The undergraduate calendar states the requirements for PNB 2XB3:
"Registration in an Honours Psychology, Neuroscience & Behaviour or Combined Honours Psychology program"
I believe they set it up so all of the "PNB XXXX" courses can only be taken by students in a specific psychology program, but the "PSYCH XXXX" courses can be taken by any student with the proper prerequisites.
The equivalent PSYCH course to PNB 2XB3 is PSYCH 2NF3, so if you're interested in something neuroscience-based you could look into that course
[Psych courses:
http://registrar.mcmaster.ca /CALEN...12/pg1837.html ]