Hallucinations and Illusions
Kornfield (1979, 1983) noted that there was a strong correlation between student reports of higher levels of concentration during insight meditation, when the mind was focused and steady, and reports of altered states and perceptions. He reported that unusual experiences, such as visual or auditory aberrations and hallucinations, and unusual somatic experiences, are the norm among practiced meditation students. Walsh (1978) reported that he experienced hypnagogic hallucinations, and Goleman (1978-79) reported visionary experiences during deep meditation. Shimano and Douglas (1975) reported hallucinations similar to toxic delirium during zazen.
... Earlier, Deikman (1966a) reported that during meditation on a blue vase, his subjects' perception of color became more intense or luminous, and that for some of them the vase changed shape, appeared to dissolve, or lost its boundaries. Maupin (1965) reported that meditators sometimes experience "hallucinoid feelings, muscle tension, sexual excitement, and intense sadness."
The contemplative literature contains numerous descriptions of the perceptual distortion produced by meditation. It is called makyo in Zen Buddhist sources, and is characterized in some schools as "going to the movies," a sign of spiritual intensity but a phenomenon that is regarded to be distinctly inferior to the clear insight of settled practice. In some Hindu schools it is regarded as a product of the sukshma sharira, or "experience body," in its unstable state, and in that respect is seen to be another form of maya, which is the illusory nature of the world as apprehended by ordinary consciousness.
In a similar manner, St. John of the Cross described the false enchantments that may lure the aspirant in prayer, warning that "devils may come in the guise of angels."  In his allegory of the spiritual journey, The Pilgrim's Progress, John Bunyan described Christian's losing his way by following a man who says he is going to the Celestial City but instead leads him into a net. In all the great contemplative manuals, one is taught that detachment, equanimity, and discrimination are required for spiritual balance once the mind has been opened and made more flexible by prayer and meditation. Illusions and hallucinations, whether they are troubling or beatific, are distractions—or signposts at best—on the way to enlightenment or union with God.