[MUSIC] So in this segment, we're gonna understand, what does stimulus mean? And this is a very tricky concept. People think they understand what it means, and I can tell you that a lot of working scientists are a little bit unclear on this concept. Because the stimulus and the perception get mixed up. So let's just go through a couple of examples. Wavelength is a stimulus that is perceived as color. Injury is a tissue damage, a noxious stimulation, noxious stimulus is perceived as pain. But the stimulus is not pain, and the wavelength is not color. It doesn't have that capacity. There's nothing inherent about the stimulus that is gonna produce a perception. So let's look at this, with color, in a little bit more detail. So does light have a color? Can we say, that's red light? We can say it as a shorthand, but what we mean is it's a wavelength, which is typically perceived as red. Okay, so does light have a color? And the answer is no. Let's just go to the spectrum. And the spectrum that human's see goes from somewhere around 400 and a little bit nanometers, which is 10 to the -9 meters. So, a millionth. Up to about 670 or so. And what I've done here, is I've placed colors, which are what a typical person would see, not a colorblind person, but a typical person would see, if you gave them this wavelength of light. But in fact, the way that the visual system works is that it actually has a probabilistic response to wavelength. So that wavelength, at this point, could activate this channel, which is called the long wavelength, but some people call it the red channel. Or could activate this, which is the medium wavelength and mistakenly referred to as green channel. So the bottom line is that the stimulus is one component of what leads us to a perception. Let's say this in another way. Do the experiment at home. I was just told about this illusion, which I had never heard of before, but it makes a lot of sense. Put your left hand in cold water. Put your right hand in hot water. So, right hand, left hand. Then put both of them, keep them there for a little bit, a minute. Then put it in room temperature water, your left and your right. What's gonna happen? I think you all know that your right hand is gonna feel cold. And your left hand is gonna feel hot. Another example is, why does 30 degrees Fahrenheit, which is 0 degrees Celsius, feel a lot colder when it happens in Florida than when it happens in Chicago, during the winter? The stimulus is the same, but we're perceiving in two different ways. Even in different parts of our body at the same time. So, we are not perceiving a stimulus. Or the stimulus does not predict the perception. The perception is dependent on so many other things. So, we wanna be clear that the stimulus is the wavelength, the perception is the color. Let's take one more example of this. Let's say you, you like sugar. And you eat a tasty jelly bean or a tasty cookie, and that tastes good. And the perception is, yum. And now, lets say, that I ask you to eat a dozen of these. And now, on the 13th sugar intake, you're like, disgusting. So, the stimulus can not predict the perception. The final thing that I wanted to mention is that the set of stimuli that we are responsive to, is not written in stone. It is adapted for the animal. So we see a certain range of colors. But, for instance, snakes see a different range. They actually see infrared, which is up here. So the infrared enables them to see the heat signatures of prey and to actually find those prey in situations where they wouldn't be visible in what's visible to humans. Another example of stretching the stimulus set away from what's present in humans is bats, and rats, and mice, all of which can hear much higher frequencies. The human range of frequencies, of sound, that we can hear ends at about 20 kilohertz, and that's only for a moment in time, as a baby. By the time you're my age, you're down to around 12 kilohertz. But bats, and rats, and mice, they can hear up to 50 kilohertz. So the set of stimuli is not written in stone. I don't think that we can find in nature, and I don't think it could ever be made, an eye that is going to see X-ray. But certainly we can see various wavelengths, various ranges of wavelengths. [MUSIC]
