The frontal lobe is the anterior most lobe of the brain and is responsible for many brain functions: reasoning, personality and moderating behavior, planning, and problem-solving. When damage occurs to the frontal lobe, such as from a head injury, the symptoms can dramatically change a person's personality and intellect.
One of the most well-known cases of frontal lobe damage is that of Phineas Gage, who suffered frontal lobe damage in 1848 when a tamping iron pierced his skull and brain:
“He is fitful, irreverent, indulging at times in the grossest profanity (which was not previously his custom), manifesting but little deference to his fellows, impatient of restraint or advice when it conflicts with his desires, at times pertinaciously obstinate yet capricious and vacillating, devising many plans for future operation which no sooner are arranged than they are abandoned in turn for others appearing more feasible. His mind was radically changed so that his friends and acquaintances said he was no longer Gage.” —J.M. Harlow, Gage's physician (1868)