When someone maps a brain, they usually map the top of it, the forebrain, and then describe it as if they mapped the whole thing.
But a large share of patients after a head injury do not complain about the forebrain. They complain about dizziness, imbalance, nausea, a loss of coordination that can make them look almost intoxicated.
That world lives lower, in the cerebellum and the brainstem. The hindbrain.
If a patient cannot walk a straight line, or their eyes drift, or their hands have lost their precision, the answer is often down there, not up top. And a map that never went there will call the case unremarkable.
So we map the hindbrain too, circuit by circuit, because the symptom has to match the finding.
You cannot explain a problem you never looked for. Mapping the whole brain means mapping the part that keeps us upright.