There's a simple cognitive test that humans tend to fail— at least, they fail when their performance is compared to the performance of rats. Why? Because our brains screw us up. But a specific brain injury can bring us up to rat-level.
The two halves of your brain are in constant communication. Severing that communication can have dramatic consequences, but in the 1950s, researchers believed that doing so could be used to treat severe forms of epilepsy. They called the procedure a corpus callosotomy.