Here is one of them:
I can't find part 1 of the study. And there was another where the control groups and etc were very detailed if I can find it. Another lady on this forum posted a couple of links as well (which is what got me started on my reading journey.)
http://lan.sagepub.com/content/31/1/1.full.pdf
Even IF the brain is capable of processing pain and fear for the last 20-30 seconds before death, that is the ABSOLUTE LEAST of the worries of a lab animal kept for testing purposes!
There is a big difference in using chemicals to sedate a lab animal before killing and using the same chemicals for a food animal.
A quick brain death through brain trauma, cervical dislocation, cervical severing, or even a quick slice of the major blood supply to the brain are all the best death you can give a food animal.