Swollen Brain is a mutation hidden within the sewers beneath freedom city, and is mutually exclusive with Brute strength. It raises your brains and senses at the cost of cool, and is also a pre-requisite to become an empath.
Swollen Brain is located in the sewers beneath Freedom City. The easiest way to reach the mutation is to head straight west on Pike then head down into the sewers from Pike Street station. Do note that to reach the mutation spot you need someone to flip a nearby switch, so bring a friend or pay someone (or pay a friend if they don't really like you).
This mutation grants:
If you have an interest in crafting or piloting stuff, take this mutation. Vampires and Zombies also may consider taking this mutation in order to offset the brains lost by taking those mutations (a hefty -4 and -6, respectively), though some find the min-maxing from brute strength more appealing. If you're a combat character that doesn't rely heavily on brawn for their damage, you might consider Swollen Brain over Brute Strength, since it helps with feints and buffs useful skills like teach, repair, and pilot.
Taking Swollen Brain results in a -1 to cool and prevents you from taking the brute strength mutation, which is often more favorable for more combat-oriented builds.
Taking Swollen Brain results in a bump appearing on the user's forehead when it is uncovered.