Your hand rests on the table, and two short vibrations arrive at your wrist — one after the other. You need to say which is stronger. The task seems simple, but the answer depends on which vibration came first. If the first is weak and the second is strong, the difference feels larger than when they are swapped. This isn't a quirk of attention but the work of a constant mechanism: the brain doesn't wait for complete information; it forms a guess and corrects it on the fly.
A new model describes this process as a pair of 'measurement plus prediction.' Each stimulus arrives with noise, like a voice on a bad phone line. The brain blends this noisy signal with what it expected to hear. And the expectation stretches from the past: the trace of the first stimulus doesn't vanish instantly but fades smoothly, like an echo in a hallway. So the second measurement lands on the still-warm background of the first, shifting the final estimate. The model reproduced both the direction of the shift and its variability across people, fitting just a few numbers — the noise level and the decay rate of the expectation.
The most surprising finding: each participant had their own internal coordinate system of perception. If you plot physical stimuli on a ruler, in the brain's guess-space that ruler bends, and distances between points become uneven. But in this curved space, comparisons follow almost symmetric rules that don't exist in the real world of vibrations. The brain seems to redraw the map of stimuli to make decisions easier.
It turns out that even the simplest comparison of two touches is not a frozen imprint but a little story with a sequel. The first event sets the tone; the second sounds different simply because the listener still remembers the previous note. And each listener has their own ear.
The order effect works not only for touch: if two loud sounds are separated by a fraction of a second, the first almost always seems quieter — the brain manages to 'turn up the volume' with expectation.