Personality regression is an uncommon and spontaneously occurring effect which often accompanies ego death. It can be described as a mental state in which one suddenly adopts an identical personality, set of mannerisms and behaviour to their past selves from a period of their life which has already occurred.
This is often capable of making one believe that they are a child again and act appropriately to this belief. There are also anecdotal reports of people speaking in languages which they have not used for many years under the influence of this effect.
Compounds within our psychoactive substance index which may cause this effect include: