Trance is a phenomenon which has received different denominations over the course of time. In societies without the State we find a form of trance practised by shamans: it is a trip out of the self during which, however, a kind of consciousness is maintained.
The possession trance, on the other hand, is characterized by the invasion of a different personality. The possessed speaks with the voice of the god or of the spirit which possesses him.
These phenomena have been studied on the field by the ethnographers and discussed by anthropologists, physiologists and psychiatrists.
The research coordinated by De Martino on the "tarantism" in the region of Salento (Southern Italy) is considered of topical importance. It is crucial to discriminate the phenomenon of trance from that of ecstasy.
It seems that trance can be shared by several persons; this raises problems of group or of mass. Freud, in his study in "Mass psychology and analysis of the Ego", spoke of a collective mental state produced by the mass. These mental states would be the effect of dissociations.
Dissociation is an important concept used first by Moreau de Tours in his book on hashish, then by Charcot and Janet to explain hysteria. Also the young Freud used dissociation in this sense, before adopting the term "repression".
We may also speak of instrumental repression, as Bleger does, that is, the ability to identify oneself and at the same time to perceive oneself identified while maintaining consciousness.
Cyberspacial dissociation will also be discussed, and the possibility of being possessed destroying the consciousness or of "tripping" and widening consciousness.