How do motivations impact our decision making? It’s a pretty important question that neuroscientists would answer the same way as everyone else: motivations somehow amplify our responses to goal-relevant stimuli. But how? Until now the only answer was the standard one: by amplifying the brain signals derived from the external stimuli (e.g., more/stronger food signals when you’re hungry).
But you can alter the impact of a signal without increasing the signal itself. We show that this how motivation biases decisions male flies make while mating (to persist or to split when conditions get rough). As the mating goes on (it lasts 23 minutes), the male becomes more and more susceptible to ending it early when the environment gets dangerous (i.e., hot or windy). We find no evidence that the male does this by increasing the signals derived from the heat or wind. Instead he increases the time signals derived from those stimuli linger in decision-making neurons.
We sometimes think of this decision as a bucket with holes in the bottom. One way to fill the bucket (i.e., end the mating) is to increase the rate that water flows into it—but another way is to plug the holes at the bottom so that the water lingers and accumulates. Since the severity of environmental cues is already playing with the faucet (is it hot or HOT?), having motivation work on those same controls could get messy (do you want it to seem hotter than it actually is based on how motivated you are to do something?). Having motivation plug (or unplug) the holes in the bucket has a lot of advantages and works just as well! So we think that scaling the temporal integration of decision-relevant inputs (the time the water lingers in the bucket) will wind up being the most common way for motivations to impact decision-making.