PSYC 441 Cognitive Psychology II

Transcription

1 PSYC 441 Cognitive Psychology II Session 4 Background of Object Recognition Lecturer: Dr. Benjamin Amponsah, Dept., of Psychology, UG, Legon Contact Information: bamponsah@ug.edu.gh College of Education School of Continuing and Distance Education 2014/ /2017

2 Session Overview The section aims at helping you to understand the background of how we come to recognize objects and patterns. We shall look at the background of object recognition and examine two important processes of pattern recognition (bottom-up and top-down) processes. These processes help us to make meaning of things we perceive. Slide 2

3 Session Objectives At the end of the session, the student will be able to Understand the background of object recognition. Be able to explain the process of pattern recognition Bottom-up process Be able to explain the process of pattern recognition Top-down process Understand some theories of perception Slide 3

4 Session Outline The key topics to be covered in the session are as follows: Topic One: Background of Object Recognition Topic Two : Perception Topic Three: Pattern Recognition Bottom-up Process Topic Four: Top-down Process Slide 4

5 Reading List Ashcraft, M. H. (2013). Cognition (6 th edn.), London: Pearson Education Int. Eysenck, M. W. (2012). Fundamentals of Cognition (2ed). Psychology Press, Sussex. Galotti, K. M. (2017). Cognitive Psychology: In and out of the laboratory (6 th edn.). Belmont, CA: Wadsworth. Hunt, R. R. & Ellis, H. C. (2004). Fundamentals of Cognitive Psychology (7 th edn.), New York: McGraw-Hill. Willingham, D, B. (2013). Cognition: The thinking animal (International edn. NJ: Prentice-Hall). Slide 5

6 Topic One BACKGROUND OF OBJECT RECOGNITION Slide 6

7 Background of Object Recognition The subject of perception is central to cognitive psychology. It reflects the study of how the body and mind work together to explain our awareness of the external world. Important questions are: What is the nature of the internal mechanism that underlies and determines our experience? How do we determine whether objects are close or far? How do we know that objects are in motion or are stationary? How do we know that a particular combination of colour texture, shape, reflects for example an orange? Slide 7

8 What exactly do new borns perceive? Our senses are bombarded by rays of light, smells, sounds. How do we manage to determine all that? The broad subject matter of perception may be divided into: visual perception auditory perception haptic (touch) perception olfactory perception (smell) gustatory (taste) perception We will concentrate on Visual Perception but let us first define the concept stimulus since it is central to our discussion. Slide 8

9 Definition of Stimulus Definition of Stimulus Stimulus is a polysemous concept in psychology. However, several ideas converge on the proper usage of the concept. A stimulus must be characterized in physical terms a ghost cannot serve as a stimulus. A stimulus must fall within the range of receptivity it must reach a threshold where it is perceptible, its presence must be felt. A stimulus must evoke a response from a receptor. For example, the sensory receptors of the eye, ears, tongue react to the presence of the stimulus. Stimulus must stimulate some action or behavior from an organism. Slide 9

10 - A stimulus must be external to the organism itself or to some internal part of the organism. Thus, we can summarise in terms of definition by saying that: A stimulus is anything that has some impact or effect on an organism such that its behavior is modified in some detectable way. Slide 10

11 Topic Two PERCEPTION Slide 11

12 Perception The central problem of perception is to explain how we attach meaning to sensory information that we receive. The important question for cognitive psychology is how do we manage to accomplish these so rapidly without error? In visual perception, cognitive psychologists attempt to describe how we acquire visual information and what we do with it in terms of processing for meaning. Slide 12

13 The importance of Visual perception is captured by Zeki (1993), who states:.the problem of vision is the problem of knowledge, knowledge about the external world acquired through the sense of vision. For why do we need to see at all, except to acquire that knowledge. Slide 13

14 Let us try to understand how we perceive objects by illustration (visual perception). Distal stimuli, proximal stimuli and perception

15 Characteristics of the Retinal Image The retinal image is two dimensional. The size of the image depends on the distance between you and the object. (The closer you are to the object, the larger the image). The image is turned upside-down and finally, the positions are reversed with respect to left and right. The retinal image is different from the real object. How do we make meaning then? The meaningful interpretation of the proximal stimulus (retinal image) is the percept (psychological process). Slide 15

16 The point made here is that the proximal stimulus (or the retinal image) is different from the percept which is experientially determined. Psychological processes are therefore needed to enable us derive meaning. Percept an impression of an object obtained by use of the senses: e,g., sense datum - impression of something that corresponds to reality (percept here is output). Perception the organization, identification and interpretation of sensory information in order to represent and understand the environment (the process). Slide 16

17 So the question now is: How does the proximal stimulus become a percept? We are going to address this through discussion of the concept of Pattern Recognition. Slide 17

18 Topic Three PATTERN RECOGNITION: BOTTOM-UP PROCESS Slide 18

19 Pattern Recognition Pattern Recognition is the process by which meaning is derived based on classification of a stimulus into a category. It is a process which interprets sensory information by matching that information to previous experiences stored in the long-term memory. Theories about how we identify patterns fall into two broad categories of 1. Bottom-up 2. Top-down perceptual theories. Slide 19

20 Bottom-up Theories Bottom-up or data driven essentially means that perceivers start with small bits of information from the environment and combine to form a percept. Theories under bottom-up processes assume that we work from the bottom up with some inputs and that through manipulation in various ways, yields a more richly or meaningful interpretation of the object or event. When psychologists speak of bottom-up processes, they have in mind information about a stimulus (from the lower level of processing) as input in the recognition process. Bottom-up processes are relatively not influenced by previous learning or expectations. Slide 20

21 Examples of Bottom-up Theories Three examples of bottom-up models of perception Template matching Featural analysis Prototype matching 1. Template matching We assume a miniature copy or template stored in the long-term memory, which corresponds to each visual pattern we know. Every object, event or stimulus we encounter and want to derive meaning from is compared to some previously stored pattern or template in our LTM. The model implies that we have already stored millions of different templates - one for every distinct object or pattern we can recognize. Slide 21

22 Template matching simply is the process of searching for a match between a new stimulus and what we know. Numbers on a cheque, credit cards may be referred to as templates, file names on the computer and so on. The ATM machines read the numbers and compare them to previously stored patterns.

23 Criticisms Template matching cannot completely explain how perception works based on the following reasons. It is impossible to store almost endless number of templates in the mind. We do not understand how templates are created in the first place in our long-term memory. In terms of sound the theory doesn t adequately explain how we perceive effectively noisy and faint patterns. Does it mean that we have templates for deep, thick and faint objects? Slide 23

24 2. Featural Analysis A pattern consists of a set of specific features or attributes. E.g, a face has different features including a nose, eyes, mouth, chin etc. The process of pattern recognition is assumed to begin with the extraction of the features from the presented visual stimulus. This is through 4 processes : Detection, Pattern Dissection, Feature comparison in memory and Recognition. Slide 24

25 The set of features is then combined, and compared against information stored in memory. Difficulties Though stimulus features play important role in pattern recognition, there are several difficulties. 1. Featural theory de-emphasizes the effects of context. 2. Recognition of Patterns may not necessarily depend on listing features of a stimulus. For example, \ / may be represented as A or other formats. Slide 25

26 3. Prototype Matching The model attempts to correct some of the weaknesses in both template and featural theories. In prototype matching, we match an input to a stored representation as in template (prototype), but not in complete whole patterns. We consider prototype as an ideal representation of the stimulus it represents. We know a prototypical dog, car, house, aeroplane etc. We can also recognize those objects that move far away from the prototypes. Slide 26

27 In perception, a sensory modality (ear, eye etc) registers a new stimulus and compares it with previously stored prototypes. An approximation close to the prototype to recognize the object. With the prototype, the more features a particular object shares with a prototype, the higher the probability of a match. Slide 27

28 The Shortcomings of the Model: How and when do we form and use the prototypes. What type of processing is involved in the matching process? These same difficulties are encountered by the template matching models. Slide 28

29 Topic Four: Top-down Theories PATTERN RECOGNITION: TOP-DOWN PROCESS THEORIES Slide 29

30 Top-Down Processes Top-down or Conceptually-driven processing refers to the use of contextual information in pattern recognition. In other words, perception of the world is guided by the perceivers expectations, concepts, experience, context, ideas, theories, beliefs and assumptions we have already. Example, understanding difficult handwriting becomes easier when you are reading them in complete sentences than when reading single isolated words. The reason is that meanings of the surrounding words provide a context to guide your understanding. Slide 30

31 Three of the biggest problems facing bottom-up processes were context, expectation effects and binding. Let s see the effect of the two concepts.

32 Top-down effects in pattern recognition. In A, we read one letter as H another as A though they are the same. In B, the letter B and 13 are identical.

33 Top-Down Theories You would agree that the successful interpretation of the above figure is a result of context and our experience. Four Examples under Top-down Processing 1. Perceptual Learning The principle that perception changes with practice and experience. What we have learnt previously guides our future perception and even enables us to make a distinction between stimuli. Slide 33

34 Example Experienced wine tasters will tell you that you need a lot of practice to taste subtle differences among wine. Novice tasters distinguish by taste (e.g., red wine, white wine or fruity and dry wines). By contrast, Experts may be able to identify the vineyard that bottled the wine, the country of origin and the production year. Slide 34

35 Though their taste buds are the same, the difference is practice and experience. Think about identification of identical twins. We simply overcome that difficulty through experience. Perceiver s experience appears to help guide what aspects of the stimulus they should focus on. Slide 35

36 2. Change Blindness It represents our inability to detect changes to an object or scene, especially when given different views of the object. An example of a film strip (motion pictures). When we look at a film strip, about 20 or more scenes might be seen as same without noticing the difference among them. The oversight reflects the idea that perception is driven expectations about meaning and we fail to keep track of every detail. The preoccupation of meaning has a survival value, it prevents visual overload. 3. Word Superiority Effect Word superiority or word advantage effect is one of the most widely demonstrated phenomena in research on recognition. Slide 36

37 We identify a single letter more accurately and more rapidly when it appears in a word than when it appears alone by itself or in a string of unrelated letters. Reicher (1969) showed that recognition accurately was significantly higher when a letter appeared in a word, such as work, than in a non-word, such as orwk. The letter s is quickly recognized in the word island, even though the s is not pronounced. An explanation for the word superiority effect is that top-down and bottom-up processing interact the letter may activate the word or the word may activate the letters. 4. Connectionist Model of Word Perception This model is from the connectionist perspective that offers explanation of word perception based on context and experience. Slide 37

38 In solving a crossword puzzle, when you see a puzzle like RAP whatever letter you fill in will depend on your experience and the context in which the puzzle is spelt out, which will also depend on activation of several candidate information from your memory. Slide 38

39 Distinction between Bottom-up and Top-down Processes Distinction between Bottom-up and Top-down Processes Bottom-up processing stresses the importance of the stimulus in pattern recognition. Information starts from the most basic or bottom level, and it works its way up until it reaches the more sophisticated cognitive processes. The combination of simple, bottom-level features allows us to recognize more complex, whole objects. Top-down processing emphasizes how a person s concepts and higher-level mental processes influence recognition. That is, our concepts, expectations, experience help in the identification of objects. Slide 39

40 Cognitive psychologists propose that both bottom-up and top-down processing are necessary to explain the complexities of object recognition. It is difficult if not impossible to ask perceivers whether they first interpret the whole or the parts. This is because they all happen at the same time. Example, you recognize a tea cup because of two simultaneous processes: (a) bottom-up processing forces you to register the component features, such as the curve, handle, size etc; (b) the context of coffee (top-down) should encourage you to recognize the handle of a cup quickly because of top-down processing. Slide 40

41 Sample Questions Compare and contrast bottom-up and top-down perceptual approaches to the study of object recognition. How will you describe perception to a friend who has never had a cognitive psychology course? Slide 41

42 THANK YOU Slide 42