Rubin’s vase is a famous set of cognitive optical illusions developed around 1915 by the Danish psychologist Edgar Rubin. The illusion generally presents the viewer with a mental choice of two interpretations, each of which is valid. Often, the viewer sees only one of them, and only realizes the second valid interpretation after some time or prompting. When the viewer attempts to simultaneously see the interpretations together, they suddenly cannot see the first interpretation anymore, and no matter how they try they simply cannot encompass both interpretations simultaneously; one occludes the other.

The illusions are useful because they are an intuitive demonstration of the figure-ground distinction the brain makes during visual perception. Rubin’s figure-ground distinction influenced the Gestalt psychologists, who discovered many similar illusions themselves. It involves higher-level cognitive pattern matching in which the overall picture determines its mental interpretation, rather than the net effect of the individual pieces.

Normally the brain classifies images by what surrounds what, establishing depth and relationships. If something surrounds another thing, the surrounded object is seen as figure, and the presumably further away (and hence background) object is the ground, and vice versa. This makes sense, since if a piece of fruit is lying on the ground, one would want to pay attention to the “figure” and not the “ground”.

However, when the contours are not so unequal, ambiguity starts to creep into the previously simple inequality and the brain must begin “shaping” what it sees. It can be shown that this shaping overrides and is at a higher level than feature recognition processes that pull together the face and the vase images. One can think of the lower levels putting together distinct regions of the picture (each region of which makes sense in isolation), but when the brain tries to make sense of it as a whole, contradictions ensue, and patterns must be discarded.

The Ganzfeld effect is a phenomenon of visual perception caused by staring at an undifferentiated and uniform field of color. The effect is described as the loss of vision as the brain cuts off the unchanging signal from the eyes. The result is “seeing black” or apparent blindness. The word is from German for “complete field”.

The effect is the result of the brain amplifying neural noise in order to look for the missing visual signals. The noise is interpreted in the higher visual cortex, and gives rise to hallucinations. This is similar to dream production because of the brain’s state of sensory deprivation during sleep.

The Ganzfeld effect has been reported since ancient times. The adepts of Pythagoras retreated to pitch black caves to receive wisdom through their visions. Miners trapped by accidents in mines frequently reported hallucinations, visions and seeing ghosts when they were in the pitch dark for days. Arctic explorers seeing nothing but featureless landscape of white snow for a long time also reported hallucinations and an altered state of mind.

In Tibetan Buddhism a dark retreat refers to advanced practices of isolation in darkness. The time period dedicated to dark retreat varies from a few hours to decades. Dark retreat in the Himalayan tradition is a restricted practice only to be engaged by the senior spiritual practitioner under appropriate spiritual guidance. This practice is considered conducive for navigating the bardo at the time of death and for realising the rainbow body. The traditional dark retreat requires stability in the natural state and is only suitable for advanced practitioners.

Ancient Egyptians and Mayans practiced a form of the dark retreat as well, traditionally lasting 10 days. Holy men would enter into the center of their respective pyramids, completely removed from light and sound, and have visions of the workings of the universe. Today, scientists have hypothesized that when the human body is deprived of visual stimulation, the brain produces a substance called Dimethyltryptamine (DMT), a tryptamine, which results in intense visions.

A phosphene is an optic phenomenon characterized by the experience of seeing light without light actually entering the eye. Phosphenes can be directly induced by mechanical, electrical, or magnetic stimulation of the retina or visual cortex as well as by random firing of cells in the visual system. Phosphenes have been reported by meditators and people who go for long periods without visual stimulation (also known as the prisoner’s cinema).

The most common phosphenes are pressure phosphenes caused by rubbing the closed eyes. The pressure mechanically stimulates the cells of the retina. Experiences include a darkening of the visual field that moves against the rubbing, a diffuse colored patch that also moves against the rubbing, a scintillating and ever-changing and deforming light grid with occasional dark spots (like a crumpling fly-spotted flyscreen), and a sparse field of intense blue points of light.

Pressure phosphenes can persist briefly after the rubbing stops and the eyes are opened, allowing the phosphenes to be seen on the visual scene. Herman Helmholtz and others have published drawings of their pressure phosphenes. One example of a pressure phosphene is demonstrated by gently pressing the side of your eye and observing a colored ring of light on the opposite side, as detailed by Isaac Newton.

Another common phosphene is “seeing stars,” from a sneeze, a heavy and deep cough, blowing of the nose, a blow on the head or low blood pressure (such as on standing up too quickly or prior to fainting). It is possible these involve some mechanical stimulation of the retina, but they may also involve mechanical and metabolic stimulation of neurons of the visual cortex or of other parts of the visual system.

Phosphenes have also been created by intense, changing magnetic fields, such as with transcranial magnetic stimulation. These fields can be positioned on different parts of the head to stimulate cells in different parts of the visual system. They also can be induced by alternating currents that entrain neural oscillation as with trancranial alternating-current stimulation. In this case they appear in the peripheral visual field. Astronauts exposed to radiation in space report seeing phosphenes.

Grapheme-color synesthesia is a form of synesthesia in which an individual’s perception of numbers and letters is associated with the experience of colors. Like all forms of synesthesia, grapheme-color synesthesia is involuntary, consistent, and memorable. It is one of the most common forms of synesthesia, and because of the extensive knowledge of the visual system, one of the most studied.

One recent study has documented a case of synesthesia in which synesthetic associations could be traced back to colored refrigerator magnets. Despite the existence of this individual case, the majority of synesthetic associations do not seem to be driven by learning of this sort. Rather, it seems that more frequent letters are paired with more frequent colors, and some meaning-based rules, such as B being blue, drive some synesthetic associations.

These experiences have led to the development of technologies intended to improve the retention and memory of graphemes by individuals without synesthesia. Computers, for instance, could use artificial synesthesia to color words and numbers to improve usability. Individuals with grapheme-color synesthesia rarely claim that their sensations are problematic or unwanted. In some cases, individuals report useful effects, such as aid in memory or spelling of difficult words.

Synesthetes often report that they were unaware their experiences were unusual until they realized other people did not have them, while others report feeling as if they had been keeping a secret their entire lives. Many synesthetes can vividly remember when they first noticed their synesthetic experiences, or when they first learned that such experiences were unusual. Writer and synesthete Patricia Lynne Duffy remembers one early experience:

“One day, I realized that to make an R all I had to do was first write a P and then draw a line down from its loop. And I was so surprised that I could turn a yellow letter into an orange letter just by adding a line.”

An interobject is a phenomenon of dreams, in which there is a perception of something that is “between” two objects. Interobjects differ from typical dream manifestations in which two objects are fused into one. Instead, the object is incomplete. An example from the literature on dreams includes “something between a record player and a balance scale.” Interobjects are new creations derived from partially-fused blends of other objects.

Interobjects, like disjunctive cognitions, would sound bizarre or psychotic as perceptions in waking life, but are accepted by most people as commonplace in dreams. They have implications for both the theory of dreaming and the theory of categorization. Interobjects show the dreaming mind grouping items together whose connection may not be apparent to the waking mind. “Something between an aqueduct or a swimming-pool” reveals the category of “large man-made architectural objects that contain water.” “Something between a cellphone and a baby” reveals a category combining a relatively new piece of technology and a live infant: both make noise when you don’t expect it, both are held close to your body, and both can give you a feeling of connectedness.

Most adults tend to regularize interobjects when discussing them in waking life. Children are better able to sustain interobjects in their original form. A child told his father a dream in which he was in trouble at sea and a seal swam up to them. They thought it was just a seal, but then they looked and under the water it was a whole boat, it was huge, so they climbed onto the seal/boat, and it brought them to the shore of the mainland. When the boy told his father the dream in the morning, the father, speaking like an adult who cannot tolerate contradictions, said to him: “So really, it was a boat, a big, safe boat.” The child, holding fast to the integrity of his dream, said, “It was a boat, but it was still a big, friendly seal.” This child had not yet learned to regularize his perceptions to fit the way the world works. Adults may learn to reject interobjects in waking life, but still retain them in their dreams.

Interobjects may have an elementary function in human thought. By transgressing the normal mental categories described by Eleanor Rosch, interobjects may be the origin of new ideas that would be harder to come by using only fully-formed, secondary process formations. They may be one example of “Oneiric Darwinism” in which new thought-mutations are created during dream-life and rejected or retained in waking life depending on their usefulness.

The Odic force is the name given in the mid-19th century to a hypothetical vital energy or life force by Baron Carl von Reichenbach. Von Reichenbach coined the name from that of the Norse god Odin in 1845.

As von Reichenbach was investigating the manner in which the human nervous system could be affected by various substances, he conceived the existence of a new force allied to electricity, magnetism, and heat, a force which he thought was radiated by most substances, and to the influence of which different persons are variously sensitive. He named this vitalist concept Odic force. Proponents say that Odic force permeates all plants, animals, and humans.

Believers in Odic force said that it is visible in total darkness as colored auras surrounding living things, crystals, and magnets, but that viewing it requires time first spent in total darkness, and that not everyone has the ability to see it. They also said that it resembles the eastern concepts prana and qi. However, they regarded the Odic force, not as associated with breath, like India’s prana and the qi of Eastern martial arts, but rather mainly with biological electromagnetic fields.

Von Reichenbach hoped to develop a scientific proof for a universal life force, however his experiments relied not on scientific instruments but on perceptions reported by individuals claimed to be psychically sensitive or psycho-kinetically adept. The “sensitives”, young women recruited from the poorer social classes, worked in total or near-total darkness, and were forerunners of the Spirit Mediums who appeared all over Europe 10 years later, in the 1850s.

The Odic force found no favor among mainstream scientists, and belief in it survives today as one among many concepts of spiritual energies associated with living things. The Odic force has been mentioned frequently in European books on dowsing, such as Reveal the Power of the Pendulum, by Karl Spiesberger.

Multistable perceptual phenomena are a rare form of visual perception phenomena which is characterized by an unpredictable sequence of spontaneous subjective changes.

Perceptual multistability can be evoked by visual patterns that are too ambiguous for the human visual system to recognise with one unique interpretation. Famous examples include the Necker cube, structure from motion, monocular rivalry and binocular rivalry, but many more visually ambiguous patterns are known. Since most of these images lead to an alternation between two mutually exclusive perceptual states, they are sometimes also referred to as bistable perception.

Transitions from one percept to its alternative are called perceptual reversals. They are spontaneous and stochastic events which cannot be eliminated by intentional efforts, although some control over the alternation process is learnable. Reversal rates vary drastically between stimuli and observers, and has been found to be slower for people with Bipolar disorder due to “sticky” interhemispheric switch in bipolar disorder. 

Human interest in these phenomena can be traced back to antiquity. The fascination of multistable perception probably comes from the active nature of endogenous perceptual changes or from the dissociation of dynamic perception from constant sensory stimulation. Multistable perception was a common feature in the artwork of the Dutch lithographer M. C. Escher, who was strongly influenced by mathematical physicists such as Roger Penrose. 

Photographs of craters, from either the moon or other planets including our own, can exhibit this phenomenon. Craters, in stereo imaging, such as our eyes, should appear to be pit-like structures. However in mono-vision, such as that of photographs, the elimination of our depth perception causes multistable perception to take over, and this can cause the craters to inverse their depth values and instead look like plateaus rather than pits. Sometimes rotating the image so that the photographic direction of the source of light matches a light source in the room can cause the correct perception to suddenly switch.

The third eye is a mystical and esoteric concept referring in part to the brow chakra in certain Eastern and Western spiritual traditions. It is also spoken of as the gate that leads within to inner realms and spaces of higher consciousness.

In New Age spirituality, the third eye may alternately symbolize a state of enlightenment or the evocation of mental images having deeply personal spiritual or psychological significance. The third eye is often associated with visions, clairvoyance, precognition, and out-of-body experiences.

In Hinduism and Buddhism, the third eye is a symbol of enlightenment. In the Indian tradition, it is referred to as the gyananakashu, the eye of knowledge, which is the seat of the ‘teacher inside’ or antar-guru. The third eye is commonly denoted in Indian and East Asian iconography with a dot or mark on the forehead of deities or enlightened beings, such as Shiva, the Buddha, or any number of yogis, sages and bodhisattvas. This symbol is called the “Eye of Wisdom”, or, in Buddhism, the urna. In Hinduism, it is believed that the opening of Shiva’s third eye causes the eventual destruction of the physical universe.

According to Max Heindel’s Rosicrucian writings, the third eye is localized in the pituitary body and the pineal gland. It was said that in the far past, when man was in touch with the inner worlds, these organs were his means of ingress thereto, and they will again serve that purpose at a later stage. According to this view, they were connected with the involuntary or sympathetic nervous system, and to regain contact with the inner worlds it is necessary to establish the connection of the pineal gland and the pituitary body with the cerebrospinal nervous system. It was said that when that is accomplished, man will again possess the faculty of perception in the higher worlds, but on a grander scale than it was in the distant past, because it will be in connection with the voluntary nervous system and therefore under the control of his will.

Some writers and researchers, including H. P. Blavatsky and Rick Strassman, have suggested that the third eye is in fact the partially dormant pineal gland, which resides between the two hemispheres of the brain. The pineal gland is said to secrete dimethyltryptamine, which induces dreams, near-death experiences, meditation, or hallucinations. Various types of lower vertebrates, such as reptiles and amphibians, can actually sense light via a third parietal eye, a structure associated with the pineal gland, which serves to regulate their circadian rhythms.