When a tree falls down in a
wood and nobody is there, does
it make a noise?
What is perception?
Do we always see what is there?
• The Lens is situated behind the pupil and completes the
process of bringing light into the eye. The lens can change its
shape to bring objects at various distances into focus.
• The retina at the back of the eye consists of three main layers
o Photoreceptors (light sensitive cells)
o Bipolar cells
o Ganglion cells.
Photoreceptors consist of 2 different types of cell –
cones and rods – which allow humans to see in a wide
range of lights.
Cones are responsible for vision under bright
conditions and colour vision.
There are about 5 million cones in each retina and
about 50,000 are packed into the small central area
called the fovea.
Rods are responsible for vision under dim lighting
Once light has been focused on the retina by the lens, it
is changed into neural impulses and sent along the optic
nerve to the brain for processing and integration.
The optic nerve leaves the eye at the blind spot – an area
at the back of the retina that is free of photoreceptors.
We are not usually aware of the blind spot – partly
because we have 2 eyes, so if the image falls on the blind
spot of one eye it will be picked up by receptors in the
The Visual Pathways
The optic nerves from each eye come
together at the base of the brain to form
the optic chiasma.
The optic nerve fibres from the nasal
retinas (closest to the nose) cross to the
opposite side of the brain at this point,
while the fibres from the temporal retinas
(closest to the temples) continue on the
same side of the brain.
After they pass through the optic chiasma,
most of the nerve fibres lead to the lateral
geniculate nucleaus (LGN) in the
thalamus. From here, they lead to the
primary visual cortex where additional
processing is carried out.
Gibson (1979) believed that there was
enough information within the pattern
of light reaching the eyes (the optic
array) for perception to occur directly.
Gibson saw perception as due to the
direct detection of environmental
invariances – unchanging elements of
the visual environment. These contain
enough sensory data to enable
individuals to perceive features such
as depth, distance and spatial
According to Goldstein (1999), Gibson’s theory is based on four
1. The pattern of light reaching the eye can be thought of as an
optic array which contains all information necessary for
2. Important information is provided by the movement of the
3. The optic array contains invariant information (information that
remains constant as the observer moves).
4. This invariant information leads directly to perception.
The Optic Array
The optic array contains all the visual information from
the environment that enters the eye.
Gibson believed that the starting point for perception
is the light that enters the eye. To perceive objects, the
light has to be structured by the presence of objects,
surfaces and textures.
According to Gibson, the optic array contains
unambiguous , invariant information about how and
where objects are situated in space. This information
comes in a number of forms...
Optic Flow Patterns
Gibson described optic flow patterns as unambiguous sources of
information concerning height, distance and speed that directly
The term refers to the changes in the light patterns that reach an
observer when either the person, or the visual environment moves.
Optic Flow Patterns
Flow patterns are created when objects in the visual field move past the
The flow provides information about position and depth (consider the way
a road sign first appears in the distance, then grows and eventually passes
to your left as you pass). Information regarding distance is also gained;
distant objects like hills appear to move more slowly while closer objects
move more quickly, this is known as motion parallax.
The optic flow also varies in relation to
the angle between an observer’s
direction of movement and the direction
of an object being regarded.
When travelling forwards – the optic flow
is quickest when the object is at 90 degrees
to the observer’s side, directly above or
Optic Flow Patterns
Johansson (1973) found that a black-clad actor wearing lights on
his joints walking in a darkened room, was seen as a moving
person, but standing still, he was not perceived as a stationary
Maher and West (1993) filmed the movement of black-clad
animals with light on their joints and found that participants
could recognise the species of animal. This demonstrates the
strength of movement information in determining optic flow
patterns and shows that there is enough sensory information for
perception to occur directly.
In this painting, the bricks and other
features of the front of the building are
mainly parallel. The building looks flat,
as if seen straight on.
Texture gradients are surface patterns providing sensory information about the depth,
shape etc. of objects.
There are several classic texture gradients – for example frontal surfaces project a
uniform gradient, while longitudinal surfaces, like roads, project gradients that
diminish with greater distance from the observer.
Texture gradient also indicates the orientation of a surface – sudden changes in
texture often signals a change in the direction of a surface, such as a wall meeting a
Linear perspective can also be identified – lines are seen to converge as they move
further away, helping us to determine depth and distance.
Here, texture gradient is used in the
form of cobblestones that get
progressively smaller to show that the
Gibson and Bridgeman (1987) showed participants
photographs of surface textures. Participants could
correctly identify objects, state their colour, identify the
light conditions and say whether they were lying flat etc.,
suggesting that there is sufficient sensory information in
surface textures to permit direct perception.
Frichtel et al. (2006) presented participants with a film of a
car driving through scenery. Evidence was found that
infants as young as 4 months old could perceive using
texture gradient, implying that the ability is innate.
Horizon ratios are another form of invariant sensory information
allowing direct perception.
They concern the position of objects in relation to the horizon.
Objects of different sizes at equal distance from an observer have
different horizon ratios, which can be calculated by dividing the
amount of an object above the horizon by the amount below the
Objects of equal size standing on a level surface possess the same
horizon ratio. When nearing an object, it seems to get bigger, but
the proportion of the object above and below the horizon stays
constant and is a perceptual invariant.
Bingham (1993) asked participants to judge the height of cylinders under
various conditions where they were provided with cues in the form of
tree silhouettes and a horizon.
The heights of cylinders were judged inaccurately when they
appeared with the horizon but without trees.
Judgements were more accurate when the cylinders also
appeared in the context of trees, demonstrating how horizon ratios are
used to estimate the height of objects in the visual field.
Rogers (1996) found horizon ratios were an effective source of
information when judging the relative size of objects in pictures, even
when horizons were not provided.
Participants could impose their own horizon ratio based on eye level and
the contents of the picture – showing the degree to which we rely on
this source of information.
Affordances are the most controversial aspect of Gibson’s theory and
involve attaching meaning to sensory information.
Affordances are the action possibilities of an object – they are what
objects suggest or mean to observers and are linked to psychological
state and physical abilities.
Gibson saw affordances as giving directly perceivable meaning to
objects, because evolutionary forces shaped perceptual skills so that
learning experiences were not necessary.
Warren (1984) studied whether participants could judge whether
staircases portrayed with differently proportioned steps could afford
to be climbed. Whether they actually could depended on the length
of a participant’s leg. It was found that participants were sensitive to
the affordance of ‘climbability’, and according to Gibson this would be
achieved by the light reflected from the staircase.
Bruce and Green (1990) found that the idea of affordances could be
used to explain the visually controlled behaviour of insects.
Stoffregen et al. (1995) tested the
affordances of the actions of others.
Participants observed tall and short
actors standing next to an adjustable
chair and in motion (such as walking on
the spot) and had to make judgements
of preferred and maximum sitting
heights for each actor.
In both conditions, participants gave
accurate judgements, supporting the
idea that affordances are available by
Horizontal lines are painted on the
road, becoming closer together as
driver approach junctions. These
markings are often seen on the
exit roads from motorways and
serve to create the illusion of
increasing speed, which causes
the driver to slow down before
joining a non-motorway road.
Gibson’s theory has practical applications – such as road markings.
Tan et al. (2006) used Gibson’s idea of optic patterns to investigate the
effects of motion on car drivers’ perception, where cars travelling
towards a viewer appear to increase in speed and size, even though their
real speed and size is constant.
This perceptual problem is increased by the fact that the viewer is also in
A camera mounted on a wing mirror was used to determine real-world
speed. The data gained from such studies is used to determine speed
limits and advise on braking distance.
As well as contributing to knowledge of perception, Gibson’s theory also
saves lives in preventing / lessening the impact of traffic accidents.
“Perception is not determined simply by
stimulus patterns. Rather it is a dynamic
searching for the best interpretation of the
available data...which involves going
beyond the immediately given evidence
of the senses”
Gregory (1966) believed that perception was an unconscious, continual
process of generating and testing hypotheses, and as an active search
for the most sensible interpretation of sensory data based on previous
Gregory saw sensory
What is perceived is much
richer than the information
contained in the sensory
Perceptual set is a concept that views individuals as biased in how they
perceive due to previous experiences, cultural factors, and emotional
and motivational influences.
In other words, people see what they want or expect to see and this
cuts down on the number of possible interpretations that data may
suggest, making perception speedier, though increasing the
opportunities for errors to occur.
Perceptual set occurs through expectations, where people perceive
what they expect to based on previous experiences.
Bruner and Mintern (1955) showed an ambiguous figure to
participants that could be seen as either the letter ‘B’ or the
Participants who saw the figure surrounded by the
consecutive numbers 12 and 14 perceived it as 13, while
those who saw it flanked by the letters A and C saw it as the
This suggests that the context in which stimuli are viewed
creates an expectation that shapes actual perception.
Leeper (1935) showed participants an
ambiguous picture, which could
either be seen as a young or an old
Participants previously given a
description or picture of a young
woman perceived a young woman,
while those described or shown an
old woman saw an old woman –
showing the influence of
Brochet (2002) gave 54 experienced wine tasters an array
of white wines, which they described as ‘fresh’ and so on.
He then gave them an array of red wines to evaluate, some of which were
white wines, made to look red by the addition of a tasteless, odourless
Not one expert spotted the frauds, describing them in terms reserved for
red wines (such as ‘intense’ ‘spicy’).
Because they had increased knowledge of wine, they were more
influenced by colour. The colour created an expectation, which
determined actual perception.
Why do you think Brochet had his participants rate genuine white wine
See Palmer’s research on p75.
and the role of hypothesis testing
Perceptual set also occurs through emotional factors, which
affect perception by creating a bias to perceive, or not, certain
features of incoming sensory data.
Perceptual defence is important here, where emotionally
threatening stimuli takes longer to perceive.
McGinnies (1949) found that emotionally threatening words took
longer to recognise than neutral ones, suggesting that perceptual
defence influences perception through emotional factors.
However, the results may be due to the embarrassment of speaking
the words aloud. Bitterman and Kniffin (1953) found no differences in
recognition time if the words were written down.
Lazarus and McCleary (1951) found that nonsense syllables presented
so swiftly that they could not be consciously perceived raised anxiety
levels if they were previously paired with electric shocks. This implies
that emotional factors have an unconscious affect.
Phelps et al. (2006) manipulated emotion by showing participants, for
a brief instance, either fearful or neutral faces in various locations on a
screen. An image then appeared briefly and participants selected in
which direction it tilted. Participants were more able to do this when
the figure was paired with fearful faces, especially those in the same
orientation as the tilted figure.
This suggests that emotion facilitates perception early in the
Draw a picture of
Perception can be influenced by factors such as thirst or hunger.
Images of food appear more
enticing as the individual becomes
What evolutionary purpose might
Solley and Haigh (1948) found that children drew a bigger Santa and sack
of toys as Christmas approached, but after Christmas Santa and his sack
Bruner and Goodman (1947) got children to manipulate a patch of light
so it was the same size as various coins. All children overestimated the
size of the coins compared to cardboard control discs, but poor children
even more so.
What does this imply?
Balcettis and Dunning (2006) briefly flashed an ambiguous figure, that
could be the letter B or number 13. Participants told that a letter would
get them a nice drink, perceived the letter, while those told that a
number would earn them a drink saw the number.
Which previous aspect does this link to?
Cultural factors influence perceptual set by predisposing individuals to
perceive environmental features in certain ways.
People from different cultural backgrounds may perceive identical sensory
information differently, as they have had different environmental
In the Muller-Lyer illusion, line A is only
perceived as longer in cultures where
individuals live in a manufactured world
of straight lines and right angles because
they unconsciously read the dimension
of depth into it.
Those from cultures where buildings are
made from natural materials do not
experience the illusion.
Segall (1963) found that Africans living in open country, where
occasional vertical objects were important features, were susceptible
to the horizontal-vertical illusion, while those living in dense jungle,
were less likely to see the illusion.
Pettigrew et al. (1978) presented a picture of one South African
ethnic group to one eye of a participant and another ethnic group to
the other eye. White South Africans were not able to distinguish
black and mixed-race people, suggesting that the cultural influence
of their racial prejudice affected their perception.
Gregory believes that visual illusions are experienced because
expectations based on past experiences are used to create and
test hypotheses from incoming sensory information – but
sometimes errors happen and false perceptions are
• Gregory's theory has increased the understanding of perception and
inspired a wealth of research.
• It seems logical that we would draw on previous experience when
viewing conditions are incomplete or ambiguous.
• The idea of indirect processing explains how individuals perceive the
same stimuli in different ways.
• Gregory give a more satisfactory explanation of visual illusions than
• However – according to Gregory’s theory, once an illusion is
understood, perception should alter so that it should not be
experienced any more.
• Eysenck and Keane (1990) believe that Gregory’s theory is better at
explaining perception of illusions than real objects, because illusions are
unreal and easy to misperceive, while real objects provide enough data to
be perceived directly.
• Most supporting research comes from lab experiments, which are biased
to favour his theory as fragmented and briefly presented stimuli are often
used, so would be difficult to perceive directly.
•Gregory underestimates how rich and informative sensory data can be.
• In general, people’s perceptions are similar. This would not be true if
perceptions were created from individual experience.
• The theory suggests that we constantly search memory to find the best
interpretation of stimuli. This would be time-consuming and inefficient.
•Gibson and Gregory both see perception as being hypothesis
based, and they both agree that visual perception occurs from light
reflected off surfaces and objects, and that a specific biological
system is necessary to perceive.
•Perhaps a combination of Gregory and Gibson’s ideas form the best
explanation of perception. Gibson’s theory works best for ideal
viewing conditions, while Gregory’s works best for less than ideal
conditions. This approach was utilised in Neisser’s (1976) Perceptual
Where is the IDA?
• Gregory and Gibson’s theories can be seen in terms of the
nature vs nurture debate.
Gregory’s indirect theory emphasises learning (nurture),
while Gibson’s direct theory sees more of a role for nature.
• Research into emotional factors can be considered unethical
by causing psychological harm through exposure to