In the case of Tan, while physicians were aware of a prior brain injury causing speech impairment for the patient, they were unable to localize it within the topology of the brain. Moreover, since all other cognitive functions of the patient remained unaffected after the injury, the task of drawing up the diagnosis and prognosis became difficult. It is only after the death of the patient that the brain autopsy revealed lesions in areas of the brain now identified to be the speech centers. (Stirling, 2002)
In contrast, in the case of Phineas Gage, the exact location of the injury was known. During a rock-blasting operation, foreman Gage’s skull was pierced by a thick iron rod above the left-prefrontal cortex and had exited through the left cheek bone. The miracle of his survival apart, Gage even managed to regain most of his functions over the next 12 years of his life. But the behavioral changes witnessed in him by close friends and colleagues indicate core personality changes. The most valuable information for neurologists and psychologists is the changes in behavior that he exhibited just after the injury – they all testify how he had lost his social skills. This manifested in recklessness, impulsivity, use of profane language, lack of manners, etc. (Stirling, 2002) In effect, Gage lost most functions of self-control and deliberation. This is consistent with the modern understanding of the role of left-prefrontal cortex, which is responsible for executive functions like delaying gratification, decision making, deliberate thought, etc.
So we see a clear differentiation between the natures of the two cases. In Tan, while the location of the injury was not known beforehand, the post-mortem analysis helped localize the speech impairment of the patient. In contrast, in the case of Phineas Gage, the locality of the brain injury was known beforehand, facilitating understanding of the role of left pre-frontal cortex in the personality make-up of an individual.