What does it mean that schemas shape what you can perceive?

Your schemas determine what you notice and what you miss.

Example: A radiologist and a first-year medical student look at the same chest X-ray. The radiologist sees a subtle opacity in the left lower lobe and immediately suspects early pneumonia. The student sees a gray image. The X-ray hasn't changed. The photons hitting their retinas are identical. The difference is the schema each person brings to the image. The radiologist's schema for 'normal lung tissue' is so refined that deviations from it pop out automatically — the way a typo pops out to an experienced editor. The student doesn't have that schema yet, so the deviation is invisible. It's not that the student isn't looking hard enough. It's that their perceptual system doesn't know what to look for.

Try this: Pick a domain you know well — your profession, a hobby, a subject you've studied deeply. Now find someone who knows nothing about it and show them the same stimulus you'd evaluate (a code review, a wine, a financial statement, a piece of music). Ask them what they notice. Write down their observations next to yours. The gap between the two lists is a direct measurement of how much your schema shapes what you perceive. Then flip it: find a domain where you're the novice. Notice what you can't see.