Schoppik Lab

How does the brain learn to balance?

We're constantly engaged in a vital struggle against gravity. Using a set of unconscious reflexes, we stabilize our bodies. Amazingly, these reflexes adapt as our bodies grow.

We study how developing brains learn to balance.

Our exquisite sense of balance relies on a nervous system that senses and compensates for destabilizing forces. To understand these neural computations, we build and use cutting-edge tools to dissect, measure, probe, and model brain activity as fish develop balancing behaviors. 

This data lets us discover fundamental principles of brain function, enabling us to understand, prevent, and treat disease.

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. Note the large eyes and ears, and pectoral fins. Notably, the fish is almost completely transparent.

One particular species, zebrafish, offers a unique set of advantages when trying to understand brain function. First, as larvae they are transparent, making it straightforward to leverage the latest advances in optical microscopy. Second, significant genetic groundwork makes molecular-level control and monitoring of brain activity straightforward. Third, their brains are much smaller. Taken together, fish are an ideal model system to uncover general principles of nervous system function.

Fish and humans use similar neural architecture and strategies to maintain postural stability. Consequentially, our work translates well to an understanding of normal human balance. Similarly, we aim to leverage the simplicity and molecular control of the fish model to understand and ultimately treat disease states.


Why study balance?

We propose that the computations necessary to balance are general in nature. By focusing on balance, we can understand the way our brains represent and utilize sensations to guide appropriate behavior.

The vestibular (or balance) system is a uniquely accessible model circuit to ask how the brain transforms sensation (forces on the body) to action (reflexive, corrective movements). Stabilizing posture and gaze are dynamic processes that can be reliably evoked by well-defined stimuli. Anatomically, we know the relevant neural projections from the sensory to motor periphery. The larval zebrafish permits molecular-level control at each stage, and the ability to monitor activity in many neurons simultaneously. We can therefore define the computations necessary for a sensorimotor transformation with rigorous control.


Tools?

As scientists, much of what we do has never been done before, and requires new technology. 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. 

Custom electronics to control and monitor a motor, accelerometer, lights, and more.

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.

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.


How do we dissect circuits?

One particular strength of the balance circuit is its comparatively constrained anatomy. Much of our work rests on a wiring diagram, elucidated over many years of work. The straightforward connectivity allows us to identify functional bottlenecks at the transition between stages.

Schematic of one side of the larval zebrafish vestibular circuit. A) Hair cells in the inner ear (blue/yellow/black lines) transduce the sensation of force into electrical activity. Afferent neurons in the stato-acoustic ganglion (SAG) relay this information to vestibular neurons in the hindbrain (VN). The midline-crossing VNs are categorized into two types, blue, which project to the oculomotor nucleus (nIII) while the yellow type project to both the trochlear nucleus (nIV) and nIII. Motoneurons that control the superior oblique (SO, red) and superior rectus (SR, green) muscles cross the midline, while those that control the inferior oblique (cyan, IO) and inferior rectus (magenta, IR) remain ipsilateral to the motor nuclei. B) Activation of both types of VN during roll tilts (along the barbecue axis of the fish, viewed head-on). C&D) Activation of a single subtype of VN to produce compensatory torsional eye rotations during nose down and nose up tilts.

Our main tool to dissect this circuit and measure the functional limits that arise at each junction is single-cell labeling. We have developed a set of protocols and reagents that allow us to molecularly target individual neurons at every stage of the circuit. This allows us unprecedented control over the balance circuit, at each node from sensation to action.


How do we measure brain activity?

We measure neural activity two ways: electrophysiologically, and optically. Together, they offer the ability to monitor the balance circuit with high spatial and temporal resolution. Electrophysiological measurements allow us to monitor the membrane potential of individual neurons at physiologically-relevant timescales. Optical recordings of genetically-encoded indicators complement these measurements, allowing us to characterize entire populations of neurons simultaneously.

Targeted intracellular recording from fluorescent cells in the vestibular nucleus

Loose-patch recording of irregular (red) and regular (black) vestibular afferent neurons in the stato-acoustic ganglion.

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.

Delivering only the finest in translational stimuli since 2015.

Delivering only the finest in translational stimuli since 2015.


How do we probe the balance circuit?

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.

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.

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.

Larval zebrafish engage in the vestibulo-ocular reflex under freely-swimming conditions. Gaze stabilization is an integral part of the larval zebrafish oculomotor repertoire. Shown here is an example of a 7 day old fish swimming normally, with an accompanying body rotation. Notably, the eye rotates remarkably little, despite large changes in the angle of the body.


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.


Limits: how does the brain decode?

Sensory neurons in the balance circuit encode information about destabilizing forces. Downstream neurons use that information to correct gaze and posture. We’ve learned quite a bit about the limits of sensory neurons with respect to encoding, or representing, sensory information. In contrast, we know almost nothing about how downstream neurons interpret and process these signals. This "decoding" step places a fundamental limit on the brain's ability to transform sensation into action. It’s largely unexplored, and we’re going to understand it.


How does this research help us treat disease?

From a public health perspective, disorders of the vestibular, or “balance system,” are a big deal: recent estimates are that one of every three Americans over 40 have experienced a balance problem. While there are treatments for some of these disorders, many remain debilitating. Understanding normal function of the vestibular system is a necessary first step towards designing targeted therapies.