Zack is likely to have suffered damage to the "frontal lobe" of his cortex.
The frontal lobe is the piece of the cerebrum that controls vital psychological abilities in people, for example, emotional articulation, critical thinking, memory, dialect, judgment, and sexual conduct etc. It is, in simple words, the "control board" of our identity and our capacity to express ourselves. Hurting this could change the personality as in this case can be seen.