Thresholds – Sensory Processing (PSY, BIO)

00:01 Okay. Let’s talk about how we sense the environment around us.

00:05 So there’s a lot of ways that we can actually do this and we’re going to walk through a whole bunch of ways we detect the things that are interacting with us.

00:16 So one of the easiest ways to determine whether or not there’s something around us is something called thresholds.

00:20 And this is a different way to -- this is a way for us to measure activation of, say, a receptor.

00:27 So if you have a threshold, you can have two types of threshold.

00:31 One is called absolute threshold.

00:33 And this is when we’ve achieved a certain level of activation so that we know that something has happened.

00:40 So, you know, in simple English, let’s say you’re in a dark room.

00:45 So you’re at your desk right now.

00:47 Turn off all the lights in your room and it’s pitch black.

00:50 Now if it’s completely pitch black, you see nothing.

00:55 Now what is it going to take for you to say there’s a light on somewhere? So if you were to flick on your phone, you would easily detect that and you can say, “Yes, there’s light right there.” But at what point, how dim does that light have to be for you to actually say, “Oh yeah, there’s a light on right now”? So that’s called absolute threshold and it can be things like sight in low light, it can be hearing slightest noise, it could be the taste of something.

01:20 So how much water when you’re drinking water, can you taste the sugar? Is it sweet or is it still tastes neutral? And you know we figured out, “Okay. We’ll, it’s roughly about three sugar cubes and a hundred liters of water.” So the other side is something called difference threshold.

01:37 And so this is when one receptor will have a measurable absolute threshold value and a measurable difference threshold.

01:43 So you can have a receptor that says, “Yes, I’m on.” And then you can have a receptor that once it’s activated, it can detect a difference.

01:52 Okay. So we’re going to walk through each, giving examples of each.

01:54 We’re going to walk through an example so this all make sense to you.

01:58 So absolute threshold is defined as the minimum stimulus intensity required to activate a sensory receptor half of the time.

02:05 So if we’re back to our light example, I think the research has shown that at roughly three miles if you have a candle in a dark and nighttime, you could detect that roughly 50% of the time.

02:21 And why do we say 50% of the time? Because that’s basically right at and above chance.

02:27 So if I have a candle lit or don’t have it lit and I say, “Do you see a candle?” If you were to guess, you’d be 50/50.

02:33 So you have to be just above that.

02:35 Okay. So we’re establishing what is a signal detection.

02:40 We also know that for most individuals there are some minor differences.

02:44 And that being said, you know, the average human would have a value, but then like with anything, there’s going to be a range.

02:50 There’s going to be some people whose signal detection is extremely great and they could find -- they could see a candle at, say, four miles and there are some people who need, you know, two miles.

03:01 So they’re not as good.

03:02 We also know that as you get older, like everything, your machinery starts to slow down and it’s not as great.

03:07 So age can be an influencing factor.

03:09 So as you get older, you’re hearing is not as good, your eyes aren’t as good.

03:13 It’s harder for you to see things, right? And so your absolute threshold can shift as well.

03:19 So now difference threshold is also known as just noticeable difference.

03:25 And what this is, is this is defined as the minimum noticeable difference between any two sensory stimulus half of the time.

03:33 And what we’re saying here is detecting change.

03:38 And if you are activating a sensory stimulus -- sorry, sensory receptor with the stimulus, you need to detect a difference at least half of the time.

03:49 So let’s go back to our candle example in the darkness in the desert.

03:53 And at three miles, I have a candle.

03:55 Now, if I was to add a second candle, would you be able to detect the fact that I have now made it brighter because there are two candles? And you would have to notice that difference of that additional candle at least half the time.

04:09 Okay? So that would be that value that you would get would be your just noticeable difference.

04:15 So the initial stimulus can influence the difference threshold.

04:18 So what are you starting with? And we’ll walkthrough examples.

04:20 This makes a lot more sense.

04:22 But let’s say if I ask you to pick up a textbook, your MCAT book that’s sitting on your desk right now.

04:30 You put that in your hand and you’re holding it and you feel a certain weight in your hand, your muscles can feel that and you know roughly what it weighs.

04:36 and you know roughly what it weighs.

04:38 Now you close your eyes and I add one sheet of paper from your printer and I put that on top and say, “Okay. Can you notice any difference?” Chances are you’re probably not going to notice the fact that I’ve added a piece of paper on top of your heavy textbook.

04:54 Now what if I was to add, say, another small book, a study notes book that went on top? Now you’re noticing there’s a difference.

05:03 And there’s a clear difference there and you would be able to notice that.

05:07 So now you can back track and say, “Okay, well, how little, how little do I have to add before you actually notice a difference?” That would be your just noticeable difference.

05:17 Now if you’re starting with a textbook and I add a piece of paper, that JND, just noticeable difference, is going to be a certain value but not much.

05:28 You’re going to need to add a little bit more than that.

05:30 Now what if you started with a piece of paper and now I’m adding maybe another sheet of paper? You might notice that difference because you were starting with something so small and you added something relatively small.

05:45 But because of the initial stimulus that you started with, that difference is going to be there.

05:49 So if you start, let’s say, the phonebook or that study book, you’re starting with something large, you’re going to need a larger proportional difference in order detect that change.

05:59 So your JND is going to be changed accordingly.