When the surface of the brain is bruised, it responds fast. Within hours and days, and by about ninety days most of that surface change has run its course.
But the story does not end there. The circuits that fed into the injured area begin to wither later, on a delay.
Think of roses cut from the root. Nothing looks wrong the moment they are cut. The bloom is still bright. Yet the clock is already running, and the wilting comes.
Deep structures behave the same way. They are not damaged at the moment of impact. They are waiting for the surface injury upstream to declare itself, and then they slowly shrink over months.
This is why a brain injury cannot always be captured in a single look. Part of it is late by design.
Understanding that delay is the difference between calling a study reassuring too early and knowing to look again.