A horse can spend up to 16 hours a day chewing hay, pretty much its entire life. One bite of banana just taught its brain that the hay is kind of garbage.
That's barely an exaggeration. Grass hay is only about 5 to 11% sugar. Pull the water out of a banana and it's close to half sugar. For a horse, that's like eating plain rice cakes every day of your life and then somebody hands you a donut.
That little freeze is the horse's brain going "wait, what?"
A neuroscientist named Wolfram Schultz figured out what happens in there back in the '90s. He recorded individual dopamine cells in monkeys while giving them drops of juice. When the juice was a surprise, those cells lit up. Once the monkeys got used to it and started expecting it, the cells barely reacted. And when the juice they were expecting didn't show up, the cells dropped below normal.
So dopamine works more like a surprise alarm. It fires when the world turns out better than your brain predicted, and then your brain quietly raises its expectations to match.
That reset is bad news for the horse.
Back in 1928, a psychologist named Otto Tinklepaugh ran a kind of mean experiment on monkeys. He let them watch him hide a piece of banana under a cup, then swapped it for lettuce when they weren't looking. These monkeys normally ate lettuce no problem. They flipped the cup, looked all over the place for the banana, and wouldn't touch the lettuce.
Rats do it too. In 1942 Leo Crespi got rats used to a big reward and then cut it down to a small one, and they ran slower than rats who'd only ever gotten the small one. Later studies with sugar water found the same thing. Drop a rat from sweet water to weaker water and it drinks less than a rat that never knew the sweet stuff existed, starting the very first day. Psychologists call it successive negative contrast, which is a fancy way of saying once you know better, the regular stuff gets harder to enjoy.
A banana weighs about a quarter pound. A horse eats something like 20 pounds of hay a day, and all that research says the quarter pound just made the other 20 a little less worth eating.
If you've ever flown lie-flat once and then gone back to a middle seat, you know exactly what this horse is having for dinner tonight.