Final answer:
The accident damaged Phineas Gage's frontal lobe, resulting in changes in his personality.
Step-by-step explanation:
The part of Phineas Gage's brain that was damaged in the accident was the frontal lobe. The frontal lobe is responsible for many important functions, including personality, impulse control, and planning. When Phineas Gage's frontal lobe was damaged, it resulted in changes in his personality, making him more temperamental and capricious.