When needs compete, love trumps thirst

A Cornell University-led study, published Sept. 27 in the journal Nature, used advanced techniques developed by researchers to track the brain's dopamine reward system and found -- for the first time - this system flexibly retunes toward the most important goal when faced with multiple competing needs.

In the study, when a lonely and thirsty male zebra finch encountered a female, his thirst waned and he instead focused his attention on her, a shift reflected in the dopamine system.

Along with expanding our understanding of how dopamine neurons and pathways influence complex behavior, the finding may also inform the development of new artificial intelligence systems that mimic neural networks and dopamine reward systems.

"What we did that was new, to my knowledge, is that we were less interested in how an animal achieves a given objective and more interested in what happens when multiple objectives are on the table," said Jesse Goldberg, associate professor of neurobiology and behavior and senior author of the study.

For more than a century, researchers have been studying relationships between dopamine and learning. Dopamine neurons fire in response to rewards meeting diverse needs such as hunger, thirst, loneliness, language, and song learning, to name a few. Upon receiving a reward, such as water when thirsty, dopamine neurons exhibit a burst in activity. All drugs of addiction work through this system, Goldberg said.

Researchers developed a technique using optical recording methods and an engineered virus. Genes in the virus drove the expression of fluorescent dopamine sensors, such that the tissue fluoresced in proportion to dopamine levels. Optical fibers placed in the brain were then able to measure dopamine levels rising and falling as birds sang songs, courted females, and drank water. While a previous system using electrodes let researchers measure dopamine signals for up to 30 minutes, this new method allowed them to measure neuron activity for up to four hours straight over weeks.

"It was a key technical advance that enabled the discovery," Goldberg said.

In experiments, Goldberg and colleagues isolated male zebra finches, which are social animals, and also made them thirsty. The male had been trained to recognize that a flashing light meant it could peck a spout and get a drink, and when the bird was alone, the cue triggered both a large dopamine signal and water retrieval. But when the female was added to the cage, the male ignored the cue and the dopamine signal ceased.

"It's kind of intuitive, but hadn't explicitly been shown before, that courtship reduces the need for thirst," Goldberg said. "And that's important because in a complex and natural environment priorities change as new opportunities arise." The shift was reflected in both the bird's behavior and in the dopamine signal, he said.

Researchers say learning centers of the brain dynamically retune on moment by moment timescales, as an animal changes its priorities in response to new opportunities in the environment.

The study was funded by the National Institutes of Health.