with potent strains at high doses i have gotten mild psychedelic-type visuals like faded geometric patterns, "sparkles" when i stare, surfaces "ripple", and other things. these effects aren't strong though and are most noticable at peak effects(about 5-10 min after smoking).
it is somewhat similar to low dose shrooms(but still a bit less visual), and visuals are more common with closed eyes. once again this only happens with potent weed at high doses.
this also happens more often of you've used strong psychedelics like LSD a few times or more in your lifetime. if i smoke strong weed the day after tripping on acid, morning glory seeds, or mescaline i definately get some visual activity.