The visual system is very complicated, and many different classes of drugs can cause hallucinations in different ways without the overall experience being similar.

Ketamine and LSD do not have similar mechanisms in the brain, nor (from what I've read) are their effects qualitatively similar. LSD is a psychedelic acting as an agonist on 5-HT_2A receptors (among other things, but that's what it shares in common with other classic psychedelics. Ketamine is a dissociative anesthetic acting as an antagonist on NMDA receptors. LSD is, however, effective against migraines at sub-hallucinogenic doses.