We use them to gather information about ourselves in the world: our physical positioning (proprioception), what's going on around us (exteroception), inside us (interoception), and basically everything (like if we're thirsty or itchy) including balance (equilibrioception) and so much more... all at the same time. 

All day our brains combine and sort all that sensory information into what it perceives as relevant or irrelevant; to process or not to process? That's the question we're always trying to answer and decide, whether consciously or unconsciously.

The decision for whether or how we'll process any one bit of information (or not) may be based on previous combinations of sensory data (experiences) categorized by the brain as relevant or irrelevant at the time of occurrence. 

Now when it comes to threatening or unpleasant stimuli, it seems the brain not only weighs its decision to respond/not-respond on the degree of perceived threat, but it also weighs in the the actual intensity of a stimulus + the combinations of other stimuli. The more neurons lit up by a stimulus = the greater the intensity.

If the intensity of this sensory information —in addition to its perceived relevance— becomes intense enough to cross a threshold, that triggers an action/behavior or reaction . That trigger-to-act is  what we call a decision.

This is different than a reflex, which is a reaction we don't get the chance to "think" about because it's processed in the spinal cord (not the brain). 

Example:

In order, to get a water wheel to turn one way or the other (make a decision), you need to funnel a minimum/maximum volume of water over it at a certain velocity and direction to get it to move, or keep it in motion. More water= a faster turning water wheel= more synapses firing.

What if water (sensory stimulus) is being funneled from other directions? The stronger funnels win and weigh more into the decision-making process.

Different behaviors involve the initiation of many different neurological micro-decisions, filtered thru various brain regions for relevance.

Let's say you weren't expecting any visitors at home today, but then you hear the doorbell. If it rings once or twice, you'll probably just ignore it.

However, if it keeps ring-ring-ringing or if whoever-it-is presses and holds it down for one long annoying ring, either of those stimuli will eventually be enough to initiate a behavior. You may get up to go to the window to see who it is, or decide to yell who is it and what do you want?!

Our behavioral decisions are routed, sort of like that. They're dictated by the provocation and intensity of several different layers of sensory input and/or experiences that have been deemed relevant.

At least that's what I gathered from this Oct 2020 study involving flies. You'll find it linked to the Meta Stuff buttons on this page.

Yes, I know we aren't flies (like Jeff Goldblum when he played Seth Brundle in The Fly), but it's still cool.

Maybe the flies can lead us to understanding ourselves better in some way? Because they're already helping to develop AI based on this.

Please correct me if I'm wrong, thanks :)