PsychologyBHP315111

MODULE 4Psychobiological

Processes- PERCEPTION

Perception…• Perception involves organising and interpreting

sensations into meaningful patterns.

• When we speak, we produce, on average, a dozen distinct units of sounds (phonemes) per second. We are capable of understanding up to 40 phonemes per second (Pinker, 1994). This requires organisation of sensations into units!

• Once we organise these units, we then need to interpret the information organised.

Organising…• A scrawl on a piece of

paper is not just a set of lines of particular orientation, but a series of letters and words.

• We interpret the lines and perceive them as letters and words.

Psych Club

Perceptual organisation• Perceptual organisation incorporates sensations into

percepts (meaningful perceptual units, like images of objects), locates them in space and preserves their meaning as you look at them from different points of view.

• If you put your textbook (percept) on the floor, it does not suddenly look like part of the floor (locate), and if you look at it from different points of view, you will still know that it is not part of the floor (points of view).

Form perception• Form perception refers to the organisation of

sensations into meaningful shapes and patterns.

• If you put your textbook on your lap, you don’t automatically perceive that it’s part of your leg, even though it might be resting on it!

• If you cover a bit of information with your pen, you don’t perceive that a piece of information has disappeared, just because your pen is blocking the information.

Gestalt principles• The first psychologists to study form perception

were the Gestalt psychologists in the early 1900s.

• Gestalt is a German word that translates ‘loosely’ to ‘whole’ or ‘form’.

Ambiguous…• Consider the lady on the

left.

• What do you perceive?

• Depending on the perceiver’s gestalt (or whole view of the picture), will depend if they see a young or old woman.

Principles…• Based on experiments from 1920s-1930s, the

Gestalt psychologists offered a small number of perceptual rules that the brain automatically and unconsciously follows as it organises sensory input into meaningful information.

• Figure-ground perception, similarity, proximity, good continuation, simplicity, and closure.

• Figure-ground perception: This is where we distinguish between figure and ground, such as words in black ink against a white page.

• The figure is the object we are viewing (penguin)

• The ground is the background or foreground the figure is in (penguin 1 or penguin 2).

Figure-ground perception

See how this uses figure-ground reversal to create an ambiguous figure, where two interpretations are possible.

Figure-ground perceptionM

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• Camouflage happens when the figure and the ground are not easily separated and they blend together.

• Military uniforms are designed to use the colours of the surrounding environment so that the figure is difficult to separate from the ground.

Figure-ground perception

Pintos, 1979, by Bev Doolittle.

Similarity • The brain tends to group similar elements together, like people arching, actually form a tree…

Artists use a technique called pointillism where they create painting using very small coloured dots, the same size. We don’t perceive dots though, we group dots of similar colours and interpret whole shapes to make sense of it all.

Proximity• This is basically ‘nearness’.

• The brain tends to group together objects that are close to one another.

• The first 6 lines have no particular organisation, but the same 6 lines in the second part are perceived as 3 pairs.

Proximity• We perceive

parts of a stimulus that are positioned close together, as belonging together in a group

Proximity• Series of letters located in close proximity to each other

will be perceptually grouped and interpreted as words.

What’s that on the road ahead?

What’s that on the road a head?

• When there is a subtle difference in the proximity of just one of the parts, it produces an entirely different interpretation of the sentence.

Good continuation• If possible, the brain organises

stimuli into continuous lines or patterns, rather than disconnected elements.

• The figure appears as an X on a circle, rather than pieces of a pie with lines extending beyond the perimeter.

Simplicity• People tend to perceive the

simplest pattern possible.

• Most people would see the figure as a heart with an arrow through it, because that is the simplest interpretation.

Closure• People tend to perceive

incomplete figures as complete. If part of a shape is missing, perceptual processes complete the pattern, like the lines below, we actually just perceive it as a square.

Closure

• Company logos and signs often require the use of closure to mentally complete an incomplete figure.

Closure

• The helicopter pilot’s failure to use closure to interpret the stranded survivor’s message, could have cost him his life!!

The Far Side, by Gary Larson. 1982 FarWorks, Inc.

Closure• Another part of closure is

called illusory contour. People see three circles and a triangle, yet none of those shapes exist.

• The brain simply fills in the gaps to perceive familiar patterns.

• The brain treats illusory contours as if they were real, because illusory contours activate the same areas of early visual processing in the primary visual cortex as real contours.

Gestalt Principles

Gestalt Principles1. Figure-ground

2. Similarity

3. Proximity

4. Good continuation

5. Simplicity

6. Closure

Biederman• Irving Biederman put forward a theory on how the brain

forms and recognises images.

• Consider this: you’re home, late at night, and you’re channel surfing. You are rapidly pressing the television remote control in search of something to watch and from less than one second’s glance, you can perceive what the show is about and whether you’ll want to watch it.

• How does the brain recognise a visual array on a screen in less than a second, and then make two decisions?

Biederman• Biederman showed that we don’t even need a half a

second to recognise most scenes! Typically a tenth of a second will do (100 milliseconds).

• He called his theory the recognition-by-components theory, where we perceive and categorise objects in our environment by breaking them down into component parts and then matching the components and the way they are arranged against similar ‘sketches’ in our memory.

Recognition-by-components• According to this theory, the brain combines the

simple features from the primary cortex (like lines and orientations) into a small number of geometrical forms (called geons – geometric icons).

• From this geometrical alphabet of 20-30 geons, the outlines of virtually any object can be constructed, just like millions of words can be constructed from the 26-letter alphabet.

These are simple geons that can be used to create thousands of different objects, simply by altering the relationships among them, like their relative size and placement.

Recognition-by-components• Biederman argues that combining what your primary visual

cortex gets, into geons, not only allows rapid identification of objects, but also explains why we can recognise objects, even when parts of them are blocked or missing.

• The brain fills in gaps in a segment of a geon, like a blocked piece of a circle. However, if the lines where separate geons connect are missing or unclear, the brain can no longer tell where one component ends and another begins.

People can rapidly identify objects, even if many parts of them are missing, as long as the relationships among their geons remain clear. When they can no longer tell where one geon ends and another begins, you can’t

identify them.

Perceptual illusions• Sometimes, the brain’s efforts to organise sensations into

accurate percepts, fails.

• This is what happens with perceptual illusions, where normal perceptual processes produce misinterpretations.

• Impossible figures are one type – They provide conflicting cues for 3D organisation. Each portion the brain works out, but as soon as it organises sensations in one way, another part of the figure changes and then you’re back to square 1!

Escher’s Belvedere (1958).

Perceptual illusions• Escher painted the first picture, the stairs, an impossible

figure. He used perceptual research.

• You’ll find that your brain tries to use Gestalt’s processes, like simplicity, but the data doesn’t allow it! The brain tries to bring order to the picture, but it is impossible!

• At times the brain’s efforts to organise percepts can sometimes produce these illusions.

Perceptual Organisation• There are four forms of perceptual organisation.

1. Form perception (which is the Gestalt principles)

2. Depth perception (which we will discuss now)

3. Motion perception (which is perceiving the movement in objects – is it a meal, mate, friend or danger?)

4. Perceptual constancy (which we will discuss later)

Depth perception• The second aspect of perceptual organisation is depth

or distance perception. This is where you perceive things in three dimensions.

• You perceive your textbook as having height, width and breadth and being at a particular distance. We make 3D judgments, based on a two-dimensional retinal image.

• We do all this so fast that we have no awareness of the computations our nervous system is making!

Depth perception• Other sensory systems (like sound and touch) give us

valuable information for depth perception, but again we focus on the visual system.

• There are 2 kinds of visual information that provide important information about depth and distance:– Binocular cues (visual input from the two eyes)

– Monocular cues (visual input from one eye)

Depth perception - Binocular• Because our eyes are in different locations, all objects

produce a different image on each retina, (apart from the most distant objects).

• Retinal disparity (which means difference) is the greatest for close objects, but diminishes with distance.

• Put your finger about 15cm from your nose and close your left eye, and then your right. You will see that each eye sees your finger in a slightly different position. If you do this with a distant object, there is only minimal differences.

Depth perception - Binocular• Most cells in the primary visual cortex are binocular

cells; they receive information from both eyes.

• When your brain sees a finger 15cm away, it uses both eyes to work out how far away the finger is; the depth/distance of the finger.

• When your brain sees a house 5km away, and the retinal disparity is minimal, it knows that it is some distance away.

Depth perception - Monocular• Although binocular cues are really important for depth

perception, you don’t automatically crash whenever you have to shut one eye, because an eyelash fell in there. You can rely on monocular cues, information received from one eye.

• Look at the following photo of the Taj Mahal as it illustrates some important depth cues…

Taj Mahal

Depth perception - Monocular– Linear perspective: Parallel lines appear to ‘meet’ in

the distance.

Depth perception – MonocularGiotto’s Flight into Egypt

Artists working in 2D media

rely on monocular

depth cues to represent a 3D world. They use

interposition and elevation

to convey depth.

Depth perception – MonocularGallipoli landing – Australian War Memorial neg. ART02161

Since the late fifteenth

century, artists have

used monocular

cues such as linear

perspective to give the illusion of depth.

Depth perception

This art, The annunciation by Carlo Crivelli is an example of Renaissance art that uses

linear perspective, as it appears to have the depth of a three dimensional painting, even though it is still just a

painting.

Depth perception - Monocular– Interposition: When one object blocks part of another (tree), the

obstructed object (Taj Mahal) is perceived as more distant. Interposition is another way of saying ‘overlap’.

Depth perception - Monocular– Texture gradient: Textured surfaces (grained wood) appear

coarser at close range and finer at greater distances. The closer you are, the better detail.

Depth perception - Monocular– Elevation: Objects farther away are higher on a person’s

view and appear higher up toward the horizon

Depth perception - Monocular– Shading: The brain assumes that light comes from above and

interprets shading differently toward the top or bottom of an object

– Aerial perspective: Objects appear fuzzier at greater distances than those nearby

– Familiar size: People tend to assume an object is its usual size and perceive familiar objects that appear small as distant

– Relative size: When looking at two objects known to be of similar size, people perceive the smaller object as farther away

Depth perception - Monocular• Page 215

Depth perception - Monocular• A final component of monocular cues is motion

parallax.

• This is where images of nearby objects sweep across your field of vision faster than objects farther away, when you are moving.

• When you’re in a car, the reflective guideposts go by much quicker than the house in the distance.

Perceptual Organisation• There are four forms of perceptual organisation.

1. Form perception (which is the Gestalt principles)

2. Depth perception (binocular, monocular)

3. Motion perception (which is perceiving the movement in objects – is it a meal, mate, friend or danger?)

4. Perceptual constancy (which we will discuss now)

Perceptual Constancy• There are three forms of perceptual contancy.

1. Colour

2. Shape

3. Size

• This fourth form of perceptual organisation refers to the perception of objects as relatively stable, despite changes in sensory receptors.

Perceptual Constancy• Imagine your grandma.

• As she walks away from you, you don’t perceive her as shrinking, even though the image she casts on your retina is decreasing in size.

• This is the same as recognising a song on the radio when the volume has been turned down; it’s still the same song even though something has changed.

Colour Constancy• This is about perceiving the colour of objects as stable, despite

changing illumination.

• An apple appears the same colour in the kitchen as it does in the sunlight even thought the light illuminating it is very different.

• If you are in a room with a red light bulb, it causes even familiar objects to appear red.

• Even though the illumination of objects change, you still know what the object is, and that it is still the same colour.

Colour Constancy

Despite the change in light intensity received by the retina,

our tendency is to maintain brightness contancy and to perceive objects within the scene as having the same

relative brightness.

Even though they look different, we still know they haven’t

changed colour.

Sh pe Constancy• This is about perceiving the shape of objects as stable,

despite a change in shape.

• The same object typically produces a new and different impression on the retina every time we encounter it, but it has to overcome this to recognise it. When you wake up with bed hair and look in the mirror and your hair is pointing in every direction, the brain knows this is the same person that looked in the mirror last night.

Sh pe Constancy• When you see an object for the first time (lets say Storm),

you find out who/what it is. When you see it a second time, you are likely to see it from a:– different position (side on)

– with different lighting (at night time)

– in a different setting (sitting in a tree)

– with different parts of it blocked from view (hair covering face)

– and even in an altered shape sometimes (standing up vs sitting, or a loss of weight)

Sh pe ConstancyWe apply shape contancy when a door opens as we

perceive that it is maintaining its rectangular

shape, despite a trapezoidal retinal image being received

as the door opens.

Sh pe Constancy• The recognition-by-components (geon theory) is one

theory to offer an explanation. As long as enough of the geons that define the form of the object remain the same, the object ought to be identifiable.

• You should be able to recognise a bee on a flower, and if it flies into your face, you should still recognise it if it still looks like a tube with a little cone at the back and wings.

Sh pe Constancy• Others offer a different theory.

• Some say that each time we view an object from a different perspective, we form a mental image of it. Each new viewpoint provides a new image, and the next time we see a similar object, we rotate it in our minds so that we can ‘see’ it from a different perspective to work out if it looks like the same object or not.

size Constancy

• This is about how objects don’t appear to change in size when viewed from different distances.

• The larger the object is, the larger an image it casts on the retina. A car 3m away will cast a retinal image five times as large as the same car 15m away, yet we don’t question how many people would be able to fit in the ‘small’ car, as our brain corrects the size based on the size of objects in the background.

size ConstancyRoad signs like this begin as a green speck in the distance, and progressively forms a larger image on the retina as we get closer to it. We perceive the size of the

sign as unchanging because of our familiarity of road

signs

size Constancy

• Helmholtz (1962, [1909]) was the first to recognise that the brain adjusts for distance when working out the size of objects, like it adjusts for colour and brightness.

• He called this unconscious inference, because you don’t have any conscious knowledge of what computations are involved.

• Although these computations are generally accurate, they can give perceptual illusions.

size ConstancyThe moon illusion

is where the moon seems

larger against a city skyline than high in the sky.

The retinal image is the same size in both cases, but when it is

high in the sky, there is nothing

to compare it to.

Perceptual Set• Look around you. Write down what you see in four sentences.

• Everyone’s sentences are likely to be different and described differently. This shows that two or more people who experience the same or similar visual input, can arrive at quite different perceptions.

• As well as visual perception principles, other psychological factors also influence how we interpret incoming sensory stimuli. These psychological factors cause us to view stimuli in a personal manner.

Perceptual Set• We have a predisposition about how we will visually

perceive objects and events. Psychologists call this a perceptual set or expectancy.

• In particular, a perceptual set is when we develop certain expectations about how things should be.

• These are learned expectations that are based on our personal, social, or cultural experiences. They automatically add information and meaning to our perceptions and can change/bias our perceptions.

Perceptual Set • When you look at this guy, you automatically add personal feelings to it. Do you like/dislike him? Do you approve/disapprove? What do you perceive his height, weight, etc. to be?

• He is a bodybuilder, but his muscles aren’t really as big as you think they are. Because of your perceptual set for bodybuilders, you expect them to be large. Perceptual sets automatically fill in information that can greatly modify our perceptions.

Perceptual Set• The culture where we are raised, influences our ideas,

knowledge, beliefs and attitudes as well as the things we are familiar with. These affect the way we perceive our environment and how we respond to it.

• Sometimes motivation can lead us to perceive in a particular way, we sometimes see what we expect, or want, to see.

Perceptual Set• The context or the physical surroundings in which the

object or event occurs, can influence our perception.

• Context is important. Imagine if you saw a fast-moving bright light in the sky that had a tail streaking behind it. How would you interpret it if you observed it in the sky over:– Outback central Australia? A war zone? Florida? Melbourne’s

Yarra River on New Year’s Eve? Cape Canaveral, Florida?

Perceptual Set• As the context of the visual stimulus changes, your

interpretation may also have changed from:– Meteorite, Missile, Rocket launch, Flare, or Fireworks.

• Past experience also comes into play when interpreting visual stimuli, as Cape Canaveral would have little effect on your interpretation, if you didn’t know from past experience that space rocket launches occur there.

Perceptual Set

• Which inner circle appears bigger?

Perceptual Set• The Titchener circles can produce a mistake in visual

perception because of the different contexts in which they sit.

• Another experiment also highlights the importance of context. Jerome Bruner and Leigh Minturn (1955) showed the stimulus figure for 80-thousandths of a second to one group, after viewing the letters L, M, Y, A. Another group was shown the same stimulus after viewing the numbers 16, 17, 10, 12.

Perceptual Set• Results from the first group showed that 92% of the

participants perceived the letter ‘B’.

• Results from the second group showed that 83% of the participants perceived the stimulus as the number ‘13’.

• Researchers concluded that a perceptual set had been established by the time the figure was to be interpreted.

13

Perceptual Set• You might be meeting a friend at the corner of a street in

Melbourne. You know that they walk fast normally, so you start looking for someone that is walking the fastest in the oncoming crowd. You start ignoring everyone that is walking slow. That’s a perceptual set; it enables you to visually perceive information quickly. You eliminate all the people in your visual field who don’t match your perceptual set. But what happens if she sprained her ankle and is walking slowly? Or you greet the wrong fast walker?

Perceptual Set• How many

aces of spades do you see?

Perceptual Set• Jerome Bruner and Leo Postman (1949) did an

experiment involving these playing cards. After briefly looking at them, most people said there were 3 aces of spades. Actually, there were 5…two were red. Because of their past experience with playing cards, the participants were predisposed to look only at the black cards, when given the task, resulting in inaccurate perceptions.

Theories• There are 3 theories we will look at in terms of visual

perception; – Richard Gregory’s indirect theory

– James Gibson’s direct theory

– Neisser’s cyclic model

• They are all theories of perceptual organisation.

• Gibson’s direct theory is commonly known as the bottom-up theory, and Gregory’s indirect theory is known as the top-down theory.

Bottom-up and Top-down• When you see a face that looks familiar or an animal that

resembles one we have seen, does our past experience actually alter the way we perceive it, or do we begin to categorise the face/animal once we have identified its features?

• A wine connoisseur…– Does his knowledge about wine alter his perception?

– Or, does he just have fancier words to describe his experience?

Bottom-up Processing• Bottom-up processing (Gibson), sometimes called

ecological theory refers to processing that begins at the bottom, with raw data, that feeds up to the brain.

• This suggests that the brain forms perceptions by combining the responses of multiple feature detectors in the primary cortex.

• It starts with detecting features from sensory data, and moves to analysing features and combining parts, before it forms a perception.

Bottom-up processing• This bicycle is partly obscured

by the lady standing in front of it, but we can still build up the image of bicycle from seeing only parts of it, as it appears to be a bicycle.

• Even if we saw much less of the bicycle, we would still complete the image.

Bottom-up Processing• Strengths of Gibson’s theory include:– Vision happens very quickly and is generally accurate

• Weaknesses…– It can be argued that people learn about the use of

objects within a cultural context, rather than their use being afforded.

– It doesn’t take into account the complex processing of visual information in the brain.

Top-down Processing• Top-down processing (Gregory), sometimes called

constructivist theory refers to processing that begins at the top, with the observer’s expectations and knowledge, our perceptual sets.

• This suggests that the brain uses prior knowledge and context to begin organising and interpreting sensations as soon as the information starts coming in, rather than waiting for percepts to form, based on the analysis of features.

• Generally we are aware of top-down processing.

Top-down Processing• Like Gestalt theorists, we presume that as soon as the brain

has detected features resembling eyes, it begins to expect a face and looks for a nose and a mouth – prior knowledge

• As your retina receives the raw sensory information that your teacher writes on the board (bottom-up), your perceptual set may enable you to interpret a misspelt word such as ‘rember’ into a meaningful word, ‘remember’, so you can make sense of what is being written (past experience and context – top-down).

• Comparison graph p167.

Top-down Processing• This processing involves conscious thinking and problem

solving. When we see the whole initially, this starts a chain of processing in the visual system.

• We often perceive a word before we perceive the individual letters in the word. However, this depends on whether the word is already being stored in our memory.

• If you see an unfamiliar word, and sound it out, this is an example of bottom-up. However, if you compare it to similar words in your memory, this is top-down.

Top-down Processing• When the sensory input is often unclear or incomplete,

we ‘best guess’ what we are seeing. Gregory called this process ‘hypothesis-testing’.

• Our past experience affects how we perceive an object or situation.

• We can sometimes produce hypotheses which are not correct, making perceptual errors. Visual illusions are an example of this.

Top-Down Processing• Strengths of Gregory’s theory include:– His theory accounts for visual illusions better than Gibson’s direct theory

– There is research evidence supporting the use of stored information to guide perception

• Weaknesses…– It over-emphasises perceptual errors; in general we perceive things accurately

– Shouldn’t people make more mistakes than they do if hypothesis-testing is involved?

– If everyone constructs their own perception of the world, why do we tend to see the world in a similar way?

– Gregory did not do much studying outside the laboratory

Which one?• Both options have support of an empirical nature.

• Without bottom-up processing, external stimuli would have no effect on perception – we would hallucinate rather than perceive!

• Without top-down processing, experience would have no effect on perception.

Which one?• Currently, it is thought that both types of processing

occur simultaneously (Pollen, 1999; Rumelhart et al., 1986).

• Figure 4.40 on page 168.

Both types of Processing• Both Gibson’s and Gregory’s theories have been influential, but

it’s likely that we use a combo of top-down and bottom-up processing.

• The Gestalt principles focus on bottom-up perceptual processing, but essentially we need interaction between both processing types, so that we are able to process new information and use existing knowledge or beliefs to give meaning to what we see.

• Whether we are looking at the raw data and then interpreting what we are seeing, or we are expecting something because of what we know, we are always interpreting visual stimuli.

Which one?• So we know that we use both processing systems to

process visual stimuli.

• One theorist attempted to combine the best of both approaches in his cyclic model; Neisser (1976).

• In his theory, top-down and bottom-up processes are used in a continuing cycle during perception.

Neisser’s theory• He assumes that perception starts with schemas

(ideas/expectations of what we expect to see), and they influence the way we explore our environment as we confirm our expectations.

• We then form a perceptual model which is a mental representation of a likely object or event.

Neisser’s theory• Then we search our sensory stimuli and if we find it

confirms the perceptual model, more details are added to the model; elaborative effect.

• If not, the perceptual model will need to be revised; corrective effect.

• Perception involves a continuous process of checking and rechecking sensory stimuli, in line with changes to the perceptual model we have formed.

Neisser’s theory• Neisser calls this process the analysis-by-synthesis theory,

because we extract information about the environment (analysis) and then generate a perceptual model (synthesis), in a cycle.

• A major strength of this theory is that it shows the interaction of bottom-up and top-down processing as important in perceiving the environment around us.

• It falls in line with Gregory’s theory of hypothesis testing, but in Neisser’s model, the hypotheses we form are much more general!

Neisser’s theory• All models show that perception is an active process.

• Our attention is selective, as we only take in what we give our attention to, and each of us will prioritise our attention individually.

• E.g. – Luke knocked a shed down and I didn’t notice it, but I did notice that he had new wet weathers!

• Males give their attention to different things, compared with females, generally speaking.

Neisser’s theory• Weaknesses:– All 3 theories are descriptive, rather than explanatory.

They doesn’t actually explain the actual processes of perception in terms of neural activity.

– It is therefore thought of more as a general framework, rather than a testable model.

– It would be very difficult to test, given the cyclical nature of the theory

– Where exactly in the cycle is perception said to occur?

Neisser’s theory

Types of ProcessingJames Gibson Direct

PerceptionBottom-up Ecological

Richard Gregory

Indirect Perception

Top-down Constructivist

Ulric Neisser (1976)

Cyclic Model Combo Analysis-by-synthesis

• These three theories are really only frameworks on how we perceive the visual stimuli around us.

Sensation: Your window to the world

Perception: Interpreting what comes in your window

REVIEW!

• Sensation & Perception• Perceptual organisation

(percepts)• Form perception

• Gestalt principles• Biederman

• Recognition-by-components

• Geons• Perceptual illusions• Depth or distance perception

• Binocular cues• Monocular cues

• Perceptual constancies• Size, shape, colour

• Perceptual set• Bottom-up theory• Top-down theory• Neisser’s cyclical model

References• Westen, D., Burton, L., Kowalski, R. (2006)

Psychology. Queensland, Australia: John Wiley & Sons Australia, Ltd.

• Cribb, B., Gridley, H., McKersie, C., Kennedy, G., Anin, N., Rice, J. (2004) Essential VCE Psychology. Cambridge, UK: Cambridge University Press.

• Plotnik, R. (2002) Introduction to Psychology. (6th ed.) CA, USA: Wadsworth Group.