John C. Lilly performed the seminal experiment about sensory deprivation. The author isolated people in a sensory deprivation tank and they reported having experienced hallucinations.

Richard Feyman reported about his own experiences with isolation tanks in the collection of essays Surely You're Joking, Mr. Feynman!

I am curious about the reason of this effect. If we are deprived of our senses (being perfectly sane and without drugs), why do we start to hallucinate? What happens inside our brains?


2 Answers 2


According to the article The Brain’s Voices: Comparing Nonclinical Auditory Hallucinations and Imagery (Linden et al. 2011):

Spontaneous activation of sensory areas (Hunter et al. 2006) has been described in cases of sensory deprivation or during epileptic seizures. Local spontaneous activity may underlie certain types of hallucinations, for example, visual hallucinations of Charles Bonnet syndrome (Ffytche 2005) and possibly imagery of music without lyrics (Kraemer et al. 2005).

An example, from Neural activity in speech-sensitive auditory cortex during silence (Hunter et al. 2006) states that the auditory cortex has a propensity to spontaneously “activate” during silence. Specifically, from their study

suggest that, within auditory regions, endogenous activity is modulated by anterior cingulate cortex, resulting in spontaneous activation during silence. Hence, an aspect of the brain's “default mode” resembles a (preprepared) substrate for the development of auditory hallucinations. These observations may help explain why such hallucinations are ubiquitous.

In the article Visual hallucinations in psychologically normal people: Charles Bonnet syndrome (Teunisse et al. 1996), states that this syndrome, where healthy people can experience visual hallucinations undergoing sensor deprivation, is more common than first realised, their study suggests that under conditions of sensory-deprivation, the slightest amount of arousal triggered the hallucinations.

This is discussed further in Spontaneous and driven cortical activity: implications for computation (Ringach, 2009), where their study suggests

the lack of a stimulus generates a weak feedforward drive and the cortical state is determined largely by its own intrinsic dynamics and top-down expectations. This explains why spontaneous activity is not noise. Further, if the attractor has been shaped (via evolution or developmental rules) to represent the manifold of natural signals, it is easy to see how sensory deprivation or weak input can lead to “hallucinations” during deprivation

Additional references:

Ffytche D, Visual hallucinations and the Charles Bonnet syndrome. Curr Psychiatry Rep 2005;7:168-179.

Kraemer D, Macrae C, Green A, Kelley W . Musical imagery: sound of silence activates auditory cortex. Nature 2005;434:158.

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    $\begingroup$ This explains that spontaneous endogenous activity is the source of the hallucinations, but it does not explain why there is spontaneous endogenous activity, therefore it does not explain why there are hallucinations. It is like saying: perceptions are the result of interpretations of sensory activity. This leaves out the important fact that an outside reality causes the sensory activity and how it does it. So the important question is: where does the "sponaneous activity" originate, and is it indeed sponaneous? $\endgroup$
    – user3116
    Commented Oct 5, 2013 at 4:50
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    $\begingroup$ @what - 2nd and final quotes in the answer. $\endgroup$
    – user3554
    Commented Oct 5, 2013 at 6:48

first, i commend you on your reading choice.

Did you know? Fun John Lilly Factoid

Despite identifying as a male, later in life Dr Lilly elected to have breast implants. Those who knew him would later note that his breasts appeared to be rotting off. Oh, that's John for ya!

for many years john lilly was my intellectual hero and i spent a lot of time pursuing his work.

i recommend you read the following, both by lilly:

the human biocomputer - really hard to follow, but it goes into the theoretical approach he took in investigating sensory deprivation

the scientist: a metaphysical autobiography - a much lighter read. very interesting things about his experiences, particularly with ketamine.

I am curious about the reason of this effect. If we are deprived of our senses (being perfectly sane and without drugs), why do we start to hallucinate? What happens inside our brains?

now, onto sensory deprivation. being such a self-indulgent wanker, i've told enough people about sensory deprivation to write it from memory. unfortunately, i have no references for you:


At the time, there was a Freudian theory that sleep is a form of unconsciousness that is instantiated once sensory input is withdrawal from the body. To test the theory, Lilly decided to construct an enclosure that attenuated all input as maximally possible.

Sensory deprivation protocol

Lilly though about this matter systemically and proceeded in a perfectly logical manner. Let's go through them one by one as it's a good way to appreciate that his goal attenuate sensory input as maximally as possible without causing any organic damage to his body. Here are the pertinent specifications:

Sight. The enclosure is completely lightproof.

Smell. Lilly was a smoker. First, smell would be detectable and be a distraction for him. However, after enough time passes e.g. 30 minutes, we tend to habituate to smells.

Touch. Conceptually, this is the most interesting. Lilly filled the enclosure up with water, then added a ridiculous amount of salt. At a density of 1.30 g/ml, the human body will float in the water as though it's weightless. Initially, salt water was used, but Lilly switched over to Epsom salts (MgSO4). A solution of Epsom salt is very silky on the skin and very therapeutic.

Temperature. Temperature is controlled with a thermostat set to 34.5 C. It was at this temperature that Lilly found that if he stayed still, he would perceive the water to be neither hot nor cold.

Sound. If sound is generated outside in the medium of air, then it needs to travel across the enclosure, into water before hitting the eardrums. Lilly found that if ge floated on the surface of the water, but with his eardrums just below the surface, the different mediums through which externally generated sound would have to travel would be dampened.

Phenomenology of sensory deprivation

I think that's it.

Now, when one enters the tank and starts floating in complete silence, the body will start to miss its normal sensory input. Just remember, your body is used to this. It's as though there is tonic firing of neurons at a particular regularity: your brain has constructed a map of what input it is meant to over the entirely of your life, but now that baseline input is gone.

Slowly, over time, your brain will start seeking sensory input. It will start by instantiating a behavioural response amenable to you giving it input e.g. you will feel like you want to move your hand somewhere, or maybe scratch yourself. Something. This is all occurring below our conscious awareness. You will probably catch yourself generating sensory input.

Now, if you stay still in the water, and your skin cannot detect either hot or cold, what happens is that the representation your brain makes of your body in space starts to lose sharpness. i.e. You will not know where your body ends, and where the water starts. In some sense, it's like when you use your smartphone and it asks you to calibrate the accelerator by rotating it and what have you. It needs to have a conception of gravity and acceleration in order for it to accurately construct a representation of your body.

Anyways: eventually, your brain finally has enough: if you aren't going to furnish it wish sensory input, then it'll make some on its own. And so, it will start generating int.

In short: you start hallucinating because your brain is used to a certain level of sensory input. In the absence of that bare minimum input from the sensory modalities, it will start generating whatever sensory input necessary, probably just so it can process it.

My impression is that if you research the phenomenon of seeing tracers when the lights are off, an account of the neurobiological substrate of this will be reminiscent of what the the brain experiences under conditions of sensory deprivation.

Did you know? Fun John Lilly Factoid

It was while he was floating that Dr Lilly came to formulate the belief that an extraterrestrial superconsciousness was communicating with him. During such communications, the beings instructed him to commence administering intramuscular injections of ketamine and await further instructions. So dedicated was he to honouring the wishes his guests that at his peak, he maintained a dosing schedule of 50 mg i.m. ketamine per hour, 20 hours a day, for three weeks. What selflesslessness!


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