Various explanations using mentalistic concepts have been proposed for apparitions of all kinds. Gardner Murphy's suggestion that apparitions might be brought on by wishing [Three papers on the survival problem: An outline of survival evidence difficulties confronting the survival hypothesis field theory and survival], and Julian Jayne's explanation of hallucinations as arising from stress [The Origin of Consciousness in the Breakdown of the Bicameral Mind] are examples from psychologists of theories positing mental events. Popular explanations of apparitions in terms of mental events include the theories that they are brought by expecting them to occur, or by vigorous efforts to produce them, or by mental depression (p. 172).Another psychological explanation comes from Carl Jung and his theory of archetypes. Wiebe writes: Archetypes are primordial mental structures that cannot be explained simply in terms of personal experience or the personal unconscious. They are "universal patterns or motifs which come from the collective unconscious, and are the basic content of religions, mythologies, legends and fairy tales"(p. 180 citing Daryl Sharp, C. G. Jung Lexicon: A Primer of Terms and Concepts, p. 29).
Wiebe continues Jung's explanation:
Apparitions, like dreams, are considered to involve psychic content from the unconscious that is forced into our conscious life. Such psychic complexes escape the control of our consciousness and appear and disappear according their own laws. Jung says: "The vision comes in much the same way as a dream, only in the waking state. It enters consciousness along with the perception of real objects, since it is an eruption of unconscious ideas into the continuity of consciousness." . . . Jung explains St. Paul's apparition experience by suggesting that Paul had already been a Christian for some time, only unconsciously. The incident in which he heard the voice speaking from heaven "marks the moment when the unconscious complex of Christianity broke through into consciousness. . . . The complex, being unconscious, was projected by St. Paul upon the external world as if it did not belong to him"(C. G. Jung, The Psychological Foundations for the Belief in Spirits cited in Wiebe, pp. 180-81).While the psychological theories do seem to have great explanatory power, there is a problem, namely, its very hard to prove cause and effect between the proposed mental state and the vision. As Wiebe points out: Establishing the plausibility of the causal claim central to a mentalistic explanation is difficult (p. 183). If this can be accomplished, it will have to be done through neurophysiology and not psychology. Even then, there could be a correlation between the mental state and the vision but not necessarily a cause and effect relationship.
Obviously, there is much more research and analysis that needs to be done. While the believer will express certainty about the reality of his vision or the visions of those whose theology agrees with his, I think its better to remain skeptical until more study is done.