In this video I introduce psychophysics and explain the difference between the absolute threshold and the difference threshold (or just noticeable difference). I also discuss the difficulties of measuring our sensitivity to stimulation and provide a simple demonstration of Weber’s Law, which is that the difference threshold relates to proportions of change rather than amounts of change.
Don’t forget to subscribe to the channel to see future videos! Have questions or topics you’d like to see covered in a future video? Let me know by commenting or sending me an email!
Need more explanation? Check out my full psychology guide: Master Introductory Psychology: http://amzn.to/2eTqm5s
Video Transcript:
Hi, I’m Michael Corayer and this is Psych Exam Review. In this video we’re going to talk about psychophysics. So what is psychophysics? Well, this is the study of our sensitivity to stimuli of difference strengths. So psychophysics essentially investigates just how sensitive our sensory organs are to different types of stimulation.
So one way we could study this is by trying to find the absolute threshold. So what the absolute threshold refers to is the smallest amount of stimulation that person can still perceive.
So it’s the minimum stimulation necessary in order to be detected. So for instance in the case of vision I might want to try to find out what’s the dimmest light that you could say that you see. So I could turn on a very very dim light and ask you whether you can see it or not. Or in the case of hearing I could play a very quiet sound and ask whether you hear it or not and I’m trying to find out how quiet can the sound be where you still hear it?
The problem that I’m going to run into is that you’re going to guess sometimes. We’re at that edge where you might think you heard it. “I think I hear that but I’m not sure.” I’m sure that’s happened to you before. You think you hear something but maybe not. Your friend says “do you hear that?” and maybe you do, maybe you don’t. For absolute threshold we want be sure that you’re really hearing something.
In order for that to happen, we need you to be accurate at least 50% of the time. That’s the point where we start getting interested, is at the 50% accuracy rate and above. If I’m playing you a quiet sound and I say “Can you hear that?” and you say yes or you say no, you’ve got a 50% chance of being right just by chance you could just guess every time and eventually probably work out to getting about 50% accuracy. So that’s sort of my minimum starting point, maybe you’re getting it right.
Maybe it’s chance but once you get about that I can be at least little bit more confident that you really are detecting the stimulation. Now this is a simple concept, this idea of absolute threshold. It seems like it would be simple; just put some headphones on play the quietest sound and somebody says they can hear it.
But what we’ll see in the next video is that this very very complicated. There’s all sorts of stuff that gets in the way. So when we think about vision, I might ask “what’s the dimmest light that you can see?” but then we might ask, how long is the light shining for? What’s the duration? What’s the wavelength of the light? How much light am I emitting versus how much is actually reaching your eye? Some of it is going to get absorbed by your eye on its way to the back of your eye. So the amount I shine versus what actually hits your retina isn’t going to be the same.
So we have to take all of these things into account as well. So you can see some estimates for absolute thresholds but remember these are just estimates and they’re usually based on supposed ideal conditions. So you might see that the human eye can see the light of a candle from 30 miles. Hypothetically that might be true but in real-life conditions, get your friend to go 30 miles away and light a candle and you probably aren’t going to see it. But of course that’s because you’re in the real world and there’s all sorts of other “noise” and that’s something that I’ll talk about in the next video.
So you can look up and find some other estimates if you’re curious. For hearing you might find something like saying you can hear a watch ticking 20 feet away. Again, well what kind of watch is it, there’s all sorts of other factors that could be involved. So that’s what absolute threshold is, minimum amount of stimulation that you can still perceive. The other thing we could be interested in, the other threshold, is the difference threshold. So the difference threshold is how much change is necessary for you to notice it. The difference threshold is about change in stimulation and they also see this called the just noticeable difference.
And this is a pretty self-explanatory term here because the just noticeable difference is the smallest difference that you can just notice. Or you can see this called the JND. So one way that you can do a little experiment with this is to have a friend get two envelopes and put two sheets of paper in one envelope and one sheet of paper in the other envelope and then you just take turns trying this: you close your eyes and a friend places one of the envelopes on your hand and then places the other one and you try to tell which one is the one with two sheets of paper and which is the one with one sheet of paper. If you do this, you’ll probably find it’s really easy.
Detecting the change in the weight between one sheet of paper and two sheets of paper is actually pretty easy. You’ll probably get it right most of the time. So then you might think, “well, this means that I can just notice the difference in weight of a sheet of paper. So how much does a sheet of paper weigh and then I can tell that much change.” But that doesn’t really work. If you then take a textbook and you have your friend place the textbook on your hand; sometimes just the textbook but sometimes with an extra sheet of paper in it and suddenly you’ll find that you can’t tell the difference anymore.
You’re not able to tell when this textbook has an extra sheet of paper in it or not. So what’s happening here? Has the difference threshold changed? Well it hasn’t really changed because when we talk about difference thresholds what we realize is something called Weber’s Law.
Weber’s Law is this idea that the just noticeable difference is not a constant amount of change but it’s a constant proportion of change. So what does this mean? It means we can tell a change in percent in weight, but we can’t tell a specific amount of change in weight. So when I compare one sheet of paper to two sheets of paper that’s a large percentage of change. The weight from the single sheet of paper to the two sheets of paper is changing pretty dramatically in terms of a percentage.
But in the case of the textbook, you know this has 480 pages which actually means it has 240 sheets of paper. But the difference between 240 sheets of paper and 241 sheets of paper is too small of a proportion of change, it’s a very small percentage that it’s changing. So this is what Weber’s Law describes. And the same is true for our other senses so can probably tell the difference, if you were in a dark room and I lit one candle vs. two candles you’d probably notice that the two candles is brighter than the one candle. You would detect this change in brightness.
But if I lit 100 candles versus 101 candles suddenly you might not be able to tell the change. So this idea that it’s a proportion, it’s relative to the original stimulus that you’re comparing. It’s not a constant amount, “I can always tell one additional candle or one additional sheet of paper” or anything else. So in the case of weight, it turns out we need about 2% change before we can detect it. So in the case of the envelopes we have much more than a 2% change in weight so we can detect it but in the case of the textbook we don’t.
Now there’s going to be some limits to this. The 2% is an estimate, if I got to the point where the weight’s so heavy that you can’t pick it up then 2% isn’t going to work anymore. I could have two boxes and one has 1000 pounds and the other has 1100 pounds, 10% change, but if you can’t pick either of them up you’re not going to be able to tell me which one is heavier. So there’s still some limits to it.
Ok, so that’s the just noticeable difference and Weber’s Law, this idea that it’s a proportion, it’s not a constant, and absolute threshold, which is the minimum stimulation necessary to detect something. OK in the next video I’ll talk about signal detection theory which is about all these problems that we have in trying to measure these things accurately. Ok so I hope you found this helpful, if so, please like the video and subscribe to the channel for more.
Thanks for watching!
