1 28 September, 2000HKU What rules are used to group objects? Definitions (Feature Lists) Family Resemblance Similarity to Prototypes Exemplar models

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4 28 September, 2000HKU Definitions and Features Dogs are animals that have four legs, have fur, bark, wag their tails…

5 28 September, 2000HKU Family Resemblance Ludwig Wittgenstein (1953) proposed that games could not be defined or categorized by features. Rather, any game shares some resemblance to some (but not all) other games.

6 28 September, 2000HKU Similarity to Prototypes Rosch (1978). Prototype is a central, average, representation (real or constructed) of a category. Tokens sufficiently similar to the prototype are considered members of that category. Memory for specific exemplars.

7 28 September, 2000HKU Exemplar Models To remember a category, just remember all the members of the category. Head-filling-up problem. Evidence for abstractions.

8 28 September, 2000HKU Perception Top-down Or Bottom-up?

9 28 September, 2000HKU The problem of perception How does our perceptual system arrive at a “best” representation of the world? Conflicting demands: –Accuracy –Efficiency

10 28 September, 2000HKU Data-driven processing (bottom-up)

11 28 September, 2000HKU Standard (naïve) assumption is that perception is entirely data-driven Why is this naïve? –Perception must be accomplished in sub-optimal conditions Need for speed Need for computational efficiency But also need for accuracy! –A common solution: “Best guess”

12 28 September, 2000HKU Concealed and bi-stable figures: Evidence for a role of knowledge in perception

13 28 September, 2000HKU Application of knowledge changes the way concealed and bi-stable figures are perceived Changes conscious processing (interpretation, object recognition) Changes unconscious processes (patterns of eye movements)

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16 28 September, 2000HKU Application of Knowledge Can be automatic –Either innately, or once learned Can also be controlled

17 28 September, 2000HKU Rabbit/Duck Jastrow (1900)

18 28 September, 2000HKU Impossible Trident

19 28 September, 2000HKU Illusions: Obligatory application of knowledge in contradiction to available data.

20 28 September, 2000HKU Application of knowledge is obligatory in these illusions

21 28 September, 2000HKU Sources of Illusions Illusions arise because our perceptual system is structured to act as if certain assumptions about the world are true. We can construct situations in which those assumptions appear to be true, but in fact are not. These situations are called illusions.

22 28 September, 2000HKU Sources of Illusions Knowledge of visual cues to distance –The Müller Lyer Illusion Filling in missing information –Object occlusion Using context –Circles of relative size –Text tricks –Shades of white –Circularity in pitch perception

23 28 September, 2000HKU The Müller-Lyer Illusion Which vertical line is longer? Neither! They are the same length! Müller-Lyer (1889)

24 28 September, 2000HKU Explanation We need to be able to adjust our knowledge of an object’s size based on our perception of how far away from us it is. The arms of the arrows make the vertical lines look like the edges of corners – one pointing toward us, one pointing away. That makes it appear as if the vertex of the corner (the vertical line) is a different distance from us in each figure.

25 28 September, 2000HKU What triangle?

26 28 September, 2000HKU Explanation Most things in the world do not have abrupt, regular discontinuities. It makes more sense to assume that the black spots are circles rather than Pac Men. It also makes sense, then, to assume that the 3 “bites” are caused by the same object which lies in front of the circles. “Makes sense” means is more likely to be a correct representation of the world.

27 28 September, 2000HKU Contextual cues Which of the two middle circles is smaller?

28 28 September, 2000HKU Is this a letter or a number?

29 28 September, 2000HKU Circularity in pitch perception Waterfall (M. C. Escher) Mac/AIFF PC/.wav

31 28 September, 2000HKU Bibliography Sheperd, R. N. (1964). Circularity in judgements of relative pitch. Journal of the Acoustical Society of America, 36, Rosch, E. (1978). Principles of categorization. In Rosch, E. & Lloyd, B. (Eds.) Cognition and Categorization. Hillsdale, NJ, Lawrence Erlbaum Associates. Wittgenstein, L. (1953). Philosophical Investigations. New York, Macmillan. Note: The illusions used in this demonstration come from various sources. When the original source is identifiable, it is given above. Otherwise, as much information as possible is provided to identify the illusion’s original author. Many illusions are so widely distributed that they appear to be in the public domain. The following texts and other sources are recommended: Deutsch, D. (1975). Musical Illusions. Scientific American, 233(4), Ernst, B. (1976). The Magic Mirror of M. C. Escher. New York, Ballantine. Ernst, B. (1992). Optical Illusions. Trans. K. Williams. Originally published as Het Begoochelde Oog. Taschen Verlag. Held, R. (1974). Image, Object, and Illusion: Readings from Scientific American. Scientific American, Inc. Houtsma, A. J. M., Rossing, T. D., Wagenaars, W. M. (1987). Auditory Demonstrations (CD). Eindhoven, The Netherlands, Institute for Perception Research (IPO) and the Acoustical Society of America (ASA).