13 — Gaze-stabilizing central vestibular neurons project asymmetrically to extraocular motoneuron pools

UPDATE: David Schoppik wrote up an awesome tweet-thread to help me be less wrong about things. I edited this post to more accurately reflect our current knowledge in this field. Thanks, David!

Full disclosure: This paper topic is a far cry from the literature in which I’m most comfortable. (You can probably expect more of this flavor in the rest of this series for just that reason.) I can’t vouch for the accuracy of everything I say; this is my read.

Larval zebrafish are interesting organisms to study because their genome is fully sequenced development happens quickly, and — pretty cool — they’re almost entirely transparent, meaning that you can record images of their brains even while they’re freely swimming.

This paper explores a population of motoneurons in the zebrafish larva responsible for eye-movement. One thing that I found particularly interesting is the discussion of the rationale possible reasons why behind why zebrafish (versus other organisms) have “nose-up sensory specialization”: The authors speculate that while Foveates — animals with areas of high visual acuity in the center of the retina…such as humans — control balance-related eye-movement with circuitry in the vestibular brainstem (the part of the brainstem responsible for detecting head-tilt, acceleration, etc), zebrafish live their lives in a forward-swimming world, where the water-surface is just up ahead (unlike other swimming organisms whose swimming angle varies more).

It might be for this reason that zebrafish oculomotion tend toward “eye-down” motion. That is, if you stimulate all of the bundle of neurons that run from vestibular nuclei to oculomotor centers, the eye looks downward, even though both “up” and “down” circuits are represented in that population. That suggests that the circuitry for reflexive eye motion is conserved, but the zebrafish specializes based on species behavior and environment.

In the words of David Schoppik (lead author on this paper),

..the key finding is that asymmetric populations like this one can help an animal encode something relevant, without skewing behavior. 5/n

— David Schoppik (@schoppik) September 3, 2017

In other words, the circuit might be conserved across many different species, but individual species (here, zebrafish) can leverage these circuits as conduits for other relevant information, such as sensory (posture) information.