When we look at something the light reflected from that thing is reflected into our eyes.  The cornea and the lens ideally focus the image of that object on our retina.  When a photon of light energy strikes the retina a complex series of chemical reactions happen within the photoreceptor and eventually nerves carry that energy to the back of our brain, the occipital cortex.  And viola we SEE.

But sometimes thing don't work exactly as expected.  Sometime disease, injury, and sometimes poorly understood normal brain functions can make us see things that are not really there.

When we dream we appear to be involved as a participant in a drama and we see images based on some fragmentary images in our associative vision centers and memory. The dream often is not logical and lacks a sense of reality.

Hallucinations are often vivid images that seem very real but are not related to any real visual input.  They can be simple spots or colors or complex images of things.

Posterior cerebral artery infarction (stroke) often leads to dramatic visual changes.  Up to one half of a persons visual field can go missing in both eyes.  Usually a stroke in the right cerebral hemisphere leads to changes in the left side of the visual field in both eyes (referred to as a hemianopsia).  It's not black, but just is not there.  As the person moves their eyes objects can appear to just disappear and then suddenly reappear. Up to 41% of patients with hemianopia's report sometimes seeing hallucinations of simple or complex images within the area of the visual field that has gone missing.   Patients with brain stem lesions also can see vivid images of things that are not there.  Patients with epilepsy often see brief simple images during seizure's.

Approximately 10% of the general population have migraine headaches and 30% of those people see simple visual hallucinations known as auras.  It usually starts as a zigzag  of shimmering lights.  It usually lasts 5 to 60 minutes and often is followed by nausea and headache.  In most cases migraine auras are seen in both the right hemi-field and the left hemi-field on subsequent attacks.  An aura that is only seen to one side during all attacks may require imaging to rule out a brain lesion.

Up to 40% of patients with Parkinson's disease have vivid visual hallucinations.  Patients with schizophrenia experience visual and auditory hallucinations and are associated with episodes of excitability. To the schizophrenic the auditory and visual hallucination appear very real and are easily confused with reality.

Charles Bonnet in 1769 documented the bizarre hallucinations that his grandfather experienced after bilateral cataract surgery.  He saw light but all the real images were very blurry.  Charles Bonnet Syndrome (CBS) refers to the visual hallucinations that alert patients without cognitive defects can experience with a severe visual deficit.  Up to 15% of patients with severe bilateral vision loss experience complex visual hallucinations.  The current theory of the cause of CBS is that when a steady stream of high quality visual input to the visual cortex stops due to severe binocular visual loss, the input is replaced with memory-based images from the associative vision centers.

Approximately 30% of healthy, normal people have hallucinations related to sleep.

Electrodes placed on a persons scalp can record the electrical activity taking place in the brain. When awake with our eyes open our brain wave is referred to as a beta wave.  Sleep has been described as a succession of five recurring stages. Stage 1 or drowsiness often lasts for five to 10 minutes.  The brain wave is transitioning from a beta wave to an alpha wave. Stage 2 is a period of light sleep. The heart rate slows and body temperature decreases. The brain wave is now an alpha wave.  Stage 3 is a period of deeper sleep.  The brain wave is now a theta wave.  Stage 4 is an even deeper sleep stage.  The brainwave  is now a delta wave.  The muscles still have their normal tonus and you can move your arms and legs.  After stage 4 the brain usually cycles back through stage 3 and then stage 2.

After about 90 minutes the brain enters REM (Rapid Eye Movement) sleep. The heart rate and respiration speed up and the face, fingers, and legs may twitch.  Our eyes begin to rapidly move about as we “view” our dream. Intense dreaming occurs during REM sleep.  During REM dreaming most of the body is usually paralyzed, preventing us from actually acting out our dreams. The first period of REM typically lasts about 10 minutes, and each recurring REM stage lasts longer, and the final one may last up to an hour. Brainwave patterns in REM are similar to that recorded during wakefulness.  Intense dreaming occurs during REM sleep as a result of heightened brain activity.  The percentage of REM sleep is highest during infancy. Infants can spend up to 50% of their sleep in the REM stage of sleep, but adults spend only about 20% of their sleep time in REM.

Up to thirty percent of normals experience hallucinations related to sleep.  Hypnagogia is the neurologic condition in which you can have lifelike auditory, visual, or tactile hallucinations. The term was coined by the 19th century French psychologist Alfred Maury. Hypnogogia is thought to happen when a person enters REM sleep directly with out passing through the 4 stages of non-Rem sleep. In many cases the individual may be aware of the same feeling of paralysis that occurs during REM sleep.  Lucid dreaming is another term used for the same phenomena. The person is totally conscious and aware of the hallucinations.  Hypnompia is the term used when the phenomena happens as you are waking up, not going to sleep.  Sometimes hypnogogia  is more common in persons with a sleep disorders such as narcolepsy.

Hinduism and several Chinese religious sects share the concept of a "Third Eye" (also known as the inner eye). It is invisible but thought to be located in the middle of the forehead slightly above the the eyebrows. It is often associated with religious visions, psychic powers, clairvoyance, the ability to observe auras, and out-of-body experiences.

Hypnogogia may account for many claims of para-normal visions and even alien abductions. Ancient yoga documents  describe several  powers that  yoga practitioners could achieve through  a diligent meditation practice.  These powers are called the yoga siddhis. They include seeing and hearing things  remotely, having unrestricted access to all places, flying and assuming any form desired.  I think that the quiet and calming effect of meditation would be the ideal situation for a person to experience hypnogogia and to “see” remote objects.

A personal note.

Forty years ago I discovered that sometimes when falling asleep I would pass through an early phase of sleep when I would "see" fleeting images. After many years of occasionally observing this phenomena when going to sleep or when meditating the images last longer and  are more detailed. My personal version is that I "see" images contained in about a  2" x 2" square (like a little television set).  It's located in my superior field of view.  I  have to look up to see it (with my eyes still closed).  And yes, that's exactly where the ancient Third Eye is said to be located - spooky.

The images that I "see" are usually of people, but I do not recognize any of them. Sometimes the images are seen looking down on a landscape as if flying.  The images are 3 dimensional and my viewpoint seems to change from front to back on its own as if I was moving.  The scene starts dark and then brightens very brightly to reveal the image and then fades on its own after a few seconds. The details are very clear and realistic.  My hypnogogia  is purely visual  There are no sounds. On occasion I have seen an image of someone speaking to me but I cannot hear the words. Pretty weird. I am a trained observer of visual phenomena and I am of sound mind and not mentally disordered.  This is for real.

I have practiced optometry for 35 years and have never had a patient tell me that they see hallucinations. I suspect that many have noticed this phenomena but were reluctant to discuss it because of the generally known association between hallucinations and mental disease.  It's also possible that many observers have just dismissed the phenomena as a dream.

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