When we are asleep and dreaming, our brain typically fires nerve impulses in the delta wave state at 1 to 3 cycles per second or hertz (Hz). When we awaken, the brain typically moves rapidly towards alpha brain waves that fire at 8 to 13 Hz. This is a resting state in which we are awake and aware, but we’re not doing anything inparticular. When we have our first espresso of the day, our brain waves move into beta-wave states that fire at roughly 13 to 20 Hz. These are high frequency brain waves in which we concentrate, do our work, and in which we function for most of our waking reality….
Below the alpha state and above the delta state of sleep is an in-between zone commonly called the theta wave state in which the brain typically fires nerve impulses between 4 and 7 Hz. This is a deep, reflective dreamlike state that has been recorded in Zen masters and transcendental meditators, in psychics and trance mediums, and in shamans in altered states of consciousness.
In the theta state, the visionary experience is easily accessed….The drum or rattle is of great service in accessing this visionary state of consciousness. As we listen to the monotonous percussion of the drum or the dry whisper of the rattle, beaten or shaken at four to seven beats or shakes per second, both halves of the cerebrum entrain to this rhythm, essentially following the sonic driving of the drum or rattle.
In response, both halves of the brain synchronize, slow down, and start firing impulses at 4 to 6 Hz., thus allowing an individual to enter the theta state of light trance. It is precisely here that we may find that numinous space between the worlds.”
Sandra Ingerman in Awakening to the Spirit World
Isogaisa 2014 Russian Sami Shaman Group