A summer meadow surfaces in memory not just as a picture — along with it come the scent of grass and the warmth of the sun. The brain glues sensations into a single image, and that image holds up better than any one sensation alone. But exactly how neurons stitch the visual to the olfactory has long been a mystery.

Biologists took fruit flies and trained them to associate specific colors and odors with a pleasant event (sugar) or an unpleasant one (electric shock). Then they tested memory: flies trained on both features at once remembered both the color and the odor better than flies trained on just one. Multisensory experience doesn't just add up impressions: it strengthens each one.

The key to the puzzle was found in the mushroom body, the insect's memory center. There, Kenyon cells (named after the scientist who discovered them) are usually divided: some respond to odors, others to visual images. After training with both color and odor, the visual cells began to help recall the odor, and vice versa. It's as if a switch was laid between two railway lines.

That switch, it seems, is built by serotonin neurons called DPM. They spread widely through the mushroom body and can connect previously separate streams. And what allows them to do this is dopamine — a substance released during reinforcement (sugar or electric shock). Dopamine temporarily lifts the brake on neighboring APL neurons, and the bridge between memory zones becomes passable. Thus, color weaves into the memory of smell, and smell into the memory of color.

Memory stops being a narrow path and becomes a wide road. A single cue, a familiar scent, can bring the whole scene to light, complete with colors and the feeling you had. That's why a meadow from childhood comes alive so vividly the moment you catch a whiff of cut grass.