▁has ▁to ▁do ▁with ▁cort ical ▁cent ers ▁responsible ▁for ▁visual ▁processing . ▁Ir rit ation ▁of ▁visual ▁association ▁cort ices ▁( B rod mann ' s ▁areas ▁ 1 8 ▁and ▁ 1 9 ) ▁cause ▁complex ▁visual ▁hall uc in ations . ▁ ▁The ▁second ▁mechanism ▁is ▁de af fer entation , ▁the ▁inter ruption ▁or ▁destruction ▁of ▁the ▁af fer ent ▁connections ▁of ▁n erve ▁cells , ▁of ▁the ▁visual ▁system , ▁caused ▁by ▁les ions , ▁leading ▁to ▁the ▁removal ▁of ▁normal ▁in hib itory ▁processes ▁on ▁cort ical ▁input ▁to ▁visual ▁association ▁areas , ▁leading ▁to ▁complex ▁hall uc in ations ▁as ▁a ▁release ▁phenomen on . ▁ ▁The ▁third ▁mechanism ▁has ▁to ▁do ▁with ▁the ▁ret icular ▁activ ating ▁system , ▁which ▁plays ▁a ▁role ▁in ▁the ▁maintenance ▁of ▁ar ous al . ▁Les ions ▁in ▁the ▁brain ▁stem ▁can ▁cause ▁visual ▁hall uc in ations . ▁Visual ▁hall uc in ations ▁are ▁frequent ▁in ▁those ▁with ▁certain ▁sleep ▁dis orders , ▁occurr ing ▁more ▁often ▁when ▁d rows y . ▁This ▁suggests ▁that ▁the ▁ret icular ▁activ ating ▁system ▁plays ▁a ▁part ▁in ▁visual ▁hall uc in ations , ▁although ▁the ▁precise ▁mechanism ▁has ▁still ▁not ▁fully ▁been ▁established . ▁ ▁Pre val ence ▁ ▁Hall uc in ations ▁in ▁those ▁with ▁psych oses ▁are ▁often ▁experienced ▁in ▁color , ▁and ▁most ▁often ▁are ▁multi - modal , ▁consisting ▁of ▁visual ▁and ▁aud itory ▁components . ▁They ▁frequently ▁accomp any ▁par ano ia ▁or ▁other ▁thought