Neural circuit ensures zebrafish will not bite off more than it can chew

drawing of zebrafish reactions

Small, prey-like objects elicit a quick response from zebrafish larvae, as excitatory brain cells fire. However, large visual stimuli covering the zebrafish's field of vision fire off inhibitory cells, and the zebrafish responds less. This mechanism allows the zebrafish to have a good hunting response to the appropriate visual cues, and not bite off more than it can chew. (Zina Deretsky, NSF)

Whether it is alerting us to danger or allowing us to spot prey, vision helps keep humans and other animals alive. But how exactly does this special sense work, and why is it easier for us to spot movement of small objects in our field of vision than to notice other things?

The complexity of the neural network that makes vision tick has long baffled scientists. With a new technology and support from the National Science Foundation, post-doctoral fellow Claire Wyart in the lab of Ehud Isacoff at the University of California, Berkeley, and Filo Del Bene in the lab of Herwig Baier at UC San Francisco have been able to follow entire populations of retinal and brain cells in their test subject, the zebrafish larva, and delve into the secrets of a neural circuit underlying vision.

Using a newly developed genetically encoded fluorescent reporter of neural activity developed by Loren Looger at Janelia Farms, the Auburn, Va., research campus of the Howard Hughes Medical Institute, Wyart and Del Bene have been able to follow how large and small visual cues translate into electrical activity in a region of the zebrafish brain.

The brain region of the zebrafish that receives input from the retina, called the optic tectum, is separated into layers. The top layer receives direct connections from retinal cells and has a population of both excitatory and inhibitory neurons. These neurons connect to output neurons that project to other brain regions, which control how the zebrafish chases prey.

Isacoff, Baier, Wyart and Del Bene revealed that a large visual stimulus covering the entire field of vision, such as large floating debris or another zebrafish, results in low output neuron activity. In contrast, small, prey-sized objects moving across the zebrafish’s field of vision at a prey-like speed activate the output neurons very well.

The basis of this “filtering” of information is that large visual stimuli massively activate the inhibitory cell population and inhibit the output cells, Isacoff said, while small moving objects activate only a small number of inhibitory tectal cells, enabling the excitation to drive the output cells efficiently.

Isacoff and Baier demonstrated that inhibition of neural activity driven by a large visual stimulus is essential for hunting prey since prey capture was disrupted when the inhibitory cells were removed or prevented from emitting neurotransmitters.

Isacoff is a professor of molecular and cell biology at UC Berkeley and a faculty scientist in the materials sciences division and physical bioscience divisions at Lawrence Berkeley National Laboratory. Baier is a professor of physiology at UCSF.