In this video, we're going to talk about some fascinating connections between brain maps and perception. In particular, we'll talk about three examples. A phenomenon called phantom limb syndrome, involving maps of the body. The phenomenon of blind spots, involving your visual maps. And these topics are included in this video. In the next one, you'll learn about connections between a map of visual motion in a different area of visual cortex and your ability to perceive the velocity of moving objects. Maps are used not only in the visual system, but also for the body. The details of our body maps were studied extensively by a Canadian neurosurgeon named Wilder Penfield in the 1930s and 40s. And Penfield performed brain surgery on patients with epilepsy and, in the course of the surgeries, he used electrical stimulation to probe the function of different brain regions. Electrical stimulation involves using small amounts of electrical current to activate neurons. You can make neurons respond even when nothing is actually happening. So, when Penfield stimulated in a region of the cortex called somatosensory cortex, a part of cortex involved in encoding stimuli on the body's surface, patients reported feeling sensations in their bodies. Furthermore, he found that where he stimulated in cortex determined where on the body's surface the patients felt sensation. So this is a drawing of the correspondence between body part and location in somatosensory cortex. So he found, for example, that if he stimulated, in this region, patients reported feeling a sensation on their fingertips. If he stimulated down here, patients reported feeling something on their tongue. And all parts of their body were represented somewhere in somatosensory cortex. Areas of the body where we are particularly sensitive to tactile stimuli, like the hands, lips, or mouth, were associated with a disproportionately large region of cortex. And this called cortical magnification, and it actually occurs in visual maps as well. In visual maps, it is the fovea that is enlarged. That's the region of the retina where we have a density of photoreceptors and the highest spacial acuity. In somatosensory maps, it is the hands and the mouth that are most enlarged. Now, a second observation is that the body is mostly, but not completely laid out in a regular fashion. We have an arm to trunk to leg to foot section here, but just past the hand region is actually the face region. And that becomes important for the next part of the story. The next part of the story involves a phenomenon called phantom limb syndrome, and this is a syndrome that occurs in some amputees. After an arm or a leg is amputated, some patients report still feeling like their missing limb is present and attached to their body. This is a phenomenon that has been investigated by neurologist V.S. Ramachandran, who has theorized that these percepts may be due to brain activity persisting in such maps in the absence of the amputated body part. Now, proof of this idea came from a fascinating observation that touching an area of the body who's brain representation lies near the representation of the missing body part may sometimes be felt as if it had occurred on that missing body part. So if the hand has been amputated, then activity may spread into the hand region from the face region, which is adjacent in that cortical map. In the next slide, I'll play a video of Ramachandran and one of his patients illustrating this phenomena. >> When I first started shaving after my surgery, I would feel my absent hand start to hurt and tingle whenever I shaved this left side of my face. >> Meeting Derek was important for Ramachandran because the explanation he came up with would rock the world of neuroscience. >> How about that? >> That's just my arm. >> The first thing Ramachandran did was to invite Derek to his lab for a simple test. >> Derek, I want to touch different parts of your body and I just want you to tell me what you feel and where you experience the sensation, okay? >> Okay. >> Close your eyes. >> I could feel that on my forehead. >> Anything anywhere else? >> No. >> Okay. >> It's on my nose. >> Okay. >> My chest. >> Your chest. Okay. >> I can feel that on my cheek and I can feel rubbing on the phantom left hand. >> On the phantom left hand in addition to your cheek. I'm going to run the Q tip across your jaw and we'll see what happens. >> I can feel the Q tip on my cheek and I can feel a stroking sensation across the phantom hand. >> You actually feel a stroking across your phantom hand? >> Mm-hm. >> Across the palm? So here is a medical mystery of sorts. Why does this happen? Why would a person, when you touch his face, claim that it was also touching his missing phantom fingers? >> That's fine. Palm. >> [INAUDIBLE] palm. >> This is just the kind of mystery that Ramachandran was drawn to. Although it would take some time to solve. One day, while Derek was making one-armed repairs on his favorite Chevy, Ramachandran turned up with his solution. It was a groundbreaking theory. >> The reason we think it happens is that, in the brain, there is a complete map of the surface of the body. The entire left side of my body, the skin surface, is mapped on to the right side of my brain along a vertical strip of cortex which we call the somatosensory cortex. Similarly, the right side of my body is represented on the left side of my brain. [MUSIC] So every point on your body surface has a corresponding point on this body map. [SOUND] Now, it turns out that the representation of the face on this map is right next to the representation of the hand. Now, that's a bit surprising, as you'd expect the map to be continuous and faithfully represent the left side of my body. But it doesn't. Now, imagine what would happen if the left arm were amputated. The part of the brain corresponding to the hand no longer gets any input and it's hungry for new sensory input, so to speak. [MUSIC]. The sensory signals from the face normally activate only the face area that's right next to the hand area. But they now invade the vacated territory corresponding to the missing hand and start activating the hand region in the brain. And so whatever is reading those signals higher up misinterprets those signals. It says those signals are coming from the missing hand, so you experience the sensations as coming from the missing fingers even though I'm touching your face. >> So, this phenomenon illustrates that what perceive is quite lit-, literally what is in our heads, not necessarily reality. Now lest you think this is some obscure observation that only applies to cases like amputation, let's turn to some phantoms that we all naturally experience. And this involves something called the blind spot. So let's turn back to eye anatomy. Due to a quirk of fate, the retina is kind of organized backwards. The photoreceptors are at the back and the retinal ganglion cells are in front. Light passes through these retinal ganglion cells to be sensed by the photoreceptors in the back layer. But the axons that form the optic nerve come from those retinal ganglion cells. So that means that the axons of the retinal ganglion cells need some way of punching through the photoreceptor layer, to come together and form the optic nerve, to send the signals back into the brain. Where they collect together and make a hole through that photoreceptor mosaic, they leave an opening where no sensors are monitoring the visual scene, and this creates the blind spot. Now, one reason you're not aware of having a blind spot is that the blind spot is in a slightly different position in your left and right eyes. It's symmetric in the two eyes, but it's situated so that it is oriented to be looking at different directions in the visual scene. So, to do this next demonstration of the blind spot, you'll need to close one eye. If you close your right eye and steadily fixate the cat, and if you hold the image at the correct distance, you should be able to position your blind spot where the mouse is located. Getting the distance right is a little bit tricky, because I don't exactly what size screen you might be watching this video on, so I can't tell you exactly how far away you might need to hold the image. But, for scale, I've printed out, I've printed out this picture on a sheet of paper, and I'm going to, going to turn sideways, and I'm going to position it in my own blind spot and hold the picture at the distance that I need to in order to make, the mouse disappear. So I'm going to close my right eye and I'm fixating the cat. And right about at this distance, the mouse disappears. A copy of this picture is available on the course website, so you can print it out and do it yourself. Now, what you should see is that the mouse should disappear when you get it located in your blind spot, but the bars of the cage will not. Instead your brain will fill in those lines to cover the gap, and it's a little like the illusory contours that we talked about last time. It's an example of the brain interpreting a pattern of activity to make inferences about the visual scene, and whenever you interpret something, there's a risk of just simply making stuff up. And that's what's going on here. Now, this is relevant for certain health problems like the eye disease glaucoma. In glaucoma, progressive damage occurs to the retina and patients are often not aware this is happening because the brain covers it up and fills in the details. That's why you might have a visual field screening test when you visit the eye doctor. Alright, so we have talked about two types of phantoms, phantom limb syndrome and the natural phantoms that we all experience associated with blind spots. And, in the next lecture, I'll tell you about another kind of phantom, phantom motion that is perceived, when a brain map for motion information is, is activated by electrical stimulation.