You can't ethically damage a healthy human brain on purpose to see what happens. For a large part of the history of neuroscience, that constraint shaped how the brain's functional map actually got drawn: rather than starting from careful observation of a healthy, intact brain at work, much of the earliest and most influential evidence about which brain regions do what came from the opposite direction — carefully documenting which specific abilities disappeared or changed after injury, illness, or surgery damaged a particular, identifiable part of the brain.
Losing a specific ability after specific damage was the original clue
Some of the field's foundational discoveries followed this exact pattern. Damage to a specific region of the frontal lobe was linked to dramatic personality change following well-documented nineteenth-century cases of localised brain injury; damage to particular regions in the temporal and frontal lobes was linked, in separate nineteenth-century clinical work, to specific, distinct kinds of language impairment — one affecting a patient's ability to produce fluent speech, another affecting comprehension, even when speech production itself remained fluent. In each case, the underlying logic was the same: if damage confined to a specific brain region reliably produces the loss of a specific ability, while other abilities remain comparatively intact, that's strong evidence the damaged region was playing some meaningfully specialised role in supporting the lost ability.
Modern imaging supplements, but hasn't replaced, this older method
Modern non-invasive brain imaging techniques now let researchers observe patterns of activity in a healthy, intact brain performing specific tasks, adding a genuinely different and valuable source of evidence to the field. But the older method — studying what specific abilities are lost after damage to a specific brain region, now often supplemented by imaging in surviving patients — remains a significant and complementary source of evidence, because it demonstrates something imaging of healthy brain activity alone can't fully establish on its own: whether a given brain region is actually necessary for a given ability, not merely active alongside it. A brain region lighting up on a scan during a task shows correlation between activity and behaviour; a documented case where damage to that same region reliably eliminates the ability provides stronger evidence the region is actually required for it.
What we're still unsure about
The general historical pattern — that studying ability loss following localised brain damage has been a foundational and continuing source of evidence about brain function — is well documented across the history and practice of neuroscience. What's harder to establish with full confidence from any single case is how cleanly a lost ability maps onto exactly the damaged region, since real brain injuries rarely respect the tidy boundaries between named anatomical regions, and the brain's genuine capacity to reorganise and partially compensate after injury means the relationship between a specific damaged region and a specific lost function is often more complex, and less absolute, than early case studies sometimes suggested.
This sits inside Brain Structure & Function (Lobes, Limbic System), one of seven topics in Neuroscience, one of four domains in Psychology, one of seventeen subjects the app can quiz you on.