So far in this class we've talked a lot about how we know where stimuli are located out there in the world. Or how we know our own body's configuration. Now we're going to switch gears and start talking about our spatial knowledge on a much greater scale. We're going to talk about how we know where we are and how we know how to get from one place to the next. Usually this happens so easily that we're really not even aware that we're doing it and it's in that context that I think that the view from your window contest on Andrew Sullivan's blog The Dish is so interesting. Every week a photograph is posted and, reader's attempt to figure out where that photograph was taken. This is surprisingly difficult to do and in fact most of the winners seem to succeed by recognizing a familiar place. Well, why is this so hard, when at any given moment, you know perfectly well where you are located? I happen to be standing in my office at Duke University in Durham, North Carolina. But I would know this, even if I couldn't see what's outside my window. The information that I use to know where I am right now comes from a variety of different factors. They involve monitoring information as we go from one place to another to update our sense of where we are in the world. So, knowing that I'm standing in my office is possible because I know, I traveled along a certain route before arriving here. And most of these kind of cues are not available to us when we're simply looking at a photograph taken by somebody else that could be anywhere in the world. So, knowing where we are involves four different factors. It involves another sensory system, the vestibular system, which is our sense of balance and self-motion. It involves visual information. It involves motor control, monitoring our commands to move about through the environment. And it involves memory to glue all these things together. In this video I'm going to talk about the contribution of the vestibular system to this process. The organs that are responsible for our sense of balance are located in the inner ear near the cochlea. There are two parts. The semicircular canals and the utricle and saccule. These different structures have both similarities and differences in terms of what they measure and how they do it. The semicircular canals are responsible for sensing rotation of the head. Whereas the utricle and the saccule are responsible for sensing translation of the head, and I'll explain more of the differences between rotation and translation in a few slides. Both structures involve hair cells, which are similar to the hair cells in the cochlea that are responsible for hearing sounds. The way these hair cells detect motion of the head is slightly different. In the semicircular canals the hairs of the hair cells are dragged by a structure called the cupula whereas in the utricle and the saccule the hairs are tickled by small rocks called otoconia. Okay, so let's talk first about head rotation and these semicircular canals. So, this is a blow up of the semicircular canals in the ear. You could see here that there are three canals. There are three canals on each side and they're oriented orthogonally to each other. One is oriented roughly like this, another is oriented roughly like this and the third is oriented roughly like that. And that allows the ear to sense the three axes of motion. That is, nodding your head, shaking your head or tilting your head. The mechanism involves fluid and a structure known as the cupula, which is a gelatinous structure and what happens is this. There are hair cells located here with their hairs sticking into the cupula. Here's a blowup where you can see the hairs embedded in the cupula a little more clearly. When the head rotates, the semicircular canal rotates but the fluid in the cupula that are in the semicircular canal lag a little bit behind and that causes the hair cells to deflect. So, if the canal rotates this way, the fluid and cupula lag a bit behind deflecting the hairs in that dir, direction and so forth. Hair is deflecting this way for a turn of the semicircular canal that way. The deflection of these hairs allows for transduction of head-turn to neural activity. That is, the physical event of turning the head becomes reflected in changes in neural signals. This is quite similar to what happens in the auditory system. The deflection of the hairs cause ion channels to open and close. This, in turn, produces changes in the resting membrane potential of these neurons. And the whole process is very similar to what happens in the auditory system. And, in fact, it may be the case that the auditory system evolved out of the vestibular system. That is, that the vestibular system arose first in evolution and that the auditory system came later. Let's turn now to the utricle and the saccule. These structures are responsible for detecting not rotations but translations. So, what's the difference between a translation and a rotation? Well, this is a rotation, like this or like this. And this is a translation, like this or like this. So, the utricle and the saccule are responsible for detecting that forward-backward movement. They're also responsible for side-to-side movement. And they're also responsible for up-and-down movement. They detect these movements because of the effects of small rocks in the vicinity of the hairs. This is a scanning electron microscope picture of these small rocks or otoconia. So, these little rocks deflect the hairs on the hair cells when you move from side to side, forward to back, or up and down. Normally our vestibular systems work very well, and we are, able to hold ourselves upright with respect to gravity, and to move skillfully about our environments. But once again, illusions can help illustrate that this isn't a magical process but one that requires careful work on the part of the brain and it's sensory inputs. So, a fairly common vestibular illusion you might refer to is simply dizziness. We all have experienced dizziness if we spin ourselves around over and over again. What happens is that the fluid in our semicircular canals starts to be set in motion. Rather than lagging behind our the motion of our head, it starts to move with us. Then, when you stop, the casing of the semicircular canal stops, but the fluid stays in motion providing a sense that you're actually turning in the opposite direction. That's what we experience when we experience that kind of dizziness. Another vestibular illusion occurs when we drink too much alcohol. And what happens here is thought to involve a different density of alcohol compared to the substances that make up the fluid in the semicircular canal, as well as in the cupula. So what is thought to happen is, that alcohol diffuses into the cupula. It changes the density of the cupula. It is a little slower to diffuse into the fluid that's filling the canal. So, during the period of time that alcohol has diffused in here but not in here there is a difference in the density of these two substances. The different diffusion rates alter the relative density of these two areas of the semicircular canal. This changes the mechanics with which they move. That in turn alters neural signals and creates a sense of dizziness. The whole process then reverse itself with the alcohol diffusing out of the cupula first and then out of the fluid in the semicircular canal. So, there can be a second period of dizziness as you're recovering from being drunk, but often that happens when you're asleep and so people don't notice it. So, the vestibular system is an essential component of our ability to sense our movement through space, but it doesn't work by itself. It works in concert with vision and with movement and these are glued together using memory and I'll begin talking about those in the next video.