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In order to receive information from the environment we are equipped with sense organs eg eye, ear, nose. Each sense organ is part of a sensory system which receives sensory inputs and transmits sensory information to the brain. A particular problem for psychologists is to explain the process by which the physical energy received by sense organs forms the basis of perceptual experience. Sensory inputs are somehow converted into perceptions of desks and computers, flowers and buildings, cars and planes; into sights, sounds, smells, taste and touch experiences.

A major theoretical issue on which psychologists are divided is the extent to which perception relies directly on the information present in the stimulus. Some argue that perceptual processes are not direct, but depend on the perceiver’s expectations and previous knowledge as well as the information available in the stimulus itself. This controversy is discussed with respect to Gibson (1966) who has proposed a direct theory of perception which is a ‘bottom-up’ theory, and Gregory (1970) who has proposed a constructivist (indirect) theory of perception which is a ‘topdown’ theory.


Helmholtz (1821-1894) is considered one of the founders of perceptual research. He argued that between sensations and our conscious perception of the real world there must be intermediate processes. Such processes would be, for example, ‘inferential thinking’ – which allows us to go beyond the evidence of the senses (these inferences are at an unconscious level). Thus Helmholtz was an early Constructivist who believed perception is more than direct registration of sensations, but that other events intervene between stimulation and experience.

An early illustration that supports the idea of perceptions as modifiable constructions rather than the direct responses to pattern of stimulation is the ‘Ames Room’. This room is of an irregular shape with a receding rear wall and decorated in a special manner.


The true wall, AC on the diagram, is decorated so as to appear to be in the position AB. Viewed from the front peephole with one eye the room appears to be rectangular but a person moving from A to C will appear to shrink.

One explanation for the Ames Room illusion is that the perceiver is in a situation of having to choose between two beliefs built up through experience – (a) rooms that look rectangular and normal, usually are just that, (b) people are usually of ‘average’ size. Most observers choose (a) and therefore consider the people to be ‘odd’.

The interesting thing about the Ames Room illusion is that it does not disappear when you learn the true shape of the room.


Gregory proposes that perceiving is an activity resembling hypothesis formation and testing. He says that signals received by the sensory receptors trigger neural events, and appropriate knowledge interacts with these inputs to enable us to makes sense of the world.

Gregory has presented evidence in support of his theory, some of which is outlined below:

  1. ‘Perception allows behaviour to be generally appropriate to non-sensed object characteristics’.

For example, we respond to certain objects as though they are doors even though we can only see a long narrow rectangle as the door is ajar.


Gregory argues that surely to do this we must be using more than just sensory inputs.

2. ‘Perceptions can be ambiguous’

The Necker cube is a good example of this. When you stare at the crosses on the cube the orientation can suddenly change, or flip’. It becomes unstable and a single physical pattern can produce two perceptions.

  1. ‘Highly unlikely objects tend to be mistaken for likely objects’.

Gregory has demonstrated this with a hollow mask of a face. Such a mask is generally seen as normal, even when one knows and feels the real mask. There seems to be an overwhelming need to reconstruct the face, similar to Helmholtz’s description of ‘unconscious inference’.

What we have seen so far would seem to confirm that indeed we do interpret the information that we receive, in other words, perception is a top down process. However:….


1. The Nature of Perceptual Hypotheses

If perceptions make use of hypothesis testing the question can be asked ‘what kind of hypotheses are they?’ Scientists modify a hypothesis according to the support they find for it so are we as perceivers also able to modify our hypotheses? In some cases it would seem the answer is yes.

For example, look at the figure below:


This probably looks like a random arrangement of black shapes. In fact there is a hidden face in there, can you see it? The face is looking straight ahead and is in the top half of the picture in the centre. Now can you see it? The figure is strongly lit from the side and has long hair and a beard.

Once the face is discovered, very rapid perceptual learning takes place and the ambiguous picture now obviously contains a face each time we look at it. We have learned to perceive the stimulus in a different way.

Although in some cases, as in the ambiguous face picture, there is a direct relationship between modifying hypotheses and perception, in other cases this is not so evident. For example, illusions persist even when we have full knowledge of them (e.g. the inverted face, Gregory 1974). One would expect that the knowledge we have learned (from, say, touching the face and confirming that it is not ‘normal’) would modify our hypotheses in an adaptive manner. The current hypothesis testing theories cannot explain this lack of a relationship between learning and perception.

2. Perceptual Development

A perplexing question for the constructivists who propose perception is essentially top-down in nature is ‘how can the neonate ever perceive?’ If we all have to construct our own worlds based on past experiences why are our perceptions so similar, even across cultures? Relying on individual constructs for making sense of the world makes perception a very individual and chancy process.

The constructivist approach stresses the role of knowledge in perception and therefore is against the nativist approach to perceptual development. However, a substantial body of evidence has been accrued favouring the nativist approach, for example:

Newborn infants show shape constancy (Slater & Morison, 1985); they prefer their mother’s voice to other voices (De Casper & Fifer, 1980); and it has been established that they prefer normal features to scrambled features as early as 5 minutes after birth.

4. Sensory Evidence

Perhaps the major criticism of the constructivists is that they have underestimated the richness of sensory evidence available to perceivers in the real world (as opposed to the laboratory where much of the constructivists’ evidence has come from).

Constructivists like Gregory frequently use the example of size constancy to support their explanations. That is, we correctly perceive the size of an object even though the retinal image of an object shrinks as the object recedes. They propose that sensory evidence from other sources must be available for us to be able to do this.

However, in the real world, retinal images are rarely seen in isolation (as is possible in the laboratory). There is a rich array of sensory information including other objects, background, the distant horizon and movement. This rich source of sensory information is important to the second approach to explaining perception that we will examine, namely the direct approach to perception as proposed by Gibson.


Gibson claimed that perception is, in an important sense, direct. He worked during World War II on problems of pilot selection and testing and came to realise:

In his early work on aviation he discovered what he called ‘optic flow patterns’. When pilots approach a landing strip the point towards which the pilot is moving appears motionless, with the rest of the visual environment apparently moving away from that point.

The outflow of the optic array in a landing glide.

According to Gibson such optic flow patterns can provide pilots with unambiguous information about their direction, speed and altitude.

Three important components of Gibson’s Theory are 1. Optic Flow Patterns; 2. Invariant Features; and 3. Affordances. These are now discussed.

1. Light and the Environment – Optic Flow Patterns

Changes in the flow of the optic array contain important information about what type of movement is taking place. For example:

  • Any flow in the optic array means that the perceiver is moving, if there is no flow the perceiver is static.
  • The flow of the optic array will either be coming from a particular point or moving towards one. The centre of that movement indicates the direction in which the perceiver is moving. If a flow seems to be coming out from a particular point, this means the perceiver is moving towards that point; but if the flow seems to be moving towards that point, then the perceiver is moving away. See above for moving towards an object, below is moving away:

The Optic Flow pattern for a person looking out of the back of a train.

2. The role of Invariants in perception

We rarely see a static view of an object or scene. When we move our head and eyes or walk around our environment, things move in and out of our viewing fields. Textures expand as you approach an object and contract as you move away. There is a pattern or structure available in such texture gradients which provides a source of information about the environment. This flow of texture is INVARIANT, ie it always occurs in the same way as we move around our environment and, according to Gibson, is an important direct cue to depth. Two good examples of invariants are texture and linear perspective.


Texture Gradient giving the appearance of depth

Linear Perspective

Parallel lines, eg railway tracks, appear to converge as they recede into the distance.

3. Affordances

Are, in short, cues in the environment that aid perception. Important cues in the environment include:

OPTICAL ARRAY The patterns of light that reach the eye from the environment.
RELATIVE Objects with brighter, clearer
BRIGHTNESS images are perceived as closer.
TEXTURE The grain of texture gets
GRADIENT smaller as the object recedes. Gives the impression of surfaces receding into the distance.
RELATIVE SIZE When an object moves further away from the eye the image gets smaller. Objects with smaller images are seen as more distant.
SUPERIMPOSITION If the image of one object blocks the image of another, the first object is seen as closer.
HEIGHT IN THE Objects further away are
VISUAL FIELD generally higher in the visual field.


Visual Illusions

Gibson’s emphasis on DIRECT perception provides an explanation for the (generally) fast and accurate perception of the environment. However, his theory cannot explain why perceptions are sometimes inaccurate, eg in illusions. He claimed the illusions used in experimental work constituted extremely artificial perceptual situations unlikely to be encountered in the real world, however this dismissal cannot realistically be applied to all illusions.

For example, Gibson’s theory cannot account for perceptual errors like the general tendency for people to overestimate vertical extents relative to horizontal ones.

Neither can Gibson’s theory explain naturally occurring illusions. For example if you stare for some time at a waterfall and then transfer your gaze to a stationary object, the object appears to move in the opposite direction .

Bottom-up or Top-down Processing?

Neither direct nor constructivist theories of perception seem capable of explaining all perception all of the time. Gibson’s theory appears to be based on perceivers operating under ideal viewing conditions, where stimulus information is plentiful and is available for a suitable length of time. Constructivist theories, like Gregory’s, have typically involved viewing under less than ideal conditions.

Research by Tulving et al manipulated both the clarity of the stimulus input and the impact of the perceptual context in a word identification task. As clarity of the stimulus (through exposure duration) and the amount of context increased, so did the likelihood of correct identification. However, as the exposure duration increased, so the impact of context was reduced, suggesting that if stimulus information is high, then the need to use other sources of information is reduced. One theory that explains how top-down and bottom-up processes may be seen as interacting with each other to produce the best interpretation of the stimulus was proposed by Neisser (1976) known as the ‘Perceptual Cycle’.


The concept of perceptual set is important to the active process of perception. Allport, 1955 defined perceptual set as:

“a perceptual bias or predisposition or readiness to perceive particular features of a stimulus”.

Perceptual set is a tendency to perceive or notice some aspects of the available sensory data and ignore others. According to Vernon, 1955 set works in two ways: (1) The perceiver has certain expectations and focuses attention on particular aspects of the sensory data: This he calls a ‘Selector’. (2) The perceiver knows how to classify, understand and name selected data and what inferences to draw from it. This he calls an ‘Interpreter’.

It has been found that a number of variables, or factors, influence set, and set in turn influences perception. The factors include:

  • Expectations
  • Emotion
  • Motivation
  • Culture


  • Bruner & Minturn, 1955 illustrated how expectation could influence set by showingparticipants an ambiguous figure ’13’ set in the context of letters or numbers e.g.

The physical stimulus ’13’ is the same in each case but is perceived differently because of the influence of the context in which it appears. We EXPECT to see a letter in the context of other letters of the alphabet, whereas we EXPECT to see numbers in the context of other numbers.

  • We may fail to notice printing/writing errors for the same reason. For example:
  1. ‘The Cat Sat on the Map and Licked its Whiskers’.


(a) and (b) are examples of interaction between expectation and past experience.

(c) A study by Bugelski and Alampay, 1961 using the ‘rat-man’ ambiguous figure also demonstrated the importance of expectation in inducing set. Participants were shown either a series of animal pictures or neutral pictures prior to exposure to the ambiguous picture. They found participants were significantly more likely to perceive the ambiguous picture as a rat if they had had prior exposure to animal pictures.


Allport, 1955 has distinguished 6 types of motivational-emotional influence on perception:

  • bodily needs (eg physiological needs)
  • reward and punishment
  • emotional connotation
  • individual values
  • personality
  • the value of objects.
  • Sandford, 1936 deprived participants of food for varying lengths of time, up to 4 hours, andthen showed them ambiguous pictures. Participants were more likely to interpret the pictures as something to do with food if they had been deprived of food for a longer period of time. Similarly Gilchrist & Nesberg, 1952, found participants who had gone without food for the longest periods were more likely to rate pictures of food as brighter. This effect did not occur with non-food pictures.
  • A more recent study into the effect of emotion on perception was carried out by KunstWilson & Zajonc, 1980. Participants were repeatedly presented with geometric figures, but at levels of exposure too brief to permit recognition. Then, on each of a series of test trials, participants were presented a pair of geometric forms, one of which had previously been presented and one of which was brand new. For each pair, participants had to answer two questions: (a) Which of the 2 had previously been presented? ( A recognition test); and (b) Which of the two was most attractive? (A feeling test).

The hypothesis for this study was based on a well-known finding that the more we are exposed to a stimulus, the more familiar we become with it and the more we like it. Results showed no discrimination on the recognition test – they were completely unable to tell old forms from new ones, but participants could discriminate on the feeling test, as they consistently favoured old forms over new ones. Thus information that is unavailable for conscious recognition seems to be available to an unconscious system that is linked to affect and emotion.


  • Deregowski, 1972 investigated whether pictures are seen and understood in the same way in different cultures. His findings suggest that perceiving perspective in drawings is in fact a specific cultural skill, which is learned rather than automatic. He found people from several cultures prefer drawings which don’t show perspective, but instead are split so as to show

both sides of an object at the same time. In one study he found a fairly consistent preference among African children and adults for split-type drawings over perspective-drawings. Splittype drawings show all the important features of an object which could not normally be seen at once from that perspective. Perspective drawings give just one view of an object. Deregowski argued that this split-style representation is universal and is found in European children before they are taught differently.

Elephant drawing split-view and top-view perspective. The split elephant drawing was generally preferred by African children and adults.

  • Hudson, 1960 noted difficulties among South African Bantu workers in interpreting depthcues in pictures. Such cues are important because they convey information about the spatial relationships among the objects in pictures. A person using depth cues will extract a different meaning from a picture than a person not using such cues.

Hudson tested pictorial depth perception by showing participants a picture like the one below. A correct interpretation is that the hunter is trying to spear the antelope, which is nearer to him than the elephant. An incorrect interpretation is that the elephant is nearer and about to be speared. The picture contains two depth cues: overlapping objects and known size of objects. Questions were asked in the participants native language such as:

‘What do you see?’

‘Which is nearer, the antelope or the elephant?’

‘What is the man doing?’’

The results indicted that both children and adults found it difficult to perceive depth in the pictures.

The cross-cultural studies seem to indicate that history and culture play an important part in how we perceive our environment. Perceptual set is concerned with the active nature of perceptual processes and clearly there may be a difference cross-culturally in the kinds of factors that affect perceptual set and the nature of the effect.


Perceptual constancies involve seeing visual objects accurately, regardless of their distance away from us, or other factors that distort the retinal image. For example, the door is ‘seen’ as a rectangular shape even when open and the retinal image is of a trapezium.


When we observe an object, the light falling on the retina is known as the ‘retinal image’. Light rays enter through the lens in the front of the eye and are focused on a particular area of the retina at the back of the eye. The intriguing question is ‘how does the brain interpret the light image on the retina to arrive at an accurate perception?’ This is particularly interesting when we try to explain how objects are perceived as the same size even when seen at a distance. This can be demonstrated:

Hold both hands in front of you, your left hand at arms length and your right hand about half way to your face. Both hands are ‘perceived’ as the same size, but the retinal image of the right hand will be much larger. Now move your right hand so that it overlaps the left hand, stin with the left at arms length and the right hand halfway towards your face. You should find that the right hand ‘swamps’ the left – this is because the two hands are actually stimulating different sized retinal images. The diagram below shows the different sizes of retinal images projected by the same sized hand at different distances.

How is it that we ‘perceive’ the hand as being the same size in spite of these differences in the retinal image? One explanation for this (ie for ‘size constancy’) is that the brain receives information both about size of the retinal image and distance of the object. The visual system seems to automatically make allowances for distance. For example, even thought the retinal image of the left hand is small, distance cues inform the brain that it is further away than the right hand and this can explain the smaller retinal image. Taking account of both size and distance in the visual system the brain would probably conclude that the hands are the same size.

Several distance cues have been identified which could aid the process of constancy scaling, and two of these are:


The retina in each eye receives a slightly different image. To demonstrate this: close one eye and line your finger up with the corner of the room. Now close that eye and open the other eye – the finger appears to move. Normally, when using both eyes the visual system calculates how far away the finger is by combining information from the differences between the two images on the retinas.


To a moving observer distant objects appear to move more slowly than near objects. For example, when you look out of the window of a moving train nearer objects like telegraph poles flash by faster than distant telegraph poles. The visual system can use this information to calculate how far away the telegraph poles are.


Knowledge of the ‘real’ shape of an object means that it is still perceived as being the same regardless of the angle from which it is viewed. For example, I ‘perceive’ the wall clock as circular even though from the angle I am now looking at it the retinal image is of an elliptical shape.


This is where familiar objects retain their colour (or hue) in a variety of lighting conditions. Knowledge of the ‘real’ colour of the object means that it is still perceived as being that colour, regardless of the actual colour wavelength of the light that reaches the eye. Thus at night we still perceive our red car as ‘red’ even under the night light.


Knowledge that objects don’t generally move means that things are seen as remaining in the same place even when the observer moves around and the retinal image changes. As we move around the environment we produce a constantly changing pattern of retinal images yet we do not perceive the world as spinning; this is due to kinaesthetic feedback. The brain subtracts the eye movement commands from the resulting changes on the retina and this helps to keep us and the environment stable.


Constancy scaling seems to happen automatically; it doesn’t require us to think about it. Normally the visual system receives accurate information about the size and distance of objects eg by the use of distance cues. Psychologists have been particularly interested in instances where the visual system makes errors as it does when conflicting information is received. Look at the Ponzo Illusion:

why we perceive the top line as longer (paradoxically, we usually perceive the nearest line as longer) is that these type of illusions contain false depth cues which trigger the size constancy mechanism inappropriately. In the case of the Ponzo Illusion the converging lines are the false depth cues which suggest the top line is further away than the bottom line. The eye is tricked by the depth cues in the converging lines into ‘thinking’ the top line is further away. An object which is further away produces a smaller retinal image. The size constancy mechanism therefore expands the perceived size of the top line. In most cases automatic triggering of the size constancy mechanism by a simple depth cue would result in an accurate perception. It is when perception goes wrong that psychologists have been given insight into how the automatic scaling mechanism might operate.


Illusions are relatively rare in the natural world and so there has been no evolutionary pressure to produce a perceptual system that overcomes this. One illusion that does occur in the natural world is the ‘Moon Illusion’.

The moon (and sun) appears larger when low down on the horizon than when high in the sky. The size of the retinal image does not change. You can ‘black out’ the moon by holding a 1/4 inch disc at arm’s length, whether the moon is high or low in the sky. Why then does it appear to be much larger when it is near the horizon? One explanation is constancy scaling. When the moon is high in the sky, there is no depth/distance information visible, so you see the moon at its correct (ie retinal image) size. However, when the moon is low down, near the horizon, depth cues operate. The horizon is as far away as it is possible to see so constancy scaling automatically increases the size. If the moon (whose retinal image remains the same) appears to be further away when it is closer to the horizon then we conclude it must be larger.


Gregory, 1983 has identified 4 types of illusions:

Distortions (eg Ponzo Illusion) where we make a perceptual mistake;

Ambiguous figures (eg the Necker Cube) where the same input results in different perceptions;

Paradoxical Figures (eg the Penrose Trident) we assume this is a 3-dimensional object;

Fictions (eg the Kanizsa triangle) where we see what is not in the stimulus, ie a second triangle.


“Everyone knows what attention is. It is taking possession by the mind, in clear and vivid form, of one of what seems several simultaneously possible objects or trains of thought…. it implies withdrawal from some things in order to deal effectively with others “

W James 1890.

Do we attend simultaneously to everything in our environment or do we attend selectively to certain types of information at any one time? The topics of perception and attention merge into each other since both are concerned with the question of what we become aware of in our environment. We can only perceive things we are attending to, we can only attend to things we perceive. Therefore some of the same questions are at issue, notably the question of whether attention is governed by ‘bottom-up’ sensory processes (as proposed by the information processing models) or whether ‘top-down’ processes like memory/expectations etc play an important part in attention.


When we are selectively attending to one activity, we tend to ignore other stimulation, although our attention can be distracted by something else, like the telephone ringing or someone using our name. Psychologists are interested in what makes us attend to one thing rather than another (selective attention); why we sometimes switch our attention to something that was previously unattended (e.g. Cocktail Party Syndrome), and how many things we can attend to at the same time (attentional capacity).

One way of conceptualising attention is to think of humans as information processors who can only process a limited amount of information at a time without becoming overloaded. Broadbent and others in the 1950’s adopted a model of the brain as a limited capacity information processing system, through which external input is transmitted.



Information processing models consist of a series of stages, or boxes, which represent stages of processing. Arrows indicate the flow of information from one stage to the next.

Input processes are concerned with the analysis of the stimuli.

Storage processes cover everything that happens to stimuli internally in the brain and can include coding and manipulation of the stimuli.

Output processes are responsible for preparing an appropriate response to a stimulus.

ATTENTION THEORIES are concerned with how information is selected from incoming stimuli for further processing in the system – therefore operate at the input processes end of the model.

Basic Assumptions of the Information Processing Approach to Cognitive Processes

The information processing approach is based on a number of assumptions, including:

  • information made available by the environment is processed by a series of processingsystems (eg attention, perception, short-term memory);
  • these processing systems transform or alter the information in systematic ways;
  • the aim of research is to specify the processes and structures that underlie cognitiveperformance;
  • information processing in humans resembles that in computers.

A number of Models of attention within the Information Processing framework have been proposed including:

Broadbent’s Filter Model (1958), Treisman’s Attenuation Model (1964) Deutsch and Deutsch’s Late Selection Model (1963)

and these will be outlined and evaluated. However, there are a number of evaluative points to bear in mind when studying these models, and the information processing approach in general. These include:

  1. The information processing models assume serial processing of stimulus inputs.

Serial processing effectively means one process has to be completed before the next starts.

Parallel processing assumes some or all processes involved in a cognitive task(s) occur at the same time.


Eg Shadowing, fate of unattended stimuli

Task Difficulty

Eg effects on automaticity

There is evidence from dual-task experiments (examples are given later) that parallel processing is possible. It is difficult to determine whether a particular task is processed in a serial or parallel fashion as it probably depends (a) on the processes required to solve a task, and (b) the amount of practice on a task. Parallel processing is probably more frequent when someone is highly skilled; for example a skilled typist thinks several letters ahead, a novice focuses on just 1 letter at a time.

  1. The analogy between human cognition and computer functioning adopted by the information processing approach is limited. Computers can be regarded as information processing systems insofar as they:
  • combine information presented with stored information to provide solutions to avariety of problems, and
  • most computers have a central processor of limited capacity and it is usually assumedthat capacity limitations affect the human attentional system.


  • the human brain has the capacity for extensive parallel processing and computersoften rely on serial processing;
  • humans are influenced in their cognitions by a number of conflicting emotional andmotivational factors.
  1. The evidence for the theories/models of attention which come under the information processing approach is largely based on experiments under controlled, scientific conditions. Most laboratory studies are artificial and could be said to lack ecological validity. In everyday life, cognitive processes are often linked to a goal (eg you pay attention in class because you want to pass the examination), whereas in the laboratory the experiments are carried out in isolation form other cognitive and motivational factors. Although these laboratory experiments are easy to interpret, the data may not be applicable to the real world outside the laboratory. More recent ecologically valid approaches to cognition have been proposed (eg the Perceptual Cycle, Neisser, 1976).

Attention has been studied largely in isolation from other cognitive processes, although clearly it operates as an interdependent system with the related cognitive processes of perception and memory. The more successful we become at examining part of the cognitive system in isolation, the less our data are likely to tell us about cognition in everyday life.

  1. The Models proposed by Broadbent and Treisman are ‘bottom-up’ or ‘stimulus driven’ models of attention. Although it is agreed that stimulus driven information in cognition is important, what the individual brings to the task in terms of expectations/past experiences are also important. These influences are known as ‘top-down’ or ‘conceptually-driven’ processes. For example, read the triangle below:

Paris in the the Spring Expectation (top-down processing) often over-rides information actually available in the stimulus (bottom-up) which we are, supposedly, attending to. How did you read the text in the triangle above?



A bottleneck restricts the rate of flow, as, say, in the narrow neck of a milk bottle. The narrower the bottleneck, the lower the rate of flow. Broadbent’s, Treisman’s and Deutsch and Deutsch Models of Attention are all bottleneck models because they predict we cannot consciously attend to all of our sensory input at the same time. This limited capacity for paying attention is therefore a bottleneck and the models each try to explain how the material that passes through the bottleneck is selected.


Donald Broadbent is recognised as one of the major contributors to the information processing approach, which started with his work with air traffic controllers during the war. In that situation a number of competing messages from departing and incoming aircraft are arriving continuously, all requiring attention. The air traffic controller finds s/he can deal effectively with only one message at a time and so has to decide which is the most important. Broadbent designed an experiment (dichotic listening) to investigate the processes involved in switching attention which are presumed to be going on internalb in our heads.

Broadbent argued that information from all of the stimuli presented at any given time enters a sensory buffer. One of the inputs is then selected on the basis of its physical characteristics for further processing by being allowed to pass through a filter. Because we have only a limited capacity to process information, this filter is designed to prevent the information-processing system from becoming overloaded. The inputs not initially selected by the filter remain briefly in the sensory buffer, and if they are not processed they decay rapidly. Broadbent assumed that the filter rejected the non-shadowed or unattended message at an early stage of processing.

Broadbent (1958) looked at air-traffic control type problems in a laboratory.

Broadbent wanted to see how people were able to focus their attention (selectively attend), and to do this he deliberately overloaded them with stimuli – they had too many signals, too much information to process at the same time. One of the ways Broadbent achieved this was by simultaneously sending one message (a 3-digit number) to a person’s right ear and a different message (a different 3-digit number) to their left ear. Participants were asked to listen to both messages at the same time and repeat what they heard. this is known as a ‘dichotic listening task’.

Right EarLeft Ear




  1. Order of presentation (ii) Ear by ear




In the example above the participant hears 3 digits in their right ear (7,5,6) and 3 digits in their left ear (4,8,3). Broadbent was interested in how these would be repeated back. Would the participant repeat the digits back in the order that they were heard (order of presentation), or repeat back what was heard in one ear followed by the other ear (ear-by-ear), He actually found that people made fewer mistakes repeating back ear by ear and would usually repeat back this way.


Results from this research led Broadbent to produce his ‘filter’ model of how selective attention operates. Broadbent concluded that we can pay attention to only one channel at a time – so his is a single channel model.

In the dichotic listening task each ear is a channel. We can listen either to the right ear (that’s one channel) or the left ear (that’s another channel). Broadbent also discovered that it is difficult to switch channels more than twice a second. So you can only pay attention to the message in one ear at a time – the message in the other ear is lost, though you may be able to repeat back a few items from the unattended ear. This could be explained by the short-term memory store which holds onto information in the unattended ear for a short time.

Broadbent thought that the filter, which selects one channel for attention, does this only on the basis of PHYSICAL CHARACTERISTICS of the information coming in: for example, which particular ear the information was coming to, or the type of voice. According to Broadbent the meaning of any of the messages is not taken into account at all by the filter. All SEMANTIC PROCESSING (processing the information to decode the meaning, in other words understand what is said) is carried out after the filter has selected the channel to pay attention to. So whatever message is sent to the unattended ear is not understood.


characteristics only.

Because we have only a limited capacity to process information, this filter is designed to prevent the information-processing system from becoming overloaded. The inputs not initially selected by the filter remain briefly in the sensory buffer store, and if they are not processed they decay rapidly. Broadbent assumed that the filter rejected the non-shadowed or unattended message at an early stage of processing.


(1) Broadbent’s dichotic listening experiments have been criticised because:

  • The early studies all used people who were unfamiliar with shadowing and so found it verydifficult and demanding. Eysenck & Keane (1990) claim that the inability of naive participants to shadow successfully is due to their unfamiliarity with the shadowing task rather than an inability of the attentional system.
  • Participants reported after the entire message had been played – it is possible that theunattended message is analysed thoroughly but participants forget.


  • Analysis of the unattended message might occur below the level of conscious awareness. Forexample, research by von Wright et al (1975) indicated analysis of the unattended message in a shadowing task. A word was first presented to participants with a mild electric shock. When the same word was later presented to the unattended channel, participants registered an increase in GSR (indicative of emotional arousal and analysis of the word in the unattended channel).

More recent research has indicated the above points are important: eg

Moray, N (1969) studied the effects of practice. Naive subjects could only detect 8% of digits appearing in either the shadowed or non-shadowed message, Moray (an experienced ‘shadower’) detected 67%.

  1. Broadbent’s theory predicts that hearing your name when you are not paying attention shouldbe impossible because unattended messages are filtered out before you process the meaning thus the model cannot account for the ‘Cocktail Party Phenomenon’.
  2. Other researchers have demonstrated the ‘cocktail party effect’ under experimental conditionsand have discovered occasions when information heard in the unattended ear ‘broke through’ to interfere with information participants are paying attention to in the other ear. For example, Gray & Wedderburn (1960) found that students could put material from both ears together so that it made sense. This implies some analysis of meaning of stimuli must have occurred prior to the selection of channels. In Broadbent’s model the filter is based solely on sensory analysis of the physical characteristics of the stimuli.


Gray & Wedderburn found that participants were able to give a category by category response

(ie one that made sense of the material heard, as shown in the diagram below). Broadbent’s Filter Model predicts this would not be possible. It is now certain that the unattended message can be processed far more thoroughly than was allowed for in Broadbent’s theory.

Ear by Ear

Jack 2 Jill

1 and 3


Selective attention requires that stimuli are filtered so that attention is directed. Broadbent’s model suggests that the selection of material to attend to (that is, the filtering) is made early, before semantic analysis. Treisman’s model retains this early filter which works on physical features of the message only. The crucial difference is that Treisman’s filter ATTENUATES rather than eliminates the unattended material. Attenuation is like turning down the volume so that if you have 4 sources of sound in one room (TV, radio, people talking, baby crying) you can turn down or attenuate 3 in order to attend to the fourth. The result is almost the same as turning them off, the unattended material appears lost. But, if a nonattended channel includes your name? for example, there is a chance you will hear it because the material is still there.



channels                                                             Inputs, including attenuated inputs are passed on for semantic analysis.

Treisman agreed with Broadbent that there was a bottleneck, but disagreed with the location. Treisman carried out experiments using the speech shadowing method. Typically, in this method participants are asked to simultaneously repeat aloud speech played into one ear (called the attended ear) whilst another message is spoken to the other ear.

In one shadowing experiment, identical messages were presented to two ears but with a slight delay between them. If this delay was too long, then participants did not notice that the same material was played to both ears. When the unattended message was ahead of the shadowed message by upto to 2 seconds, participants noticed the similarity. If it is assumed the unattended material is held in a temporary buffer store, then these results would indicate that the duration of material held in sensory buffer store is about 2 seconds.

In an experiment with bilingual participants, Treisman presented the attended message in English and the unattended message in a French translation. When the French version lagged only slightly behind the English version, participants could report that both messages had the same meaning. C1early, then, the unattended message was being processed for meaning and

Broadbent’s Filter Model, where the filter extracted on the basis of physical characteristics only, could not explain these findings. The evidence suggests that Broadbent’s Filter Model is not adequate, it does not allow for meaning being taken into account.

Treisman’s ATTENUATION THEORY, in which the unattended message is processed less thoroughly than the attended one, suggests processing of the unattended message is attenuated or reduced to a greater or lesser extent depending on the demands on the limited capacity processing system. Treisman suggested messages are processed in a systematic way, beginning with analysis of physical characteristics, sy11abic pattern, and individual words. After that, grammatical structure and meaning are processed. It will often happen that there is insufficient processing capacity to permit a full analysis of unattended stimuli. In that case, later analyses will be omitted. This theory neatly predicts that it will usually be the physical characteristics of unattended inputs which are remembered rather than their meaning. To be analysed, items have to reach a certain threshold of intensity All the attended/selected material will reach this threshold but only some of the attenuated items. Some items will retain a permanently reduced threshold, for example your own name or words/phrases like ‘help’ and ‘fire’. Other items will have a reduced threshold at a particular moment if they have some relevance to the main attended message.


  1. Treisman’s Model overcomes some of the problems associated with Broadbent’s Filter Model,

e.g. the Attenuation Model can account for the ‘Cocktail Party Syndrome’.

  1. Treisman’s model does not explain how exactly semantic analysis works.
  2. The nature of the attenuation process has never been precisely specified.
  3. A problem with all dichotic listening experiments is that you can never be sure that theparticipants have not actually switched attention to the so called unattended channel.


When does selectivity occur? Does it happen in the early stages of recognition – when constructing a description of the input – or during the later stages, when comparing the input’s descriptions to those of stored objects? The issue is important because it concerns whether we can selectively ignore something before we know what it means – EARLY SELECTION – or only after we know its meaning – L ATE SELECTION.

Broadbent and Treisman agree that selection of a single channel occurs at an early stage before recognition processes begin and so their models are called EARLY SELECTION MODELS.

An alternative view is that information from all channels is transmitted to the semantic analysis recognition stage and it is only after this that a selection is made. The general framework for a late selection theory of this kind was first proposed by Deutsch and Deutsch (1963) and was later elaborated by Norman (1968).


DEUTSCH AND DEUTSCH (1963) solved the problems posed by the Broadbent model in a different way to Treisman. Their model suggests that all inputs are subject to high level semantic analysis before a filter selects material for conscious attention. Selection is therefore later because it occurs after items have been recognised rather than before as in Broadbent’s model. Selection is also ‘top-down’ as opposed to Broadbent’s and Treisman’s Models which are known as ‘bottom-up’ in that an item which has relevance to you, your name for example, or is in context, is likely to be selected. Material is identified or recognised, its relevance, value and importance weighed and the most relevant is passed upwards for conscious attention.



DEUTSCH AND DEUTSCH (1963) proposed a more radical departure from Broadbent’s position in their claim that all inputs are fully analysed before any selection occurs. The bottleneck or filter is thus placed later in the information processing system, immediately before a response is made. Selection at that late stage is based on the relative importance of the inputs.



  1. Some support for a late selection model is offered by research which shows an unattendedmessage in a dichotic listening task can affect behaviour even though the listener has no conscious awareness of hearing the unattended message. For example, Moray (1969) paired an electric shock with a word over several trials so that the person became conditioned to produce a detectable change in GSR (Galvanic Skin Response) when the word was spoken. He found that several of his participants produced a change in GSR when the word occurred in an unattended message even though they were not aware of hearing it.
  2. McKay (1973) using ambiguous words like ‘bark’ instructed participants to shadow anambiguous sentence while, in the unattended ear a word was played which could clarify the meaning of the sentence. Later, participants who were quite unaware at a conscious level of the word in their unattended ear, chose meanings for the ambiguous sentence they had shadowed which were in line with the unattended word.
Right Ear (Attended and Shadowed Ear)

“the bark was not like anything she was familiar with”

  Left Ear (Unattended


Either (a) tree       or (b) dog.


  1. More recent studies have also shown that under some circumstances unattended material mayreceive some degree of analysis. For example, Wexler (1988) found that a GSR response varied not only according to ear of presentation but also according to the personality of the listener, indicating that processing of unattended material is more complicated than the early models suggest.


  1. However, the assumption made by Deutsch and Deutsch that all stimuli are analysedcompletely, but that most of the analysed information is lost immediately, seems rather uneconomical.
  2. The research to support the Deutsh and Deutsch Model can also be explained by Treisman’smodel. A word in the unattended ear could have a reduced threshold because of its relevance. Physiological evidence also supports Treisman. When a measure of brain-wave activity known as the evoked potential is recorded, it typically shows that the initial response to the unattended message is much weaker than the response to the attended message, suggesting attenuated processing of the unattended message.
  3. Treisman and Geffen (1967) asked participants to shadow one of two simultaneous messages,and at the same time monitor BOTH messages in order to detect target words. Detection was indicated by tapping. According to Treisman’s theory, detection on the unattended message should be less than the shadowed message, whereas Deutsch and Deutsch’s Model would predict no difference (as both messages would be fully analysed). As Treisman’s Model predicts, detection was significantly higher on the shadowed message.

Perhaps on grounds of economy and explanatory powers of the available experimental data, Treisman’s is the model most appropriate at present.


Researchers interested in attention have suggested a distinction between AUTOMATIC and CONTROLLED processing (Posner ~ Snyder, 197S). The basic idea is that some mental and physical processes are under an individual’s conscious control, while others tend to occur automatically, without conscious awareness or intention.

A frequently quoted example of this is learning to drive a car. When you first learn to drive, such things as steering, braking and changing gear, all require a great deal of concentration. Problems often arise for the learner driver when they are required to do two or more things at once eg brake and change down gear. Also, as any driving instructor will tell you, learner drivers can become so engrossed in such things as changing gear that they fail to attend to what is happening on the road in front of them! Yet to drive competently frequently requires a driver to do two or more things virtually simultaneously.

How does the transformation from learner to expert occur? The concepts of AUTOMATIC and CONTROLLED processing have been used to explain this transformation. The basic idea is simple – with practice, skills which initially required a considerable amount of attention become virtually automatic. The development of automatic processing has a major advantage in that it reduces the number and amount of things that we have to attend to consciously. Thus the scarce resource of conscious attention is released for other tasks.

However, psychologists such as Gleitman (1981) have pointed out that automatic processing can produce interference which actually lowers performance on certain tasks. A classic example of this is the STROOP EFFECT, named after JR Stroop (1935) who devised a colour naming experiment. The experiment involved participants naming colours as quickly as possible. In one condition participants named patches of colour, in a second condition participants had to name the ink colour in which words were printed, but the words themselves were colour names. For exarnple, participants would see the word BLUE but it would be written in red ink and their task would be to say RED’. Stroop found that participants were much slower at naming the ink colours when the stimuli were themselves colour words.

One explanation for the Stroop Effect is that we automatically process the meaning of words. Thus when a participant sees the word BLUE but is supposed to respond to the ink colour and say RED, the name of the word is automatically processed. This interferes with the participants’ ability to process and name the ink colour (RED), thus delaying their response. In particular it has been suggested that the Stroop effect produces a ‘mental race’ between the 2 processes involved in naming colours, the reading response wins the race and slows the colour naming. The difficulty experiences in naming the ink colour of the colour words is therefore the consequence of an overlearned skill, and cannot be brought under conscious control.


Can we do two things at once?


An obvious factor determining how well we can perform two tasks together is their level of difficulty. However, a task that is difficult for one person may be straightforward for another (eg when we first learn to drive). We should also consider the difficulty of each of the tasks separately.


While experienced drivers can converse and drive at the same time, learner drivers have difficulty doing the 2 tasks. Spelke et al l976 demonstrated the value of practice with 2 subjects (Diane and John) who were given approximately 90 hours of training on a variety of tasks. The students were first of all asked to read short stories for comprehension while writing down words at dictation. To begin with their reading speed and their handwriting during dictation both suffered substantially. After 30 hours of practice, however, their reading speed and comprehension had both improved up to the levels they displayed when not taking dictation and their handwriting was also better quality.


It may well be that the inability to report much about the non-shadowed message in the shadowing situation is due to the great similarity between the 2 inputs – both English prose passages presented in an auditory fashion.

Allport et al 1972 found when 2 shadowing tasks were dissimilar – for example, the standard shadowing task and the task of learning pictorial information – 90% of the pictures were recognised.

The extent to which 2 tasks can be performed successfully together seems to depend on a number of factors:

  1. Two dissimilar, highly practised and simple tasks can typically be performed well together, whereas
  2. two similar, novel and complicated tasks cannot.

Dual task experiments imply that some well-learnt skills are virtually automatic. Once a decision has been made to drive somewhere, the actual driving of the car goes into ‘autopilot’. And yet some form of unconscious monitoring of environmental requirements must be going on to enable us to deal with sudden emergencies.


Kahneman (1973) proposed that there is a certain amount of ATTENTIONAL CAPAClTY available which has to be allocated among the various demands made on it. On the capacity side, when someone is aroused and alert, they have more attentional resources available than when they are lethargic. On the demand side, the attention demanded by a particular activity is defined in terms of MENTAL EFFORT; the more skilled an individual the less mental effort is required, and so less attention needs to be allocated to that activity. If a person is both motivated (which increases attentional capacity) and skilled (which decreases the amount of attention needed), he or she will have some attentional capacity left over.

People can attend to more than one thing at a time as long as the total mental effort required does not exceed the total capacity available. In Kahneman’s model allocation of attentional resources depends on a CENTRAL ALLOCATION POLICY for dividing available attention between competing demands.

Once a task has become automatic it requires little mental effort and therefore we can attend to more than one automatic task at any one time, e.g. driving and talking.

Kahneman’s Capacity Model of Attention

  • Attention is a central dynamic process rather than the result of automatic filtering ofperceptual input.
  • Attention is largely top-down process as opposed to the Filter Models which suggest abottom-up process.
  • The focus of interest is the way the central allocation policy is operated so as to shareappropriate amounts of attention between skilled automatic tasks and more dimcult tasks which require a lot of mental effort.
  • Rather than a one-way flow of information from input through to responses, attentioninvolves constant perceptual evaluation of the demands required to produce appropriate responses.


  1. Cheng (1985) points out that when tasks have been learnt we change the way we process andorganise them, but this is not necessarily ‘automaticity’. For example, if asked to add ten two’s you could add 2 and 2 to make 4, add 4 and 2 to make 6, add 6 and 2 to make 8, add 8 and 2 to make 10 etc. Indeed young children when first learning arithmetic would do just this. When we have more arithmetical knowledge and realise that adding ten two’s is the same as multiplying 2 x 10, the solution can be produced in one step. The answer is quicker because we have processed the information differently, using different operations, not because we have added ten two’s ‘automatically’.
  2. A MAJOR PROBLEM with Kahneman’s theory is that is does not explain how the allocationsystem decides on policies for allocating attentional resources tasks.

The need for a homunculus to make decisions is a weakness of a psychological theory, as what makes the homunculus make decisions – another little person inside him(her) ????

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