Neural signals, muscles work together to control eye movements
Enter a cluttered room or examine a great work of art and your eyes dart quickly around from object to object. These eye movements are governed by a three-dimensional rule called Listing’s law.
When eye movements follow Listing’s law, the head remains in a stable position, the eyes move, and torsion (rotation around the line of sight) is kept at zero. But this raises a question. Does the brain control these torsional movements, using complex, internal calculations? Or can the muscles keep torsion to a minimum on their own?
Researchers at Baylor College of Medicine recently ended that long-standing debate by demonstrating that it is actually a little bit of both. The findings appear in the Journal of Neuroscience.
"Initially, the thinking was that the brain does everything," said Dr. Eliana M. Klier, assistant professor of neuroscience at BCM. "But over the last decade, new anatomical findings have shown that the eye muscles, and their surrounding tissue, play an important role in implementing 3D kinematics. Some have even gone on to speculate that the brain only operates in 2D, focusing only on horizontal and vertical movements."
Researchers were able to stimulate the eye muscles while subjects were performing the vestibular-ocular reflex—an eye movement that does not obey Listing’s law. The stimulation elicits an eye movement and this eye movement provides a snapshot of how the eye muscles were arranged at the time of stimulation. If the eye muscles are solely responsible for the eye’s 3D kinematics, then the elicited eye movements should show torsional components.
"We found the opposite. Listing’s law (i.e., zero torsion) was still obeyed at the level of the eye muscles during this non-Listing’s behavior. This implies that the eye muscles are actually set up like a gimbal system (picture the movements of a camera on a tripod) that is obligated to implement Listing’s law no matter what," said Klier. "When other non-Listing movements are needed, neural signals must be sent down from the brain to override this ocular gimbal arrangement."
These findings are important for surgeons who perform procedures that change the relative locations of the eye muscles, such as surgeries to correct what is known as lazy eye (i.e., strabismus). Klier also said that while her research with Dr. Dora Angelaki (professor and chair of the Department of Neuroscience at BCM), focuses on the eye, these same principles may also apply to other body movements, like those of the head, arms and legs. How much of their control is neural and how much is mechanical? This research could one day lend itself to understanding and correcting disorders or illness involving motor control and neural activity.