We can understand the functions of the brain’s different parts by observing the consequences of brain damage.

Imagine you’re an engineer, and you’ve been given a complicated machine. Your job is to figure out how it works. As you examine it closer, you see that it has many different parts working together in seemingly mysterious ways. So, how would you go about it?

Well, you could start by removing one of the parts and seeing what happens. If you take out thingy X and the machine stops making sparks, you could conclude that thingy X has something to do with spark production. Repeat the same procedure with the rest of the parts, and you can gradually figure out how the machine works.

The same logic applies to understanding the machinery of the human brain – with one important caveat.

Of course, it would be extremely unethical to go into someone’s brain and remove a part just to see what happens. Fortunately, for science, we don’t need to.

Brain-damaging injuries, tumors, and diseases can sometimes affect specific regions of the brain. And, if they happen in just the right way, they can more or less wipe out a single part of the brain without damaging anything else – almost as if an evil scientist came along and scooped it out with a scalpel.

Assuming the victim survives, his brain will continue to function, but in a different way from before. For instance, damage to a part of the brain called the third frontal gyrus leads to a language disorder known as aphasia. People suffering from this condition struggle to understand or express themselves in speech. Evidently, the third frontal gyrus plays an important role in the brain’s ability to process language.

In this way, by comparing how the brain functions before and after one of its parts have been damaged, we can start mapping out the role that part normally plays in the brain’s overall machinery. This is how the field of experimental neuropsychology approaches the study of the brain.