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Perception (PSY)

by Tarry Ahuja, MD
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    00:01 Okay. So now, we’re going to get into the topic of perception.

    00:05 How do you perceive things? And this is a process that I think a lot of us just take for granted, right? We don’t ever think about, “How am I going to perceive this image?” Or “How am I going to perceive the situation?” It kind of just happens and you just deal with it.

    00:18 Let’s start with an image.

    00:20 So take a look at this image.

    00:21 What do you see? Take a minute, think about it.

    00:25 And you’re reaction might be, “Oh, I see.” So you might see two triangles opposed to one another with one on top of the other, right? How are you seeing that? Because in reality what’s drawn there are a couple of broken lines and a couple of green Pac-Mans which aren’t connected to anything.

    00:45 But you’re still seeing two triangles.

    00:48 So you’re perceiving a complete picture you’ve completed in your mind.

    00:53 How does that happen? Look at this one.

    00:55 What are you seeing here? Take a minute.

    00:58 Get up close to your computer screen.

    01:00 Get up close to your computer screen.

    01:02 Now, there's two possibilities here.

    01:05 You have two Taylor Swift looking ladies on either end with a candelabra in the middle.

    01:12 Or do you see two people? Let’s go with a guy and a girl. Maybe it’s two girls. Who knows? About to kiss in the middle. Okay? So what do you see? Again, you could see either, you could see both.

    01:25 And if I’m directing your attention to one versus the other, you might see it that way but you’re perceiving a picture based on broken pieces of information.

    01:35 So theoretically, on paper, this shouldn’t make sense.

    01:38 How am I seeing a few odd things and realizing that that’s a woman’s face? How am I looking and seeing two girls kissing when really that’s not what was drawn there.

    01:47 Okay? And this comes back down to perception.

    01:50 So what we’re going to do is we’re going to look at two ways we can process that information.

    01:55 And the first one is going to look at the information coming in.

    01:58 So what are we sensing or sensation? So the info that’s coming in, we’re going to take that and piece it together and come up with a concept.

    02:07 So let’s take a look at these two diagrams here.

    02:09 We have what looks like the letter A, one kind of broken apart.

    02:13 We have what looks like, you know, some lines here.

    02:16 What is that? I don’t know.

    02:17 So using bottom-up processing, we’re saying we’re taking the sensory information or information that’s coming in through our senses, in this case, our eyes, from the different receptors and we’re going to take all those bits and pieces of sensory information, integrate that at the brain, in the CNS, which stands for central nervous system, and come up with an idea of what we think it is.

    02:38 Okay? So this first one on the right, the one on the right that you’re seeing here, we see these lines, we’re saying, “That could be, I don’t know, train tracks.” And, you know, what is that? I don’t know.

    02:51 How about this one right here? Here’s an example of you taking your perception and coming up with an answer.

    02:57 So, what’s around us? This now might become a train track.

    03:02 And what you’ve done here is you’ve added context.

    03:05 You’ve added some surrounding information to now change your perception of what I’m seeing, okay? So what was a couple of lines now might be train tracks, it might be a ladder that allows you to climb the tree. I don’t know.

    03:17 So another way of looking at it is top-down processing.

    03:20 And that’s where we’re using experience and expectations to interpret sensory information.

    03:25 to interpret sensory information.

    03:27 Your eyes are seeing the lines, but your mind is saying, “In the past I know that lines in that orientation typically means train tracks or a ladder or it might be two girls kissing or it might be, you know, a triangle on top of another triangle because I’ve seen that before.” So you’re using previous experience and your expectations of what that should be.

    03:50 The lines weren’t actually drawn together, weren’t closed, but you saw two triangles, you saw two girls.

    03:56 So you’re making that leap based on your previous experiences along with looking at that sensory information.

    04:03 So where top-down kind of refers to mind down versus bottom-up which is sensory information coming in through the receptors going up to your mind to get processed.

    04:13 So that kind of differentiates bottom-up versus top-down.

    04:18 So let’s walk through that bottom-up processing, use the sensory information from the receptors and works up to integration at the central nervous system.

    04:25 So coming in the eyes, getting integrated up at the brain, and we make our idea of what we think that is.

    04:30 Top-down is the other way around.

    04:31 We use the experiences that we have saved in our mind to take a look and figure out what we just saw.

    04:37 Now in reality, as humans, we use a combination of both.

    04:42 So you know that you saw a couple of broken lines and that gets integrated in your mind as triangles but you also know you’ve seen a triangle before.

    04:50 So what I’m trying to say to you is I don’t think it’s so clear cut.

    04:54 It’s not so black and white.

    04:55 It’s not so we only use bottom-up for this or we only use top-down for this.

    04:58 It’s actually a combination of both which is good.

    05:02 And it also allows our perception to be quite fast.

    05:05 So there’s a term that you need to know for the MCAT and that’s, I guess, a German term called “gestalt” which means whole.

    05:12 And that’s a principle that states that humans perceive objects rather than individual features.

    05:17 So if I were to show you a triangle that is not completely drawn together, you would still know that that’s a triangle because in your mind, you have saved a gestalt of what a triangle normally looks like.

    05:30 When you see your mama’s face, and you could be in a crowded room, or you see her from a mile away and you see your mama, you know right away, “That’s my mama.” You don’t have to sit there and say, “I’ve seen that lady before.

    05:43 I think she gave life to me. Yeah, that’s my mom.” Instead, you right away know that’s mama, okay? And this can apply to so many different things.

    05:51 It can apply to street signs.

    05:54 It can apply to names.

    05:55 It can apply to anything.

    05:56 And the ideas in our mind, we have saved these gestalts or these whole diagrams of things that we know to be true, instead of saving all the individual features that we need to memorize and know to realize what it is. Okay? So the human mind forms a gestalt and this has a reality of its own.

    06:13 So again, we’re saving these nuggets of information and that allows us to be very, very fast with our perceptions.

    06:19 Which is why in a crowded room, you can very quickly pull out people and things that you know as opposed to having to sit there and wait for your mind to get all that bottom-up, top-down processing to happen.

    06:33 So collectively, putting all these pieces together, this explains how we, as humans, use various facets and ways to process to bring our perceptions together.


    About the Lecture

    The lecture Perception (PSY) by Tarry Ahuja, MD is from the course Sensing the Environment.


    Included Quiz Questions

    1. Top-down processing
    2. Bottom-down processing
    3. Gestalt principle
    4. Sensory Adaptation
    5. Threshold
    1. Gestalt principle
    2. Muller's doctrine
    3. Weber's law
    4. Fechner's Llaw
    5. Signal substitution theory
    1. Sensory receptors
    2. Frontal cortex
    3. Hippocampus
    4. Cognition
    5. Motor efferents

    Author of lecture Perception (PSY)

     Tarry Ahuja, MD

    Tarry Ahuja, MD


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