In this video I describe perceptual constancy, which refers to the idea that we perceive a relatively stable and unchanging world despite the fact that sensory information is changing dramatically. I explain how this applies visually to size, brightness, and shape. We often forget this learning process because it occurred during infancy, but it is demonstrated clearly in previously blind individuals who must slowly learn how to see following eye surgery.
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An Anthropologist on Mars – Oliver Sacks: http://amzn.to/2mjlibz
Video Transcript:
Hi, I’m Michael Corayer and this is Psych Exam Review.
In this video I want to explain this idea of perceptual constancy This refers to the fact that the information coming in from our senses is constantly changing and yet somehow we’re able to perceive a world that is relatively stable.
We don’t feel like things are constantly changing all the time even though in terms of our sensory information they are changing dramatically from moment to moment. So we can use vision to demonstrate this idea. If you think about watching your friend approach from the end of a hallway. What are the things that change as she approaches?
Well, her size is going to change dramatically on your retina. At the end of the hallway she’s this tiny part of your retina and as she moves towards you this grows and yet you don’t have this experience that she is actually growing larger. You can sense that she’s moving closer to you.
This shows is that we experience a sense of size constancy, we we think that things are stable in their size even though their image grows on our retina all the time. Even as you walk down the hallway, the size of all the things on your retina that you’re passing is changing dramatically and yet you don’t feel anything is growing or shrinking or changing size or anything like that. You perceive it as being relatively stable.
So we experience this sense of size constancy. We also experience brightness constancy and color constancy. This refers the idea that as you move around through the world the brightnesses of things and the colors of things change all the time. Even just as you walk around a room, the change in lighting and shadows and things like that changes what you’re seeing and yet you don’t experience that things are changing.
You perceive them as being stable. So for instance if I were to suddenly shine a light onto my face here you don’t perceive that my face has changed color in any way, you perceive that there’s a light shining on it and so you correctly assume that my face is the same, it’s just the lighting that has changed.
A last example of this, and perhaps the most impressive one, is shape constancy. So the shape of things that you see is constantly changing on your retina. Even as I talk to you as I just turn my head the shape of my head on your retina is changing dramatically and you’re able to correctly interpret that it’s just moving, you’re seeing it from a different angle but it’s not actually changing in any way.
This is fairly impressive. All of these things that make up this idea of perceptual constancy have to be learned. This is because you have lots and lots of experience seeing the world. When you were an infant, you weren’t able to do all these things. To use a famous quote from William James, the infant’s world is a “blooming, buzzing confusion”.
It’s this idea that there’s so much sensory information and the infant’s brain doesn’t yet know how to figure it out, how to correctly interpret it. This actually takes a very long time.
For you, you simply don’t remember that you had to learn all these things but you did have to learn them. One way that we can demonstrate this is we can see people who have their vision restored, so they’re born blind they haven’t been able to see and now certain modern surgical techniques are able to overcome this.
So for instance, people might be born with a lens that’s very cloudy, it doesn’t allow light through, so they can’t see. But now we can surgically replace the lens, so we can restore the vision in people who haven’t been able to see for their entire life. So you can have somebody who’s 40 years old and they can undergo the procedure and they can now see for the first time and it’s not like you know, the clouds have cleared and I can suddenly see everything and it’s amazing. It’s actually very confusing and very difficult for them. They have to learn how to see and they realize this actually takes a very long time and is a very slow difficult process.
In his book “An Anthropologist on Mars” Oliver Sacks describes a patient named Virgil who underwent this procedure and found it very very difficult and actually became depressed after this because he could see but it was difficult, it was described as like he was blind without being blind. Technically he could see but he couldn’t make sense of all this information.
And people expected that, you know, he can see now that he’s going to suddenly being able to do all sorts of things instead of realizing that actually he’s still essentially blind because he’s can’t make sense of all this information and he’s going to have to learn it. And it’s going to be fairly slow.
OK so in the next video we’ll look at another aspect of our vision that we have to learn that’s quite impressive and this is the idea that we can perceive depth. We can perceive depth pretty well compared to a lot of other animals. How we do this? We have a flat surface of our retina essentially. It’s like a screen that the lens is projecting the light onto. And yet somehow we’re able to perceive a world that has depth. How do we go about doing this? What sort of clues do we use to figure out how far away things are from us, just based on a flat image on our retina.
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Thanks for watching!