Pratyahara is derived from two Sanskrit words: prati and ahara, with ahara meaning food, or anything taken into ourselves, and prati, a preposition meaning away or against.
Pratyahara or the ‘withdrawal of the senses’ is the fifth element among the Eight stages of Patanjali’s Ashtanga Yoga, as mentioned in his classical work, Yoga Sutras of Patanjali composed in the 2nd century BCE. It is also the first stage of the six-branch yoga (ṣaḍaṅgayoga) of the Buddhist Kālacakra tantra, where it refers to the withdrawal of the five senses from external objects to be replaced by the mentally created senses of an enlightened deity. This phase is roughly analogous to the physical isolation phase of Guhyasamāja tantra.
For Patanjali, it is a bridge between the bahiranga (external) aspects of yoga namely, yama, niyama, asana, pranayama, and the antaranga (internal) yoga. Having actualized the pratyahara stage, a practitioner is able to effectively engage into the practice of Samyama. At the stage of pratyahara, the consciousness of the individual is internalized in order that the sensations from the senses of taste, touch, sight, hearing and smell don’t reach their respective centers in the brain and takes the sadhaka (practitioner) to next stages of Yoga, namely Dharana (concentration) and Dhyana (meditation), and Samadhi (mystical absorption), being the aim of all Yogic practices.
From Wikipedia Article Pratyahara
An isolation tank is a lightless, soundproof tank inside which subjects float in salt water at skin temperature. They were first used by John C. Lilly in 1954 to test the effects of sensory deprivation. Such tanks are now also used for meditation and relaxation and in alternative medicine. The isolation tank was originally called the sensory deprivation tank. Other names for the isolation tank include flotation tank, float tank, John C. Lilly tank,REST tank, and sensory attenuation tank.
The flotation tank was developed in 1954 by John C. Lilly, a medical practitioner and neuro-psychiatrist. During his training in psychoanalysis at the US National Institute of Mental Health (NIMH), Lilly commenced experiments with sensory deprivation. In neurophysiology, there had been an open question as to what keeps the brain going and the origin of its energy sources. One hypothesis was that the energy sources are biological and internal and do not depend upon the outside environment. It was argued that if all stimuli are cut off to the brain then the brain would go to sleep. Lilly decided to test this hypothesis and, with this in mind, created an environment which totally isolated an individual from external stimulation. From here, he studied the origin of consciousness and its relation to the brain.
Peter Suedfeld and Roderick Borrie of the University of British Columbia began experimenting on the therapeutic benefits of flotation tank usage in the late 1970s. They named their technique “Restricted Environmental Stimulation Therapy” (REST).
From Wikipedia Article Isolation Tank