The shudhis are tantric meditative techniques for
withdrawing the consciousness from the lower levels of
mind and concentrating it for the journey to the higher
levels, back to its source.
The first corresponds to shutting down the reticular
activating system (RAS) in the brain stem, so that all
external sensory stimuli are blocked from awareness.
This is done by essentially imagining the void, or the
state of no sensory input. Since the mind cannot tell
the difference between an external image, or an internal
image this leads to the experience of the void.
The second corresponds to blocking the internal sensory
signals originating in the body itself. In this process
the awareness is completely withdrawn into the thalamus,
[are these body signals also processed by the RAS? or
by some other physical organ?]
The third cleans the subconscious and introduces the
thought of the infinite, the goal of the meditation.
Thus the meditation starts from the