What do you do? If it were not for your top-down processing, you would probably have to get out of the car and peer around the corner to check that the road does, in fact, continue and that there are no other unexpected occurrences such as that in Figure 2. An example such as this, while superficially rather silly, emphasises just how much we rely on what we know to influence almost everything that we do. It is now worth defining what we mean by sensation and perception.
The individual may be consciously aware of the perception arising from incoming sensory information, or they may not subliminal perception. Sensation and perception thus lie at opposite ends of the visual process and may well be quite different.
It is beyond the scope of this chapter to discuss subliminal perception, so the following discussion will focus on perception as a conscious awareness of the output of the visual system. It is possible that not all the information that is sensed will reach perception at all.
Some of the sensory information entering the visual system may be filtered out by attentional processes discussed briefly in Chapter 1, and in detail in Chapter 3 and will not form a part of our perception. This chapter has also considered that what we already know may influence what we perceive and so perception may represent not only a filtered, but also a modified, version of the original sensation. A simple diagram of the route from sensation to perception is presented in Figure 2.
Different parts of the process may also interact; what we know may influence the way that attentional filtering operates, as well as our final perception of the sensory information. It should thus be noted that the process described above is a very simplified version of what appears to be happening in the processing of visual input. Kant argued that we can never truly access the numena, only the Attention phenomena. That is, we can never know the world as it truly is, only our perception of it after it has been filtered and modified by our senses and cognitive processes.
Raw sensory but as we are. Note that information is seen as flowing top-down as well as bottom-up at all stages. Generally, those individuals who are most likely to have an Key Term accident while driving lie at the ends of the age range for drivers Numena Claret et al.
See also between have a lower risk of being involved in an accident. Numena as we perceive them. This is where the distinction between sensation and perception becomes particularly important. Undoubtedly, some accidents are due to a failure of sensation. Sensing an object, however, involves more than just detecting the light coming from it. Even if light from Figure 2.
Source: Photograph courtesy of Gloucestershire Constabulary. Earlier in the chapter the Gestalt approach emphasised the importance of picking an object out from its background, and Marr provided a possible process by which this could be done.
But what if it is difficult, or impossible, to discriminate an object from its background? The and the light from the background, may pedestrian on the left of the picture is silhouetted against be too similar to tell one from the other. The pedestrian on the right presents against a from the background see Figure 2. The term used to describe how easily an object can be detected by the senses is sensory conspicuity, and Key Term refers to the intrinsic properties of an object such as shape, colour, brightness, amount of noise that it is making that are likely to be Sensory conspicuity registered by the senses — usually as a result of increasing contrast with The extent to which the background.
Thus, a pedestrian can often increase their sensory aspects of a stimulus conspicuity by carrying a torch, or wearing reflective material. Cole and Hughes suggest that in order the senses. The discussion of attention above, and that The interaction of in Chapter 3, suggests that much information that we sense is not aspects of a stimulus attended to.
There is an interesting distinction that can be made here such as colour, between sensory conspicuity and attention conspicuity. As mentioned luminance, form above, sensory conspicuity relates mainly to aspects of the object being with aspects of an perceived brightness, etc.
Broadly speaking, sensory pre-conceptions that conspicuity relies primarily on bottom-up processing whereas attention determine how likely conspicuity is more heavily influenced by top-down processes. At most a stimulus is to be road junctions, for example, the class of road user that a driver is most consciously perceived. Thus if a driver is not expecting a motorcyclist at a particular location they may drive into them, even if the motorcyclist has high sensory conspicuity.
It is this class of accident that may be referred to as LBFS. In accidents of this kind the object that is hit may be of very high sensory conspicuity, and it appears almost certain that the driver would have looked in the general direction of the object and, at a sensory level, detected it. Is there any evidence that accidents of this kind occur?
Martin Langham and his co-workers Langham et al. Despite having a full range of conspicuity enhancers reflective and retro-reflective materials, flashing lights, cones stationary police cars have been hit by drivers who subsequently claimed that they did not see them. The police car, while having high sensory conspicuity, has low attention conspicuity for those individuals.
Langham et al. To do this, they gathered survey data from a variety of sources, obtaining details of 29 vehicle accidents. This survey identified a number of interesting aspects of LBFS accidents that will now be considered in relation to the discussion above on the role of knowledge in perception. The factors that seemed to be important in LBFS accidents were: 1. It is easy to speculate that the orientation of the car may influence the perception of that car. Experience tells us that most cars that we see on a road have the same orientation as the other cars and, crucially, that they are moving.
Thus a car parked in-line may well be perceived as a moving car — until it is too late. There is much less ambiguity with a car that is parked in echelon; it is an unusual or even impossible!
Deployment of warning signs and cones did not guarantee detection. The deployment of such aids would almost certainly raise the sensory conspicuity of the police car still further, but not enough to prevent an accident. Again, it is possible that this finding is the result of experience. Drivers are familiar with the environment and the roads around their home.
They may have driven the same route every day for years — and never seen a police car parked in the road. Thus, when they do see a stationary police car, they assume it is moving, with disastrous consequences. The offenders were all, except one, over the age of As discussed above, this is highly unusual as it is usually the younger drivers that are more likely to be involved in accidents.
The finding emphasises the role that knowledge and experience are likely to play in these accidents. More experienced drivers have learnt that cars on the motorway are nearly always moving, and so do not pay sufficient attention to cars that are not moving.
It seems highly likely that they detect them, but they do not perceive them appropriately. It would thus appear that one explanation for LBFS accidents is that more experienced drivers are placing more reliance on what they already know and this is affecting what they perceive or do not perceive. Edgar et al. The pilot apparently believed that he was attacking enemy forces that were nearby.
Note the clear markings on the side of the vehicle and the top of the vehicle was also marked. To try and drive excuse the pun the message home and to allow you to experience a clear example of knowledge influencing perception, have a look at Figure 2.
What do you see? It is most interesting if you just see a pattern of light and dark shapes. Does it change your perception of the picture if you are told that it is, in fact, a picture of a cat? If you could not see the cat initially but now can after being provided with extra information about the picture, then this is a clear example of knowledge influencing perception.
If you still cannot see the cat, have a look at the 2. The sensory aspects of the original picture have not changed at all. It is still a collection of light and dark blobs. What has changed is what you know about the picture, and this has changed your perception of it. Even if you did see the cat immediately, your perception of it will be forever changed hopefully by knowing that it has been used as an illustration in a textbook.
The next issue to consider is just why we make so much use of stored Figure 2. Given that the consequences of using what we Answer at end of chapter. So why does stored knowledge appear to have such an influence on perception? One of the theories of the way in which perception operates and which deals explicitly with why we make so much use of stored knowledge is the constructivist theory which was initially proposed by Irvin Rock , and Richard Gregory Thus we use what we already know to fill in the gaps and interpret the sensory information Constructivist coming in.
The cat picture used previously can the world from be used again to give an idea of how this works. The picture is not incomplete at all clear and may be difficult, at first, to resolve into anything that sensory input.
See makes sense. Once the hypothesis fits the constructivist the sensory information, the image is then recognised, hopefully approach, in which correctly. The interaction of object are tested the two determines what is perceived. Source: Photographs courtesy of Graham Edgar. Gregory, in particular, has demonstrated that we can perhaps learn as much about the perceptual processes when things go wrong as when they go right. Once again, when things go wrong, it seems to be previous knowledge that is to blame.
Gregory uses a nice demonstration that illustrates this point Gregory, , Look at the faces in Figure 2. Under certain viewing conditions, whichever view of the mask we take, it still looks like a solid face — not a hollow face. Some evidence for this perception being based on acquired knowledge is provided by studies Tsuruhara et al. It only seems to lead to trouble, so why not ignore what we already know? Apart from the obvious answer that we would not even be able to recognise our own grandmother, there are other reasons for involving knowledge in the process of perception.
We have already touched on a possible answer when we first considered the constructivist theory. The incoming information is limited in two ways. One way has been considered in Chapter 1 and will be covered in much more detail in Chapter 3 and this is that our cognitive resources can only cope with a certain amount of incoming information, so that a proportion of it is filtered out by our attentional 2.
Another factor limiting the completeness of the incoming sensory information as already mentioned is the fact that our senses may not provide a full picture in the first place. This is illustrated by the lower part of Figure 2. Our visual acuity is not constant across our field of view and the scene in the upper picture has been progressively blurred in the lower picture to represent the effect of the reducing acuity of the eye with increasing distance from the high-acuity centre the fovea.
What this means is that much of the visual information coming in is actually of quite poor quality. Thus the constructivist theory seems to be making a reasonable assumption in proposing that we need to use prior knowledge to help us to interpret the rather blurry image that we receive from our retina.
Photographic process of generating hypotheses and manipulation by David Brookes. This is not a problem if the incoming sensory information remains constant or if the observer is, at least, continuing to look at different bits of the same object , but what happens if it changes?
The hypothesis testing constructivist approach would predict that a sudden change in the visual input would disrupt processing Key Term and make it more difficult to recognise an object, and this appears to be exactly what happens. Di Lollo et al. A typical target procedure of following stimulus and mask are shown in Figure 2. In a masking paradigm, a briefly presented a second stimulus can prevent recognition of an earlier stimulus if the stimulus by random mask follows very soon after presentation of the stimulus.
It is not visual noise or even necessary for the stimulus and mask to be at the same position fragments of other in the visual field i. A mask that surrounds stimuli. Interferes with the stimulus as in Figure 2. This term is used to describe the finding in neuroscience research that communication between different areas of the brain is never in one direction only. If a signal goes from one area to another, then there is Target Mask sure to be one coming back the other way Felleman and Van Essen, Thus the flow Figure 2.
Indeed, masking could be conceptualised as drawing attention away from the initial target stimulus so that cognitive resources are no longer allocated to processing it. Key Term Certainly, masking provides support for the constructivist approach. Hupe Re-entrant et al. Incoming sensory information Information flow flowing bottom-up is used to generate an initial hypothesis.
The accuracy between brain regions of this hypothesis is then checked against the continuing sensory input bidirectional. The constructivist theory of vision is thus very appealing, elegantly combining bottom-up and top-down processing. One slight puzzle remains, however. If this approach is so good, why does it make so many mistakes?
They are, thankfully, quite rare. One criticism of the constructivist approach is that previous knowledge appears to be so important due to the kinds of methods and stimuli used to test perception. Many investigations of perception are done in the laboratory using deliberately simple, and often static, stimuli. Thus, the sensory input is a very impoverished version of what an individual would normally be exposed to in the real world.
Some of the stimuli used to illustrate the use of knowledge are even deliberately difficult to recognise, such as the cat picture in Figure 2.
Thus, it could be argued that if you ask people to view a static two-dimensional, impoverished, image under laboratory conditions, you will force them to use knowledge to try and make sense of it. If you allow people to move around in the world, with a rich flow of sensory information coming in and changing as the individual moves and looks around and as objects in the world move around 2.
Thus the lower picture in Figure 2. Gibson , Gibson, rather than considering how perception operates, was much more concerned with what perception is for. That is, Gibson proposed that perception should be considered in terms of how it allows us to interact with the world we live in. In the theories of Gibson there is a strong link between perception and action with perception being referred to as direct.
The basis of direct perception is that the sensory information available in the environment is so rich that it provides sufficient information to allow a person to move around, and interact with, the environment without the need for any top-down processing.
Gibson would claim that the results obtained in laboratory studies are misleading in that they are studying indirect perception of static 2-D representations of the world. That is, laboratory studies and visual illusions do not demonstrate how we interact with the world, merely how we react to impoverished representations of it. For Gibson, moving within the environment and interacting with the environment are crucial aspects of perception.
One problem, of course, with denying the use of stored knowledge in perception is that it becomes rather difficult to work out how we can interact with objects in the world without recognising them in the way that would be proposed by the constructivist approach.
Gibson developed his theories by suggesting that we are able to interact with objects in the world because they afford their use. For instance, consider Figure 2. It is a picture of a hammer, and Gibson would suggest that a hammer would afford hitting things. If you think this is an unreasonable assumption, try giving a hammer to a two-year-old who has never seen a hammer before, and see what they do with it.
Actually do not try that, although it would almost certainly be a good example of an object affording its use. Key Term Direct perception Perception without the need for top-down processing. Source: Photograph courtesy of Graham Edgar. Handy et al. One of the pictures was of a tool and the other was of a non-tool. Objects that could be grasped such as hammers drew attention and functional magnetic resonance imaging fMRI of brain function indicated activity in dorsal regions of premotor and prefrontal cortices.
Also, as suggested by Bruce et al. All it needs to do is to sense the small flying object and use that sensory information to guide its tongue to allow it to snare it although it could be a nasty surprise if it is not a fly. Thus the constructivist and Gibsonian theories seem to conflict, one emphasising the centrality of stored knowledge in the perceptual process, the other denying that it is necessary at all.
The question, of course, is which one is right? Well, it is not giving too much away to say that it looks as though both theories could be right and that both types of processing could be occurring in perception. To illustrate this, we shall have a look at the structure of the visual system. Dorsal stream A pathway which carries visual information about the spatial location of an object. Even very early in the visual system there appear to be at least two distinct streams of information flowing back from the retina Shapley, These streams are referred to as the parvocellular and magnocellular pathways e.
Shapley, , the names deriving from the relative sizes of the cells in the two pathways. These pathways carry information back to the primary visual cortex. After the visual cortex, the visual information is still maintained in at least two distinct streams see Figure 2.
One stream is termed the ventral stream and leads to inferotemporal cortex and the other, leading to parietal cortex, is known as the dorsal stream. For instance: 1. The ventral stream is primarily concerned with recognition and identification of visual input whereas the dorsal stream provides information to drive visually guided behaviour such as pointing, grasping, etc. Ungerleider and Mishkin, ; Goodale and Milner, Source: Drawing courtesy of David Groome.
The ventral system is better at processing fine detail Baizer et al. The ventral system appears to be knowledge-based using stored representations to recognise objects whilst the dorsal system appears to have only very short-term storage available Milner and Goodale, ; Bridgeman et al. The dorsal system is faster Bullier and Nowak, We appear to be more conscious of ventral stream functioning than dorsal.
For instance individuals may report awareness of ventral processing, while manifesting different dorsal processing. The perception is illusory, but the action e. This difference in dorsal and ventral processing will be discussed in more detail below. The ventral system aims to recognise and identify objects and is thus object-centred.
The dorsal system is driving some action in relation to an object and thus uses a viewer-centred frame of reference Goodale and Milner, ; Milner and Goodale, Norman , , following on from similar suggestions by Bridgeman and Neisser , has suggested a dual-process approach based on the characteristics of the two streams outlined above. In this approach, it is suggested Goodale and Milner, that the dorsal and ventral streams act synergistically, with the dorsal stream largely concerned with perception for action, and the ventral stream with perception for recognition.
The function and characteristics of the two streams thus seem to fit rather nicely with the two theories of perception outlined above, with the dorsal stream appearing rather Gibsonian in the way that it operates, and the ventral stream rather constructivist. Thus, we appear to have a fast system ideally suited for driving action, but which makes relatively little use of stored information the Gibsonian dorsal stream combined with another slower system that uses stored knowledge to analyse fine detail and recognise objects the constructivist ventral stream.
The study used a hollow face like the one in Figure 2. Of course, just because the two streams appear to process the hollow face independently, it does not mean that the streams do not interact.
It is interesting to speculate, as we finish this discussion of visual perception, just how these two types of processing may act together to allow us to perceive our world. To do this, it is worth considering our experience and consciousness of what we are perceiving, i. The founder of the phenomenological tradition was a German philosopher-mathematician called Edmund Husserl who suggested the concept of intentionality, whereby the mind reaches out to the stimuli that make up the world and interprets them in terms of our own personal experience, which 2.
As an example of this, consider once more the pictures in Figure 2. At any one moment the sensory information coming in from the world gives us a view of the world rather like that in the lower picture. Our phenomenological experience of the world, however, is more like that of the upper picture. We have the impression that we have a clear and accurate perception of the world surrounding us at any one time.
An analogy for this is the light in your refrigerator Thomas, It always appears to be on because whenever you go to the fridge and open the door, the light is on the irreverent magazine Viz once suggested that it would be a good idea to drill a hole in the door of your refrigerator so you can really be sure that the light does go off when you close the door!
The experience of the real world is much the same. Whenever you look at any object in the real world it appears clear and sharp assuming your eyesight is good because as soon as you become interested in some part of the visual world, you tend to move your eyes so that the image of that part falls on the high-acuity central region of the retina. Thus, you tend not to be aware that the rest of the time that part of the world is just a blur in the same way that you never see the refrigerator light off.
Stored knowledge allows us to maintain this phenomenological percept that the world is sharp and clear. Having looked at something, we can remember it as sharp and clear, even when we look away and the sensory information coming from that information is actually blurred. The constructivist ventral stream would seem to be ideal for building up, and maintaining, our representation of the world, recognising objects as they appear in central vision and generating stored representations of those objects for when we are looking elsewhere.
As long as everything remains unchanged, our perception of the world should be fairly accurate. To maintain that accuracy, however, we need a system that will warn us if some part of the visual world changes. This is one of the functions that the dorsal stream could serve. Key features: A new chapter on Emotion and Cognition, written by Michael Eysenck, the leading authority in the field Completely revised and updated throughout to provide a comprehensive overview of current thinking in the field Accessibly written and including new authors, including Sophie Scott, Tom Manly, Hayley Ness, and Elizabeth Styles, all established experts in their field.
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