A rise-to-threshold signal for a relative value deliberation

Abstract Whereas progress has been made in identifying neural signals related to rapid, cued decisions1–4, less is known about how brains guide and terminate more ethologically relevant deliberations, where an animal’s own behavior governs the options experienced over minutes5–8. Drosophila search for many seconds to minutes for egg-laying sites with high relative value9, 10 and neurons, called oviDNs, exist whose activity fulfills necessity and sufficiency criteria for initiating the egg-deposition motor program11. Here we show that oviDNs express a calcium signal that rises over seconds to minutes as a fly deliberates whether to lay an egg. The calcium signal dips when an egg is internally prepared (ovulated), rises at a rate related to the relative value of the current substrate being experienced, and reaches a consistent peak just prior to the abdomen bend for egg deposition. We provide perturbational evidence that the egg-deposition motor program is initiated once this signal hits a threshold and that sub-threshold variation in the signal regulates the time spent deliberating and, ultimately, the option chosen. These results argue that a rise-to-threshold signal guides Drosophila to lay eggs on substrate options with high relative value, with each egg-laying event representing a self-paced decision similar to real-world decisions made by humans and other mammals.