Reply To: Which channels to use for study that visual and auditory corteces are involved

P3 and P4 are not used as normal channels – they are the CMS (reference point) and DRL (feedback noise cancellation point) respectively and they are not monitored. It may be better for your experiment to use the mastoid locations as references instead of P3/P4 – move the sensors the default location to the sockets behind the ears.
Auditory and visual signals will be received in the temporal and occipital lobes respectively, and processed along with other sensory inputs in the parietal area. So your starting point should be those sensors – T7, P7, O1, O2, P8, T8, but note that other frontal and prefrontal locations can also show responses, so don’t discount them…