The Foundations of the Float Tank

In 1954, John Lilly built a floatation tank at the National Institute of Mental Health to ask what the brain does when you take away the outside world: light, sound, temperature, gravity. Floating in the dark, the mind wandered off: first in a dream-like way, then into full-fledged hallucinations for those who stayed in long enough.

Seventy years later, how float tanks affect consciousness remains largely anecdotal, a line in Lilly’s memoirs about floating with ketamine or convincing pioneers such as Richard Feynman to enter his Malibu tank.

Clinical Populations Enter the Tank

Our paper, out now in Neuroscience of Consciousness, does something paradoxical and bordering on absurd: take treatment-seeking adults with clinical anxiety and depression and leave them alone with their thoughts for an hour. Rinse and repeat six times.

Seventy-five people were randomly placed into three groups: one that floated six times on a fixed one-hour weekly schedule, a second that floated six times but picked its own session length and spacing, and a third that reclined in a zero-gravity chair. That third group, our active control, isolated the water component of the floatation tank. All three lay in a dark, quiet room, so the water was the differentiator: buoyancy, water and air held at skin temperature, and the disappearance of the feedback you usually receive from your body while lying down or walking. We administered a variety of affect and anxiety measures to each of these groups before and after the sessions and at the beginning and end of the trial.

A scale borrowed from the land of James and psychedelics

At the suggestion of our co-author and Principal Investigator Dr. Justin Feinstein, we also administered the 5D-ASC, a scale built for psychedelic research whose intellectual lineage runs back to William James’ The Varieties of Religious Experience (1902). This questionnaire was part of an attempt to answer the question Lilly posed: if something is happening in the tank, what is it, and what is it for?

One dimension separated the two floatation groups from the chair, repeatedly: Oceanic Boundlessness. The float groups scored roughly double the chair group on it, and it was the largest group difference in the study. Rescoring the same items with an eleven-factor framework (the 11-ASC) showed where floatation’s effects took place, specifically in Disembodiment, Experience of Unity, and Spiritual Experience.

We call this state aquahenosis, or ‘unity with water’ (a term coined by co-author Joel Frohlich; more on this is in his recent blog post). We also found from this questionnaire that the tank was not uniformly pleasant: a negatively-valenced dimension, Anxious Ego Dissolution, went up too.

An embodied-disembodied paradox

We also measured interoception, the sense of your internal organs (think of noticing your stomach growling—that too is a sense, but a different one from the five classic senses). We looked at interoception because floating has already been shown to make people attend more closely to their bodies, and because interoception goes awry all over psychiatry; the sweaty palms and pounding heart of a panic attack, or the bodily preoccupation that comes with OCD. 

This effect reproduced in our cohort: participants in the floatation tank listened to their bodies more. Body Listening and Emotional Awareness from the Multidimensional Assessment of Interoceptive Awareness (MAIA) both rose, and ratings of breath and heartbeat intensity went up enormously. 

Yet what about the body dissolution uncovered by the ASC? How do we reconcile floatation making participants pay more attention to a body that is simultaneously expanding and dissolving?

Our guess is that these aren’t opposites, but rather a sequence. Early in a float, there is nothing else to attend to—no music, no colors and shapes hitting our retina—so attention goes inward. Sustained attention to this unchanging signal makes it less salient over time, until it thins out completely to ‘Oceanic Boundlessness.’ A similar interpretation has been recently hypothesized in body-scan meditation as well.

Mediation of the floatation experience

As a follow-up analysis, and to answer the ‘what for’ question, we ran mediation tests on every phenomenological (5D-ASC) and interoceptive (MAIA) variable against affect, anxiety, and depression. Only one mediator survived correction: Oceanic Boundlessness. It mediated improvements in positive affect and clinician-rated depression. Anxious Ego Dissolution, also heightened in the tank, mediated nothing, which is worth pausing on: In psychedelic trials, Anxious Ego Dissolution predicted bad outcomes. In the tank it went up, yet it did not affect clinical outcomes in the same way that its positively-valenced counterpart, Oceanic Boundlessness, did. Thus, the answer to Lilly’s question is that a floatation tank will sometimes produce a particular state of aquahenosis that does real therapeutic work.

A psychedelic feeling without the psychedelic

Compared against publicly available data on psilocybin and ketamine from the Altered States Database, floating converged with ketamine on experiences of disembodiment, unity, and bliss. It diverged on essentially every perceptual one, the ones psilocybin heightens: elementary and complex imagery, synesthesia, and changed meaning of percepts.

This matters practically, as altered states are increasingly understood as part of how these treatments work, and the routes to them are narrow, blocked by ‘bad trips,’ contraindications, scheduled substance classifications and more. Characterizing non-pharmacological routes to altered states is central to the IACS mission, and aquahenosis can now be considered part of that list.