Did you see that? The Mysteries of Sensory Deprivation

When I was a kid swimming in the neighbor’s pool, I loved to float on my back, eyes closed, with ears submerged so that I could only hear the muted sounds of the water around me. Though I found the experience relaxing, I also felt profoundly strange. We are constantly bombarded with sensory information from our eyes, ears, nose, mouth, and skin, as well as the canals in our ears and organs in our muscles that help us keep track of our balance and limb position. What happens to our minds in the absence of these inputs?

Sensory deprivation, the blocking of the ability to use one or more of the senses, has a complicated history. Despite its long history of alleged use in torture, sensory deprivation in a different form has become both treatment and fad. It is now being studied by medical and neural scientists for its potential benefits under the term flotation restricted environmental stimulation therapy (REST).


An example of an open sensory deprivation tank. The tank is big enough to comfortably accommodate an adult laying down. (source: Wikipedia)

Here’s how it works: you can visit any number of ‘float spas’ (a quick Google Maps investigation revealed at least 5 separate companies with multiple locations in San Diego alone). After a shower and informational briefing, you will insert ear plugs and enter a chamber filled with 1 foot of Epsom salt-saturated water. The salt content of the water increases its density and ensures that you will be able to float without any effort – many people even fall asleep during the 1 hour floating session without any issues in buoyancy. The chamber is long and wide enough to accommodate an adult floating on their back, but narrow enough so that you can reach out and touch each side. Once the chamber door is closed, you are left in complete darkness and silence. Clean, filtered air enters the chamber so you can breathe comfortably, but it is devoid of odors. You will have already removed all clothing and jewelry so that the only thing touching your skin is the salty water, warmed to your external skin temperature. During the float, you are in complete control. The experience can be stopped at any time, although a majority of floaters do not exercise that option. Now that you are in the tank, simply relax, quiet your mind for one hour, and… float.


Do you see what I see?

While it seems that the consensus on floating is of positive physical and mental health effects, one phenomenon often overlooked in flotation REST research is the propensity for multisensory hallucinations within the one hour floating session. Visual experiences are most common, followed by auditory and somatosensory events. Norlander, Kjellgren & Archer [1] actually attempt to quantify these experiences in the laboratory setting through detailed questionnaires for study participants before and after a session of flotation REST. Approximately 75% of the participants reported the feeling of their body disappearing. 64% reported seeing images. 35% heard noises in the complete silence, and 75% of the participants compared their physical experiences to flying.

While these numbers are astounding, the most interesting reported data from the group includes what these participants were seeing and hearing. Common visual hallucinations include flashes of lights or colors, often called phosphenes. However, in some individuals, full visual scenes become clear. For example, one participant reports seeing “horses and a woman that fell off and hit her head.” Five subjects reported meeting “strange beings” – one participant in particular met and received advice from an older wizard-type figure named Araknion. Six heard inner voices of strangers or family members, and many felt themselves become one with the universe. Perhaps most strangely, four participants relayed the experience of being born, in which they were initially in a dark, constricted space and were delivered into the light, some even with the bonus experience of breastfeeding.


Artist depiction of a type of phosphene, or pattern of light or color that is perceived in the absence of visual input. These can often happen by rubbing closed eyes. (source: Wikipedia, By Al2 – self-made, using GIMP and Inkscape., CC BY 3.0)


The neuroscience of deprivation-induced hallucinations

Ok, these hallucinations that can occur with flotation REST are strange, but are commonplace of anecdotal experience with sensory deprivation. Why would our brain respond in this way to deprivation of the senses?

Our first clue that sensory deprivation can cause visual hallucinations comes from the study of Charles Bonnet Syndrome, in which people with vision loss from conditions such as macular degeneration can develop persistent visual hallucinations from phosphenes to detailed figures or scenes. Additional studies have correspondingly shown that normally sighted subjects with no history of hallucinations who have been blindfolded for 5 full days often exhibit visual hallucinations within the first few days of deprivation [2].

The way visual information is processed in the brain has been highly studied in neuroscience and is a logical place to look for how visual hallucinations might be manifesting in the brain. Information from the retina of the eye is sent through the optic nerve to the thalamus, which acts as a switchboard for routing many types of sensory information. From there, the visual information is sent off to the cortex where different areas work together to encode various aspects of the visual information and integrate it with other processes happening in the cortex. What happens in the cortex and thalamus during visual sensory deprivation?

Visual Pathway Image

Schematic of the visual pathways in the human brain. Information in the visual field enters the brain through the retinas of the eyes and the optic nerve. The lateral geniculate nucleus (LGN) of the thalamus routes the activity to the primary visual cortex located at the back of the brain (source: http://mriquestions.com/visual.html)

Historical theories of visual deprivation effects on the visual cortex suggest two possible causes. First, in an “irritative” process, in the absence of visual input to the primary visual cortex, nearby cortical areas may provide “excitatory discharges” – essentially, nearby neurons may make the neurons of the visual cortex more active when they shouldn’t be. Second, visual deprivation may provide “release” for the normal inhibition on the visual cortex. In this case, the hypothesis is that neurons in the visual cortex always have signals coming in to decrease their activity, and sensory deprivation disrupts those signals and allows the visual neurons to be active at the wrong time [2]. In either case, the visual cortex is active when it shouldn’t be. Using fMRI to measure the amount of blood flow into the visual cortex as a sign of increased cellular activity, researchers found that the visual cortex is in fact more active following 1 hour of light deprivation [3].

A lack of activity in the thalamus may be responsible for this errant activity in the visual cortex. The thalamus, that sensory switchboard, provides both excitatory input that makes neurons in the visual cortex active and excitatory input to inhibitory neurons in the same area. Put more simply, the thalamus passes visual information to the cortex and simultaneously provides input to silence the cortex right after the information gets there [4]. Sensory deprivation decreases the activity of the thalamus, which decreases its control on the activity in cortex, allowing the cortex to become active in the absence of real sensory input. This phenomenon is also thought to be the cause of hallucinations when subjected to white noise and a completely uniform visual field, known as the Ganzfeld effect – check out this awesome 2014 NeuWrite article with a description of thalamocortical contributions to this effect!

While most of the research on deprivation-induced hallucinations is focused on the visual cortex, flotation REST customers also often report auditory and somatosensory hallucinations. These same thalamocortical neurological pathways could similarly explain these hallucinations – both auditory information from the auditory nerve and somatosensory information is relayed through the thalamus on the way to the cortex. Interestingly, the sense of smell is the only sense that does not route through the thalamus, but the prevalence of olfactory hallucinations is not clear.


Health benefits of flotation REST and sensory deprivation

Despite the potential for a weird hallucination or two, proponents of flotation REST and frequent float spa customers swear by the technique’s abilities to promote health and well-being. Flotation REST can cause modest decreases in diastolic blood pressure, the pressure in your blood vessels between heart beats. A more striking effect on physical health is floating’s benefit for patients with chronic pain. In 32 patients with pre-existing stress-related muscle tension, researchers found a decrease in multiple pain measurements such as normal pain intensity, frequency of pain (self-reported daily pain transitioned to weekly pain), and number of painful areas. Interestingly, REST had a great effect on relieving back pain, which is often seen as one of the most difficult-to-treat pain areas [5].

Floating is also often used as an effective stress reliever. As might be obvious from its description, flotation REST provides time away from every possible external and environmental trigger of stress and disrupts harmful behavioral patterns. In many studies, self-reported stressful feelings and the behavioral effects of stress, such as muscle tension, headaches, and insomnia, decrease after floating. Additionally, physical markers of stress such as elevated cortisol, a hormone released in response to stress, in the blood and urine have been observed to decrease after flotation REST as compared to non-stress related hormones [6]. From a comparison of 27 different flotation REST studies involving 449 patients, it is clear that floating also improves measures of “well-being,” such as optimism and positivity, and measures of “performance,” which involves the increase in proficiency of a task (e.g. “archery performance”) [7].

Flotation REST in a sensory deprivation tank provides a truly unique experience to unplug from the typical stresses of daily life and encounter your mind in the absence of sensory input. While this environment can be confusing to the brain, our sensory processing machine, the relaxation benefits advocated by experienced floaters appear unparalleled.





  1. Norlander T, Kjellgren A, Archer T (2001) The experience of flotation-rest as a function of setting and previous experience of altered state of consciousness. Imagination, cognition, and personality, 20(2):161-178
  2. Merabet LB, Maguire D, Warde A, Alterescu K, Stickgold R, Pascual-Leone A (2004) Visual hallucinations during prolonged blindfolding in sighted subjects. J Neuro-Ophthalmol, 24:109-113
  3. Boroojerdi B, Bushara KO, Corwell B, Immisch I, Battaglia F, Muelbacher W, Cohen LG (2000) Enhanced excitability of the human visual cortex induced by short-term light deprivation. Cerebral Cortex, 10:529-534
  4. Miller KD, Pinto DJ, Simons DJ (2001) Processing in layer 4 of the neocortical circuit: new insights from visual and somatosensory cortex. Current Opinion in Neurobiology, 11:488-497
  5. Bood SA, Sundequist U, Kjellgren A, Nordstrom G, Norlander T (2005) Effects of flotation-restricted environmental stimulation technique on stress-related muscle pain: What makes the difference in therapy – attention-placebo or the relaxation response? Pain Res Manage, 10(4):201-209
  6. Suedfeld P, Borrie RA (1999) Applications of chamber and flotation restricted environmental stimulation therapy (REST). Psychology and Health, 14:545-566
  7. Dierendonck DV, Nijenhuis JT (2005) Flotation restricted environmental stimulation therapy (REST) as a stress-management tool: A meta-analysis. Psychology and Health, 20(3):405-412

Cover image source: https://floatationlocations.com/blogs/nudity-hallucinations-and-relaxation-my-dip-into-an-isolation-tank/