[MUSIC] Okay, in this unit, we’re gonna talk about perception, and we’re gonna take two examples and look at them in more detail. We’re gonna take vision and hearing but we're gonna start off, and today, what we'll look at is, what is perception, some of the features, shared features, of perception, then we'll go to vision and we'll look at how the eye gets the light into the retina. How the light, which reaches the retina is turned into a neuro signal. Then, we'll look at how, when the retina sends on the information to the rest of the brain, how that information gets interpreted into a perceptual, percept. And finally, we're gonna talk about the fact that we're not born knowing how to see. We learn how to see and the importance of early experience on seeing correctly but in this segment, what I wanna concentrate on is some terminology and specifically let's define perception. Well, we have to start with a stimulus, and a stimulus is defined as any change. A change in, it can be in the environment, it could be inside the body. So I just ate a bagel. Well I'm starting to feel that. My stomach is starting to expand. That's a change. If a light flashes outside that's a change. So a stimulus can be inside or outside and it can result in either a sensation or a perception. In a sensation, it will result in a stimulus that is of adequate strength, will always be sensed, but that sensation may not turn into a perception. So, these are not synonymous terms. A perception is a conscious appreciation of some stimulus or energy. Now we also actually can perceive without our stimulus and one easy example of that is amputation. Phantom limbs sometimes painful, sometimes just a phantom limb, just the sense that it's there when it's not there. So there often times, we have perceptions, and there are other examples that we'll look at in a moment, where we have a perception without a stimulus. So a stimulus does not necessarily create a perception, it does typically create a sensation. Sensation. We have all these senses. Things like blood gasses. So, how much oxygen, how much CO2 is in our blood. Never reaches consciousness. Can't even access it. Can't learn to access it. We have no idea how much O2 or CO2 is in our blood. On the other hand, I can look out and see whether there's a bird flying across the sky. So some sensations reach consciousness and produce a perception. Some sensations don't do that. They do other things like evoke reflexes or adjustments in the body. Okay, so in the next segment what we're gonna do is understand what the biggest issue is in perception. [MUSIC]