The damage to Phineas Gage's brain
Within weeks of Phineas Gage's accident differences of opinion emerged among those who examined him about the extent and location of the damage to his skull and brain. Over time these differences increased. There are two problems: first, can the path of the tamping iron be estimated accurately from the damage to his skull, and, second, can the damage to his brain be inferred from that path?
Differences over the damage to the skull
There are three places where Gage's skull is damaged. There is a relatively small area under the zygomatic arch (or cheek bone) where the tamping iron first entered his head. The second place is the orbital bone of the base of the skull behind and below the eye socket. After healing, this area is about 1 inch wide by 2 inches in the anterior-posterior direction and its size at the time of injury is difficult to estimate by eye alone (upper left of a, below). The total area of bone damage caused by the tamping iron where it emerged is truly enormous. As can be seen in (b), there is an unhealed irregular, roughly triangular shaped area of total bone destruction at the top of the skull. Lying mainly to the left of the midline, it is about 2 inches wide and 4 inches in circumference, and there is another on the lower left side about 2.7 inches in circumference. Between them there is a flap of frontal bone about 2.5 inches long and about x 2 inches wide at the widest point (c). Behind the main area is a second flap of parietal bone about 2 wide and 0.75 to 1.5 inches long. Harlow replaced both flaps; the parietal (rear) reuniting so successfully that it is actually difficult to see from outside the skull. The image in (c) had been made by copying a photograph of the inside of the skull and 'pasting’ on the outside.
In summary, the main injury to Gage's skull was at the exit, where the tamping iron created an irregular area of damage about 3.5 inches long and 2 inches wide. The main problem in estimating the trajectory of the iron is to know exactly through which part of each of these areas the iron passed. It is a problem that is most acute for the exit area on the top of the skull.
Differences over the passage of the tamping iron
John Martin Harlow and Edward Higginson Williams, the two physicians who saw Gage on the day of the accident, said nothing specific about the entry under the zygomatic arch or the damage at the base, but Harlow was definite that the tamping iron had emerged at the junction of the coronal and sagittal sutures, and in the midline. Phelps, who examined Gage six weeks later, thought the point was about 0.5 inches in front of the junction and 1 inch to the left of the mid line.
Henry Jacob Bigelow apparently drew no conclusions from his examination of Gage late in 1849, about a year after the accident, but when he drilled holes through a demonstration skull to show that the passage was possible, he may have arbitrarily placed the centre of the hole in the base 1 inch from its midline, and that of the exit to the front of the junction and to the right of the midline.
The differences were not resolved when Gage's skull was brought to Massachusetts in 1868. Harlow placed the entrance in the base of the skull 1.25 inches from the median line. He was now rather less certain about where it emerged saying only that it was to the front of the junction and in the midline. Eugene Dupuy, in criticising the concept that different functions were localised in different parts of the brain, used John Barnard Swets Jackson's 1870 description, and possibly the photographs, from the Catalogue of the Warren Museum to conclude it had emerged frontally and to the left of the midline. After later seeing the skull itself, he seemed to maintain the left sided point of emergence but now moved it to behind the junction. In developing his reply to Dupuy's criticism of his work on localisation, David Ferrier first judged Bigelow's placement of the entry in the base to be too far the left of the midline, and the exit too frontal and too far to the right. In an important later discussion he omitted mentioning an exit point; it was almost as if he had given up trying to determine one.
Stanley Cobb prepared a sketch in 1940 on the basis of a visual examination of the skull. He implied that the exit was between the frontal bones and that some right-sided damage had been caused.
Rick and Ken Tyler concluded from their 1982 CT study that all that could be specified was a range within which the tamping iron could have emerged. As can be seen on their scan below (d), it included the area of total destruction so that the small area to the right of the midline marked one extreme limit of that range.
Hanna Damasio and her colleagues used a more complex computer based method in 1994. They first made a computer model of a skull like Phineas' by 'deforming' the image of a normal skull. They then located an a priori most likely exit point at the top of skull and identified other possible exit points around it. These points all fell within half of the tamping iron's diameter from the edges of what they called the area of total bone damage, but which, in their case, excluded the rear flap. They then projected likely trajectories from these mainly right frontal points through the centre of the hole in the base to the entry area under the cheek bone. Each possible trajectory was examined for its compatibility with known anatomical damage, and seven were considered viable. On further examination two of these were rejected leaving five to be used in estimating the brain damage. A notable peculiarity of the exit points generated by the five trajectories arrived at by Hanna Damasio and her colleagues is that, as can be seen on (f) above, all of them lie to the right of the midline and so far forward of the junction as to have the tamping iron emerge under the reunited but otherwise undamaged frontal flap.
Peter Ratiu and his colleagues compared computer-generated three-dimensional reconstruction of a thin-slice computed tomography scan (CAT) of Gage's skull with the actual skull, rather than a model of it as had Hanna Damasio and her colleagues. They paid especial attention to two aspects that had not been considered by previous workers. First, there was a continuous line of fracture that began from the parietal area well behind the back of the area of bone loss on the top of the skull and ran down to the lower left side of the jaw (e). Second, they noted that the area of bone loss at the entrance and in the eye-socket was smaller than the maximum diameter of the tamping iron. On this view, the skull must have hinged open to allow the iron to enter and then have been closed by the contraction of the soft tissue of the head after it had emerged through the hole in the unhealed area at the top of the skull.
Jack Van Horn and his colleagues have since verified that the damage was left-sided.
Differences over the damage to the brain
The different views over the passage of the tamping iron naturally led to different views about which parts of Gage's brain were damaged. Put simply, perhaps over simply, the further to the left of the midline of the base and the further to the right of the midline at the top, the more of the right lobe would be involved. Similarly, if the exit point is placed in front of the coronal and sagittal junction, the greater the frontal involvement; if behind it, the frontal damage is less.
Harlow concluded that only the left hemisphere had been affected and that the right was 'intact.' Bigelow was equally clear that there was some right-sided damage. Dupuy accepted that the trajectory was left sided but placed it less frontally; claiming that the more posterior motor and language areas should have been destroyed. That Gage had no motor impairment or aphasia was prime evidence for Dupuy's anti-localisation arguments. In rebuttal, Ferrier showed fairly conclusively that the passage was not so posterior and that both motor and language areas had been spared. He also concluded that the only damage was to the left hemisphere, a conclusion that seems not to have been disputed for about 70 years.
Cobb's diagram, the first of the 'modern' reconstructions, showed damage rather like that supposed by Bigelow: both hemispheres were damaged with the left suffering more than the right. That conclusion, although less detailed, is broadly congruent with that of the Tylers. When Hanna Damasio and her colleagues simulated the passage of the iron along their five selected trajectories through a three dimensional model of Gage's brain, they found the damage to be even more frontal and right sided. Ratiu and his colleagues concluded that the damage was limited to the left frontal lobe, did not extend to the right side, and did not affect the ventricular system or vital blood vessels inside the skull -- conclusions that seem to provide the most likely reconstruction of the damage. Van Horn and his colleagues stress the left-sided nature of the damage.
What to make of the differences?
The disagreements have a number of different bases, not the least of which is what David Ferrier called “the inexactitude and distortion ... by men who have some pet theory to support.” But, even were there no bias and it were possible to determine the path of the tamping iron exactly, there was more damage than that caused by its passage. As the Tylers noted, hemorrhaging and damage from it was massive, the iron pushed fragments of bone through the brain that were not recovered (Gage was still vomiting some of them several days after the accident), and a massive abscess took further toll. Further, almost all the above methods, especially that used by Hanna Damasio and her colleagues, make the unlikely assumption that Gage's brain and its position within the skull can be estimated from the skull itself, and that its functions were localised in an average way.
Suppose all these problems were overcome. We would still not be able to learn much more from Gage's case than that the massive injury to his brain caused a massive change in his behaviour. Some 160 years after the accident we know only a little more about Phineas' pre- and post-accident behaviour than Harlow told us in 1868. What we do know now is not enough from which to infer any detail about the localisation of behavioural functions in the behaviour. Phineas Gage's case is important for what it pointed to, including the possibility of a reasonable psychosocial adaptation, rather than what we can learn of the details about the relation between brain and behaviour.