Today, researchers have only Harlow's description of the injury and examinations of Gage's actual skull and the tamping rod to provide evidence of the type of injury that was sustained. Several different studies have been conducted to try to determine exactly how much of Gage's brain was affected by the projectile. One 1994 study suggested that both prefrontal cortices were affected, while a 2004 study indicated that the damage was limited to the left frontal lobe. In 2012, a new study estimated that approximately 11-percent of Gage's frontal lobe was destroyed and that 4-percent of his cerebral cortex was impacted.
While we will never be able to tell the exact extent of the damage, we do know that a significant portion of his frontal lobe was damaged.
The frontal lobe plays a vital role in problem-solving, decision-making, and planning. The area known as the prefrontal cortex is associated with the expression of personality. Other functions associated with the frontal lobe including reasoning, judgment, and impulse control.
In Harlow's descriptions of Gage after the accident, he suggests that Gage would often make plans but fail to carry them out and that many of his friends described his personality as greatly changed, to the point that they felt he was "no longer Gage."