I think it would be a mistake to assume surgery was performed. It's perfectly possible the scar is from an incision and nothing else, and the intention is to confuse and intimidate. But putting that aside...

I'm hardly an expert on neurosurgery, but it would have to be the temporal lobe. That area controls the formation of memory, but the surgical techniques don't exist to affect it with enough precision that we'd fail to notice a problem immediately.