Week 4: The VOR
Introducing all of you to the Raymond Lab’s modus operandi, the vestibulo-ocular reflex!
We do get OKR (optokinetic reflex) data, but VOR data is our staple here. Get ready for your weekly lesson on mouse eye movements!
Two weeks ago we talked about how mice move their eyes to follow objects moving in their visual field. Here, we’ll be discussing how mice move their eyes in response to head motion. Try keeping your eyes fixed on a certain object. If you rotate your head horizontally, while keeping your eyes fixed on that object, you may notice that your eyes need to move in the opposite direction to compensate for the head motion. This is the VOR, and it is the movement of eyes in response to vestibular input (hence, vestibulo-ocular).
The VOR exists to promote image stability at the back of the retina. Thus, if the image moves, and the eyes do not follow it, the brain interprets this as an error. Our experimental treatment for increasing the mouse’s eye movement in response to vestibular motion is shown below. As you can see, the chair and the drum move in opposite directions. In the real world, the image remains unmoving, but by moving the drum (which is the mouse’s visual input), the mouse is forced to move its eye more than normal to keep its gaze fixed on the same point.
Suprisingly, mice display this reflex even in the dark, when they have no visual input. Because the mouse’s VOR will attempt to keep whatever image it sees fixed on the retina, if it sees nothing, we can get a sense for its innate reflex. This is how we test whether our training has had an effect on the mouse or not.
A sample Spike2 file for the VOR is below. Here, we are moving the drum and chair in opposite directions, so as to get the mouse to move its eye more. This is called gain UP training, and the sine waves for the drum and chair positions are exactly opposite phases. We also do gain DOWN training (less often) to decrease the gain (gain is the amplitude of the mouse’s eye movement, and is fittingly measured by the amplitude of the sine wave fit to the mouse’s eye movements during data analysis; see last week).
I’ve been collecting VOR data for two weeks now, and there are two more weeks to go. At the end of that week, I’ll do data analysis for the VOR data, and may soon begin work on my final product.
This post took me from not knowing what VOR is to being extremely excited to see what your data shows.
Does VOR account for rotating the shoulders as well? If I rotate my shoulders without twisting my neck, I still have to move my eyes to keep them fixed.
The VOR does account for shoulder rotation as well. Our scientific terminology says that it is in response to “head motion,” whether that motion is due to rotation of just the skull or the entire body.
Our mice are head-fixed for just the duration of the experiment, and are free to move the rest of their bodies during that time (i.e. if they were to move their shoulders, their head would still remain fixed). This ensures that their only vestibular input is from the motion of the chair (the platform that they are on, as shown in the video), which serves as an important control in our experiments.
So are you analyzing OKR and VOR separately in different experiments? Or are these two related in another way?
Yes, I’m analyzing both. They are slightly related, but OKR is more of a control. We know that OKR is very dependent on the cerebellum, but VOR is a little less charted territory. We’ll be comparing the two and seeing what we can find out.
Nice work! I’m excited to see your collection of VOR data.