Complex Sound Discrimination in Zebrafish: Auditory Learning Within a Novel "Go/Go" Decision-Making Paradigm

Previous anatomic and physiologic studies of the peripheral and central auditory system, with rare exceptions, have relied on the use of tonal stimuli. Here, we test the hypothesis that zebrafish, Danio rerio, can detect and discriminate between two 6 s long complex sounds-a sequence of five multi-harmonic, noise-embedded constant frequency (NCF) tone pips and a chirp sequence consisting of six rapid downward frequency-modulated (DFM) sweeps. To test our hypothesis, we develop an associative conditioning assay, requiring prediction of an unconditioned stimulus (US). A video clip of a shoal of free-swimming zebrafish presented on an LCD screen serves as a desirable or rewarding US and a bullfrog with inflating and deflating vocal sacs serves as an aversive or fearful US. Within our novel "Go-to/Go-away" (or Go/Go) assay, sound discrimination allows an animal to decide to go/swim towards the desirable US and away from the undesirable US within a short time window preceding each US. We use markerless tracking of fish locations following twelve training runs and six test runs to determine if zebrafish can discriminate between the two sounds. We discovered that on average, fish move closer to the LCD screen in response to the sound paired to the rewarding CS and farther away from the screen in response to the sound paired with the aversive US. Differences in locations and longest swim trajectories occur in the 3 s time window between the CS and the US. These differences are largely retained on the second day of testing, suggesting overnight memory consolidation. We conclude that adult zebrafish can both perceive and rapidly learn to discriminate between complex sounds and that our novel assay can be implemented for high throughput screening of drugs targeted for alleviating memory and attention deficits as well as other neurodegenerative disorders.