Our exquisite sense of balance relies on a nervous system that senses and compensates for destabilizing forces. When it fails, we fall. To understand how it all works, we build and use cutting-edge tools to measure, probe, and model behavior and brain activity as fish develop balancing behaviors, stabilizing both their eyes and their bodies.
To learn more about our mission, click the links in the statement above, or read on.
Why study fish?
A four day old larval zebrafish, viewed from above. It’s got large eyes (black ovals), ears (clear, with two black spots in each), and pectoral fins. The big black circle in the middle is the swim bladder, an air-filled organ that helps fish balance. Note how this baby fish is almost completely transparent.
The neural architecture and strategies to maintain balance are over 500 million years old. That’s older than Saturn’s rings, insects, and trees on land! All vertebrates, from fish to humans, stabilize posture similarly. Fish have a lot to teach us about balance.
We use zebrafish. You’ve probably seen them in pet stores. In the lab, they offer many advantages. First, they are small and transparent. We can just look into their heads under a microscope and see their tiny brains at work. Second, we can access and control different parts of the brain. Lastly, they grow up and learn to balance in under a week. That’s perfect for impatient scientists.
Why study balance?
You can’t move through the world without keeping your balance. That’s fine when you’re 3 months old, but it’s devastating when you’re older. 1/3 of American adults over 40 experience dizziness or vertigo, symptoms of ailing balance systems. Falls in the elderly are a top-20 cause of death. Understanding balance is the foundation of impactful treatments.
For neuroscientists interested in how the brain works, the balance system is a tractable model to make progress on general questions. The vestibular (or balance) system is a uniquely accessible model. To balance, the brain transforms sensation (forces on the body) to action (reflexive, corrective movements). Interested in learning & memory? What could be more fundamental than learning how to deal with gravity. Curious about neural circuit formation? Vestibular circuits are a wonderful place to explore development — from fate specification to circuit assembly to myelination. Want to know about neurodegeneration? There’s no better place to investigate how molecular disturbances disrupt behavior. Finally, balance is integral to both navigation and locomotion, if you’re interested in those amazing behaviors.
Eye movements?
If you nod your head, this sentence stays in focus. Why? Your eyes reflexively counter-rotate to keep the visual scene stable. All vertebrates perform that same reflex. In the video below, a fish swims from right to left and tilts its nose-up. A close-up of the eye in the upper right corner shows how little the eye rotates. What the fish sees would appear stable and in focus.
We study how eye control develops. Vertebrates all have the same six eye muscles, and the same neural architecture to control those eye muscles. By understanding how zebrafish move their eyes, we learn about how we do.
Stable Posture?
Fish face three major challenges: First, they sink, because they aren’t as dense as water. Second, they rotate nose-down, because they are top heavy. Third, they rotate belly-up.
In the image to the left, an anesthetized baby zebrafish was placed in the water. Starting at the top, the series of poses illustrates how the fish body rotates and sinks. If you were anesthetized and propped upright, you’d fall down. This is what it looks like when a fish “falls.”
How do zebrafish stabilize their bodies? They swim! Swimming counteracts rotational torques. Swimming with a slightly nose-up posture also adds a little upward nudge to every movement, counteracting sinking. Finally, they reflexively keep their backs up, so they don’t roll over on their bellies.
As they grow, and their bodies change, fish must learn to adjust these behaviors. These behaviors must also be flexible enough to navigate up/down in the water. We’re deeply interested in understanding how the brain permits such flexibility.
What tools?
Apparatus to deliver tilt sensations to larval zebrafish. Apologies for the shaky video. A rotating platform is contained between two posts. The rightmost post has a motor that spins the table in small, discreet steps. The movie focuses on the clear acrylic cuvette holder mounted on a manipulator. Also visible are the cameras on the left side that image the larval zebrafish in the cuvette, the infrared LEDs that allow imaging in darkness. On the right, there are dedicated electronics that report the velocity and acceleration profiles of the platform.
Custom electronics to control and monitor a motor, accelerometer, lights, and more.
As scientists, much of what we do has never been done before. We need cutting-edge technology to understand the brain. In service of our science, we innovate in many domains, from genomic engineering to machining novel apparatus. Often, this innovation happens together with our collaborators, who bring a domain of expertise to the table. We could not be happier to discuss such collaborations; we’ve got a lot to learn.
We believe that the more we share the technologies we develop, the stronger the field. To that end, we’ve found it most effective to host people to come learn how we do what we do. Please contact us if you’re interested.
How do we measure brain activity?
Activity of vestibular nucleus and hindbrain neurons during movement. A transgenic zebrafish expressing an indicator of calcium in select vestibular nucleus and hindbrain neurons is mounted loosely under a multiphoton microscope. The fish faces left. When it moves, the neurons increase their fluorescence, giving rise to a "flash" that corresponds to increased activity.
We measure neural activity optically and electrophysiologically. Together, they offer the ability to monitor the balance circuit with high spatial and temporal resolution. Optical recordings of genetically-encoded indicators characterize entire populations of neurons simultaneously. Electrophysiological measurements measure membrane potential of individual neurons at physiologically-relevant timescales.
How do we probe the balance circuit?
A typical eye movement in response to activating vestibular nucleus neurons. The left eye of a four day old larval zebrafish is imaged, with the fish facing left. The blue dot in the upper right corner indicates when blue laser light stimulation is provided to the fish. This particular fish expresses an ion channel in the vestibular neurons that makes them light sensitive: when the blue light is on, they become active. The fish appears to receive the sensation that it has been tilted in the nose-up direction, and rotates its eye counter-clockwise to compensate.
The Torsional Vestibulo-ocular Reflex in Larval Zebrafish In Response to Pitch Tilts. One complete set of steps, plotted in time, with the accompanying raw video of the left eye. The table position is plotted as a grey line; each step is 10°, starting with nose-down tilts away from the horizon until 60° (nose-down), then reversing direction, moving until -60°(nose-up) and returning to the horizon. The inter-step interval for this movie is 2.5 s, instead of 5 s as in the paper, to reduce the duration of the video. The eye movements are similarly plotted, scaled 2x to be comparable to the table position, and color-coded so responses to nose-down positions are in cyan, and nose-up in magenta. Note that the peaks corresponding to the initial eye rotation following a step are considerably larger in response to steps that take the fish nose-up (magenta) away from the horizon relative to their nose-down (cyan) counterparts.
We interrogate the balance circuit in two ways: with natural stimuli, and with optically-gated ion channels. We read out the results of our perturbations both by monitoring behavior and its associated neural activity.
Fish mature?
Larval zebrafish rapidly develop a set of sophisticated behaviors. Stable gaze and posture are vital to this repertoire, which includes meeting complex challenges like hunting and predator avoidance. Over the first few days of life, the nervous system rapidly wires the necessary balance circuitry. We aim to relate the molecular-level events necessary to establish the balance system to its fundamental functional limits. We believe that defining this relationship is crucial to understand disease.