical Structure – the Eye
Let there be light!
• Receptor cells respond to 380 nm (violet)
to 760 nm (red) of the electromagnetic
radiation spectrum.
• This is the visible light spectrum to which
humans are able to respond.
• To remember the pattern of light in order,
think ROYGBIV (red, orange, yellow,
green, blue, indigo, violet).
Eye structure and function
• CORNEA – light enters through the cornea and
is bent onto the lens.
• PUPIL – light passes through the SPACE called
the pupil.
• IRIS – a band of muscle that contracts/expands
to manage the amount of light entering the pupil
and hitting the lens.
• LENS – the lens bends the light more and
focuses them on the retina – in particular, on the
FOVEA. The lens can bulge or stretch to help
the light reach the fovea.
Eye structure and function…
• RETINA – registers the electromagnetic energy,
processes the incoming information and
transduces the energy to a form the brain can
interpret.
• BLIND SPOT – where the optic nerve attaches
to the retina – no photoreceptors.
• PHOTORECEPTORS – 2 types:
Rods – 125 Million
Cones – 6.5 Million
Rods VS Cones
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•
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•
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•
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RODS
Black and white vision
Operate well in low level light
– night vision
Sensitive to brightness,
darkness and movement
Mainly located in outer part
of retina
Poor acuity (low sharpness
and focus)
Excellent for peripheral
vision rather than direct
19x the number of cones
•
•
•
•
•
CONES
Daytime and colour vision
Excellent visual acuity –
sharp and clean images
Concentrated in the fovea
Not useful at night – can’t
discriminate colours
Outnumbered by rods 19:1
GESTALT Principles
• Theory that the whole is greater than the sum of
its parts – Gestalt means ‘form’ or ‘shape’.
• We group individual elements of a visual
stimulus into a complete form.
• This allows us to perceive objects in the most
simple way possible.
• BOTTOM-UP = building an image from the parts
into a whole
• TOP-DOWN = working to move from the whole
image to the parts it is assembled from
What are the 4 Gestalt
Principles?
• Figure Ground
• Closure
• Similarity
• Proximity
Figure-Ground
• We tend to separate the
important aspects of the figure
from the surroundings
(background). We focus or
give our attention to the
figure.
• Real or imagined contour lines
separate the figure and ground
• Camouflage depends on
difficulties with establishing
contour lines
• Reversible figures change the
ownership of the contour lines
Love and Death?
Proximity
Proximity is the grouping of elements that
are close to each other to form an overall
figure or pattern. Also known as
‘nearness’.
Similarity
When the elements of a stimulus or pattern have
similar features (size, shape, colour) we tend to
group them together. Similarity is stronger than
proximity if the two are together in a stimulus.
Closure
What do we see?
We perceive an object as a whole, despite it
being actually incomplete. We group the
individual elements to make ‘one’ by filling in
the missing contour lines so that it makes sense.
Fido?
Depth Perception
• Depth cues which are internal and
involve the eye and the brain and are
known as primary cues.
• Depth cues which are external to the
body and part of the environment or
pictures are known as secondary cues.
Binocular Cues
• Binocular cues are primary cues and involve
the eye and its functioning.
• Binocular cues ALWAYS involve BOTH eyes
• Convergence
• Retinal Disparity
What do I need to know about
convergence?
• We use this for objects up to 6m away
from our eyes.
• When we view objects close to us, our
eyes turn inwards or ‘converge’ so that
a single image is formed on both retinas.
• Changes in muscle tension are detected
and interpreted by the brain and used to
determine the depth and distance of an
object.
• Our brain detects that the closer the
object, the greater the convergence.
How does retinal disparity work?
• Because our eyes are 6-7cm apart, the retinas
receive slightly different images. An object must
be under 10 metres away for us to use retinal
disparity.
• The retinal images are combined together and
compared by the brain.
• Any disparity or difference between the 2
images provides us with information about the
depth of the object and its distance from us.
• You can check this by only using one eye, in
turn, to view the same object on your desk.
Monocular Depth Cues
Accommodation:
This is a primary monocular depth cue that
involves the lens of the eye changing shape.
Information about how much the lens bulges or
flattens is used by the brain to determine depth
and distance of an object.
Object close? Lens bulges to direct the image
clearly onto retina.
Object distant? Lens flattens or elongates to
direct the image clearly onto retina.
Secondary Monocular Depth Cues
• Linear
Perspective
• Interposition
• Texture
Gradient
• Relative size
• Height in the
visual field
Pictorial Cues are
secondary cues, as
they occur in the
environment – they
are NOT a function
of the eyes!
Secondary Monocular Depth Cues
Linear Perspective
• Parallel lines appear
to converge towards
the horizon and an
imaginary’ vanishing
point, creating a
sense of distance.
Interposition
• The object that is
closer obscures part
of another, more
distant object.
• It is useful to indicate
which objects are
closer than others
but not actual
distance.
Secondary Monocular Depth Cues
Texture gradient
Relative Size
• The amount of detail in a
scene decreases as the
distance increases or
moves away from the
viewer.
• The foreground is highly
detailed and the
background is less
detailed and less
focussed.
• If separate objects are
predicted to be the same
size, then the one that
appears larger, is
closer.
• The retinal images of the
objects are different and
we can only use relative
size if we KNOW the size
of an object to be able to
compare.
Secondary Monocular Depth Cues
Height in the
Visual field
• NEVER just call this
‘height’!
• If land objects appear
smaller and closer to the
horizon, they appear to
be further away.
• If air objects appear
smaller and closer to the
horizon, they appear to
be further away.
• HOWEVER – land based
objects will be LOW in the
visual field if close and air
objects will be HIGH in
the visual if close.
Henri Cartier-Bresson
Concert crowd - Bristol, UK
Perceptual Constancies
• These are learned visual perception
principles that let us perceive or make
sense of stimuli.
• This means when objects change in
shape, size or brightness, we still see
them as being what they are – a ‘constant’
and familiar thing.
• Our perception stays the same even
though the retinal images may have
altered; we are familiar with the objects.
Size Constancy
• We maintain an object’s perceived size
even though the size of the retinal image
alters due to the object’s distance from us.
• If we look at a truck outside our homes, we
KNOW that the truck isn’t smaller if we
stand and look at it as it drives down the
street. We don’t think it ‘shrinks’ as it is
driving away from us.
Brightness Constancy
• We maintain the perceived brightness of
an object, despite actual changes in the
amount of light being received by the
retina.
• Because we look at objects in their
immediate environment, we perceive that
everything else has altered by the same
amount of brightness too and so know that
the brightness has remained constant.
Shape Constancy
• We maintain an object’s perceived shape
even though the image cast on the retina
changes if the object is observed from a
different angle.
• If we see a closed door, a half open door
and a fully open door, we still understand
that the door has a rectangular shape,
even if it appears different when closed,
half open and fully open.
Orientation Constancy
• This is our tendency to maintain an
object’s location in ‘space’ as constant –
and perceive the world as being upright
and vertical.
• If we hang upside down on monkey bars,
we don’t believe that the world is no longer
vertical and that the trees are upside down
and sideways! Again, we compare and
use our body’s signals to help us work out
what’s going on.
Psychological factors affecting Visual
Perception
Perceptual Set or ‘Expectancy’
• We interpret stimuli by comparing it to
what we already know and have
experienced = prior experience.
• We tend to see what we EXPECT to see –
this includes ignoring some stimuli and
giving attention to others.
• Factors like emotions, motivation, context
or setting, cultural background and past
experience all affect our perceptual set.
Physiological factors affecting Visual
Perception
Alcohol… Slows us down, changes
coordination, reduces attention,
concentration and ability to make
judgements.
Old Age…cataracts, long-sightedness,
age-related macular degeneration, retinal
detachment, diabetic retinopathy,
glaucoma. Our eyes age as we do and
become more susceptible to disease and
damage.
OPTICAL ILLUSIONS
OR
HOW EASY IT IS TO
TRICK HUMAN BEINGS….
How do Optical Illusions work?
• A visual stimulus misleads our perception
(or meaning-making) of that stimulus.
• This happens because we APPLY
perceptual constancies to what we are
seeing – they are our RULES.
• We make a false judgement because we
misjudge length, position, speed, direction
or curvature.
MULLER LYER Illusion
There are 2 theories to explain this illusion!
The Muller Lyer illusion consists of 2 lines that are
equal in length to each other.
•One has ‘arrow-heads’ attached.
•One has fish-tails’ attached.
Theory of Perceptual Compromise
1. The length of the solid horizontal lines of
each figure are actually equal in length.
2. BUT the length of each WHOLE figure is
not the same.
3. The lengths between the added tips
(heads or tails) is very different. We use
closure to estimate the length and
compromise somewhere between the
actual length and the perceived length.
Diagram of Perceptual
Compromise
The Apparent Distance Theory
• We use our knowledge from depth cues and
size constancy.
• We liken the line with ‘fish-tails’ as being the
inside corner of a room in a building.
• We liken the arrow-heads as the closest
outside corner of a building .
• BOTH lines cast the same size image on the
retina.
• As the fish-tails are perceived to be further away
(inside the building), we apply size constancy
and interpret that the line must be longer.
Diagram of Apparent Distance
Theory
The Ames Room
• We must only use MONOCULAR depth cues in
order to perceive the Ames room as an illusion.
• It is a trapezium - a shaped room that is much
higher and longer on one side than the other,
however, it LOOKS like a rectangle to the
viewer.
• The back corner on the left hand side is double
the distance away from the viewer, compared to
the back right hand side of the room.
The Ames Room
• The roof is also higher on the left hand
side of the room, compared to the right
hand side of the room.
• All of the decorations, windows and
furnishings are also not ‘square’ – there
are no right angles to be seen. Even
clocks are not circles but odd ovals.
Ames Room construction
The Ames Room
• A person standing in the back left hand
corner of the room is going to appear very
small.
• A person standing in the back right hand
corner of the room looks like a giant.
• If a person crosses the room, they appear
to shrink or grow in size.
• This creates a conflict for the viewer!!! The
retinal images of the room’s corners
(etc) are the same shape and size.
The Ames Room
EXPLANATIONS?
SHAPE CONSTANCY:
If the viewer maintains shape constancy,
they continue to perceive the room as
SQUARE or RECTANGULAR – this
means they see the person shrink and
grow. Most people do this.
EXPLANATIONS?
SIZE CONSTANCY:
It is difficult to maintain size constancy in
the
Ames room as it means that the brain
must remain totally confused! The retinal
information (the same sized retinal image
of the room’s corners) cannot be corrected
due to the lack of accurate depth
information – even if we know what we are
‘seeing’ is not possible.
Perception Deception
Is this picture
REALLY moving?