And yet there are plenty of experiments that show that brain waves continue far longer after the "modern" method than with the exsanguination method. I have yet to see a single study with both procedures properly executed that show anything else.
How was this Johnson guy doing it wrong? Can you link the details of his methodology or whatever you're looking at?
I read the paper years ago, and don't have access to a full copy.
There are four major blood vessels in the neck - two carotid arteries, and two jugular veins. Proper kosher/halal butchery severs all four of these completely and effectively simultaneously (Jewish law in particular specifies that the knife must be at least half as thick as the animal's neck to guarantee this), causing the animal to pass out within seconds and die seconds later. Even if the animal feels the knife in the first place (far from guaranteed - a sharp enough knife leaves an essentially painless cut), it takes so little time that it isn't going to register as pain before the animal passes out. Despite claiming to have "matched" the procedure, the study group does not appear to have complied entirely, with little effort to make the cuts complete or simultaneous. While the sharpness of the knife used is probably without question, it was a smaller knife than that used in traditional butchery, which makes a great amount of difference to how humane the procedure may be. To get proper data, they should have prepared the experiment and had a halal or kosher butcher come in to perform the kills - this would eliminate any errors due to ignorance or bias. Even then, this study shows the animal to usually be unconscious within five to ten seconds, at which point the importance of the pain readings becomes much less important.
More importantly, I can find no evidence the study was ever repeated. A thousand small experimental errors could have slipped into the procedure - anything from a bad restraint that rubbed the wrong way after the cut was made to the way the personnel reacted during the kills could have thrown it off. The scientific method requires reputability to eliminate the possibility of such things, which has not been shown.