In this video I describe touch sensation and haptic perception, which refers to exploring the world by grasping. I outline the types of receptor in the skin including mechanoreceptors, thermoreceptors, and nocireceptors, and then discuss how signals are mapped out according to sensitivity in the somatosensory cortex of the parietal lobe. This mapping research was initially conducted by Wilder Penfield and can be used to create a “homunculus” or a representation of a little person in the cortex, which can be done for both the somatosensory cortex and the motor cortex.
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Sensory and Motor Homunculus Diagram
http://willcov.com/bio-consciousness/diagrams/Homunculus%20(Topographic)%20Diagram_files/image295.jpg
Sensory and Motor Homunculus Models from the Natural History Museum, London.
http://piclib.nhm.ac.uk/results.asp?inline=true&image=001914&wwwflag=3&imagepos=1
http://piclib.nhm.ac.uk/results.asp?inline=true&image=001915&wwwflag=3&imagepos=2
Video Transcript:
Hi, I’m Michael Corayer and this is Psych Exam Review. In this video I’m going to talk about touch sensation and I’m also going to talk about what’s called haptic perception. So haptic perception refers to the idea that we can explore the world through touching and through grasping. We can feel objects, squeeze them and apply pressure and explore the world this way.
So how exactly do we have this process of transduction happening? How do we go from the physical properties of these objects and somehow by touching them we’re able to convert that into neural activity in the brain? Well, you probably partially know the answer already, that’s through our skin surface.
So your skin, you’ve probably heard, is the largest organ in your body and so it’s not just a separation from the outer world to keep invaders out. It does do that but it has many other types of receptors in it so it can do a number of different things.
Let’s look at some of the things that the skin can do. First, we have things called mechanoreceptors, these are receptors that detect properties of the things that we touch. So we can detect things like pressure We have cells that are specialized to respond to different amounts of pressure and then we can also sense vibration. So we have specialized neurons in our skin that respond to certain frequencies of vibration.
Then we can also, when we touch things, we can feel texture. Rather than just feeling millions of isolated little points, we recognize textures when we feel things. Along with that we have this idea that we also sense patterns.
So these are four properties that we can detect with our skin and these use different types of receptors. We’re not going to go into detail on all these different types of receptors and how they’re laid out in the skin it’s very complicated and actually not really all that well understood, how all these systems work together.
But we also have other receptors that detect other things. You could probably think about, well what can I sense with my skin, I probably have some type of receptor for it. So another thing that you can sense is temperature.
That’s going to use specialized receptors that are called thermoreceptors. So thermoreceptors are able to detect temperature.
Then you also have receptors that respond to pain. So you have these nocireceptor these are also called nociceptors, a shortened version but it means the same thing, and so these nocireceptors detect pain. These are very important and in fact I’m going to go into detail on pain in the next video we’ll talk about different types of pain receptors and why they’r so important. We also have, in some parts of our skin, we have things called chemoreceptors that can detect chemicals, most notably you have these on your tongue so we’ll talk about those in a separate video as well.
So we have a number of different things we can detect on the skin surface, so then what happens is all these messages go up the spinal cord and they get to the brain. They get to a region called the somatosensory cortex. This is a strip of cortex on the parietal lobe, it runs down either side of the parietal lobe.
There’s two main things you should know about the somatosensory cortex. The first you already know, is that it’s contralateral. So remember this idea that the left side of the brain controls the right side of the body. That applies to your skin sensation. So processing the information from the right side of your skin surface happens in your left hemisphere and the opposite is true for the left side of your body happening in the right hemisphere.
So remember that the somatosensory cortex is contralateral. The other thing to know about it is that the greater amount of sensitivity that we have, that means we have more receptors there, that means we’re going to need more cortex in the brain to process that information. The idea here is that greater sensitivity (I’m not sure what’s happening with my pen here)
greater sensitivity means that were going to need more cortex.
This is pretty logical. You can demonstrate this, you can demonstrate these different regions of sensitivity in your skin with a test like this. So you get two little points, in this case I’ve straightened out two paper clips, you could use two sharpened pencils or something. You need a partner for this.
What you want to do is you’re going to take turns poking each other. Either with both of the points at once or with just one of the points. You have to close your eyes and try to guess was that two points or just one? You find if you do this on your fingertip it’s actually pretty easy to tell when it’s two, you can put them really close together and it still feels different than the one. But if you try it on other areas of your skin surface you find that suddenly you can’t do this very well. So if you try this on your arm suddenly they can be fairly far apart and it will still feel like just one point.
And if you do it on your back or your shoulder you’ll find you’re actually really bad at this. You can put them 1/2 inch apart and to you it just feels like one poke. That’s because you just don’t have as many receptors there, it’s not as sensitive. That means that in your brain you have less space in your somatosensory cortex for processing information from your back in your skin surface compared to your hands which take up a huge amount of space because they’re so sensitive, right? They have lots of receptors in them so we need a lot of brain to process that.
This was first shown by Wilder Penfield, who was a neurosurgeon. I’ll bring up a picture of Penfield here. Penfield was doing brain surgery on patients and he took advantage of this to study, he was able to do this while people were undergoing a surgical procedure you can be conscious while you’re having brain surgery because the brain doesn’t have pain receptors or anything, or really any of the receptors I’ve been talking about. So he found he could stimulate the brain of people who were awake and they could tell him what they felt.
That’s exactly what he did and in doing this he was able to map out the somatosensory cortex. He would stimulate a brain region and people would say “I feel that on shoulder” and then he would stimulate another area and they’d say “I feel that my hand, it’s on this part of my hand here” and so by doing this with a number of patients Penfield was able to create essentially a map of the human body on the cortex. This is called a homunculus. Homunculus is Latin for “little man”.
So if we look at the somatosensory cortex which is here, this is a sensory homunculus. We can see that it shows the different parts of our body mapped out onto the surface of the cortex here. And you can see that some areas take up a lot of space, your lips take up a huge amount of space, they’re very sensitive. Your hands are massive in terms of how much cortex they require, and then the rest of your body is this little shriveled up part of it. That’s because you don’t have that much sensitivity in most parts of your body.
This is a version of the homunculus that Penfield published in the early 1950s and it’s still pretty much the same today. His research really has held up very well. The only exception I can think of this is there’s been some dispute about whether the genitals are actually underneath the feet as he has them here and the evidence has been sort of mixed, people have tried to see if that’s actually where they’re located. Maybe it’s hard to get people to come into lab and study this. Otherwise his work is really still used.
Ok, let’s look on the other half of this picture though. This is a motor homunculus. We can also map this out according to motor movements. So the motor cortex is another strip of cortex just in front of the somatosensory cortex. Somatosensory cortex is in the parietal lobe then just in front of it at the frontal lobe is the motor homunculus.
You can see this same pattern applies. More complex movements require more brain space. In this case, it’s really striking. Your hands are massive, you’re basically just mouth and hands in the motor homunculus. That’s because you make lots of very precise delicate movements with your lips and your tongue in order to speak. If we didn’t have that we wouldn’t be able to speak articulately or really at all, and then your hands, of course you can move them in a seemingly infinite number of ways.
Whereas the rest your joints, the rest of your body movements are pretty simple. Your elbow, right? How much can you really do with that? The answer is, not very much. You can bend it, you can straighten it, that’s about it. So that’s not going to require very much brain space. You can see that here, the elbow, one little part of the brain there for controlling that movement whereas the hands are huge.
Then we can take these maps here, these of maps of the somatosensory cortex and the motor cortex, and we could create a model of a person. So if you looked like your brain surface, in other words if those sizes corresponded to your actual size of your body then the motor cortex would look something like this. You’d have these massive hands and a very large mouth and the rest your body would be this sort of shriveled remnant of a body.
So this is a map, this is from the London Natural History Museum they have a little model here of a motor homunculus. Then on the other side we have a sensory homunculus. So, same idea except that this is for the somatosensory cortex, so the skin sensitivity in different areas. You can see they’re pretty much the same.
Really the only noticeable difference is, the genitals on the sensory homunculus are much larger than on the motor homunculus. That’s because these are parts of the skin surface that are obviously very sensitive but don’t do very much in terms of movement.
OK so that’s touch sensation and somatosensory cortex. I hope you found this helpful, if so please like the video and subscribe to the channel for more.
Thanks for watching!
