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intro to psych lectures 7-9

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McGill University
PSYC 100
Daniel J Levitin

PSYC 100 INTRO TO PSYCHOLOGY Set #3 (Lectures 7, 8, 9) 1 Lecture 7 September 22 2009 Perception and mental representations: Today I want to talk about perception. At first, this issue of perception might begin with the question: why do we bother studying perception? Perception is the means by which we link to external world to our inner mental world. If it sounds jargon, its a bunch of language that comes from cognitive psychology. And it traces its origins back to William James and the idea is that there is this external world out there and we take it in with our senses. Lets set monist and dualist arguments apart and move forward. The idea is that, what were building up in our minds, in our brains, is some representation of that world. Were representing the external world, internally. These internal representations allow us to interact with the world. You expect that when you buy chocolate ice cream from Ben and Jerrys, its going to taste like it did last time. And so youve developed an internal representation for the taste of the ice cream and the sub-extent that relies on memory, but the idea is this mental representation allows you to detect whether something in the world is changed or not. If you buy an ice cream and if its a sour awful taste that you dont remember being there before, you might think that its spoiled. Its the mental representation that allows you to think that. We have mental representations about things like gravity. When you see something thrown, you expect it to fall eventually. We have mental representations about other people, what we expect them to do. It allows to recognize them either through their voice or through their face or their behaviors. A person could tell you a story about somebody at a party without naming them and you might be able to say Oh, that sounds like Sasha. Only she would do that. So youve got a mental representation of your friend. The fundamental idea here is perception is necessary for us to do mental representations. And that might not be obvious at first, but it is John Lock, the British philosopher, who stated that everything we know, we know through our senses. We cannot know anything else. His challenge was to try to imagine a smell that youve never smelled before. This is called Locks challenge. Lock argued that you cannot do this. We can imagine conjunction of things. So, if I ask you to imagine peanut butter and pickle ice cream, you can probably imagine it because you can imagine peanut butter and you can imagine pickles and you can imagine ice cream. And you might even have had peanut butter ice cream, so you can imagine that conjunction, but thats different than something completely new. If I ask you to imagine a flavor that youve never tasted, Lock predicts that youll be unable to do it and nobody has shown him to be wrong yet. This underlines the importance of our senses: they are the gateway to external world. Its true that you can have thoughts, desires and ideas that are entirely in your own mind. You can 2 imagine things that youve never done, but generally speaking these are based on real things that youve sensed with your body, real experiences which youve had. Another fundamental principle of perception that may not be obvious is that perception is constructive. Im going to try to persuade you into this because its not intuitive for most people. By constructive, what I mean is that if you look out across the world, it seems like the world is jut there and you simply see it. You see, interpret colors, shapes and it seems as though your eyes just deliberate it to you. This is an illusion. You can imagine seeing something when your eyes are closed. Those things feel as thought theyre projected on some kind of screen inside your brain. Thats also an illusion. There is special purpose processing devices in the brain that process things like the color of an object and they are in the separate locations in the brain. Weve seen patients whove suffered from damage to one system. You can take such patients and hold a green apple and a red apple and they could tell you that there is something red and there is something green, but they dont know where the red is and where the green is. Theyve got a disconnection between the color part and the location of shape parts of the brain. Some people lose color vision entirely. Some of the people lose the part of a brain, which processes color, location or movement. Your brain is using parallel processing to put together and it presents you world and all happens so quickly that you dont realize. But the brain really is constructing this real world for you. It does in each of the senses in slightly different ways. Wavelengths and vision: We tend to think that our senses give us an unbiased view of the world, but this isnt really true. In terms of the visual world in color, you could ask what color are light waves? It turns out they have no color. Newton understood this. Light waves present themselves to you with different frequencies. This is not obvious and it is counter intuitive, but this is the way it is. The light wave itself has no color. If you see a red apple, its because those light waves that are reflected off the surface of the apple are of particular neural wavelengths that your brain interprets as color. Similarly, when molecules are vibrating, theyre not actually making a sound. Its a chain of electro-chemical events that occur after these molecules in your eardrum and they cause you to feel a sense of itch and a sense of a timber of the instrument. Your brain is constructing all of these. If you want to be accurate about it, there are light waves of varying frequencies. There are sound waves of varying frequencies. But as soon as you talk about color or pitch, youre talking about things that are not in the physical world. Theyre entirely in the mental world and they are constructed by the brain. 3 The same is true with loudness. Something is only loud when you hear it. There is no such thing as loudness of the world. Loudness itself is in the brain. You recall Berkeleys question: if a tree falls in the forest, it doesnt make sound. It may excite the molecules, but if there isnt a brain there to interpret the vibration, there is no sound and theres no loudness. This is the threshold that was very carefully acquired from human subjects in an experiment looking at different wavelengths of light. I filled in the colors that are associated with those wavelengths just because you may not have memorized it. What you need to know for the midterm, which is that the 400 nanometers of wavelength is at the bluish violet end and the 700 nanometers is at the reddish end. And as the wavelengths get shorter, the colors move from red to orange to green to violet. These thresholds are very carefully obtained by human subjects who saw lights projected at a constant illumination in terms of how much energy the light has been putting up, measured with a photon-meter. It turns out, although the lights were emitting exactly the same energy, human perception was that they were not emitting the same energy. In fact the sensitivity curve shows you that people are more sensitive to green light than they are to red or violet light. What that means is that a green light with an equal luminance appears brighter than a red light or violet light. There are some odd things here. The wavelength as you can see is varying in a strictly linear fashion: the distance between 400 and 500 is the same with the distance between 500 and 600 on this chart that Ive made, but you dont see a continuous change. Its not a gradual change where each new nanometer gives you a slightly different color. Perception therefore is categorical: we see things in categories. Youll notice also some colors are represented across a larger wavelength than others. Yellow is rather narrow compared to green. The other thing we know is that when we ask people to tell us which colors are most similar, in general people say that red is more similar to violet than it is to yellow. Theres this weird effect where the things are at the opposite end seem more alike to each other than they do to things in the middle leading what we call a color circle. Your mind is imposing a kind of two dimensional structure on this one dimensional attribute of the world. The world presents you with one dimensional variation wavelength and your brain perceives it in two dimensions. We see the same thing in music. Lets repeat what were the biases of the visual world. Its linear, its not continuous, its categorical and its 2 dimensional in the mind whereas its reflecting only 1 dimensional change 4
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