Deprivation and Overstimulation – Rhythmic dancing, monotonous singing, chanting and drumming, prolonged fasting, hyperventilation, flagellation and other forms of pain, and light deprivation all mark ways in which shamans may deprive or stimulate the senses in order to induce altered states of consciousness. Such practices are not universal to shamanisms, but they are a consistent feature since such practices facilitate altered styles of communication— enabling intimate relationships with other-than-human persons.
SOURCE:
Historical Dictionary of Shamanism by Graham Harvey and Robert J. Wallis 2007