Like a blues musician tapping his foot to the music, zebrafish can learn to flap their tails to the rhythm of a flashing light--and remember the beat after the light has gone out. The surprising discovery, reported online today in Nature, offers insights into how animals gauge time.
Humans and other animals keep time via an internal biological clock known as the circadian rhythm, which regulates 24-hour cycles of sleep, eating, and other activities. But what about smaller increments of time? Animals must be able to tell, for example, how quickly a potential predator is approaching. However, "the mechanism that controls the seconds-to-minute clock is still a mystery," says neurobiologist Mu-ming Poo, of the University of California, Berkeley.
Poo's team set out to investigate how neurons in the brain perceive time on these shorter scales. The researchers exposed zebrafish larvae--a popular laboratory animal model--to lights flashing at different durations and frequencies. Then they looked at neural activity in the tectum, the brain area that processes visual information, before and after the stimulus. When the flashing stopped, the team noted that specific patterns of neural activity in the tectum persisted for up to 20 seconds, suggesting that the larvae remembered the flashing lights. Indeed, when the lights went out, the larvae continued to flap their tails to the same rhythm with astonishing precision, much like a metronome. "The zebrafish larvae can reproduce the rhythm almost perfectly after the lights have gone off but only for short periods of time," says Poo.
The study provides an "elegant" example of the link between acquired behaviors and neural activity, says neurologist Dean Buonomano of the University of California, Los Angeles. But Buonomano, who investigates how the brain keeps track of time, says that, because the fish in the study only acted like metronomes in response to an artificial stimulus, it isn't clear that they have a true internal clock for measuring small time intervals.