For centuries, "magic" mushrooms have been both celebrated and reviled for their mind-expanding properties.

Research and popular use of psychedelic drugs like mushrooms and LSD surged in the 1960s, when the substances first entered the American cultural consciousness on a large scale, and came to define '60s counterculture. At this time, thousands of studies were conducted to determine the properties and potential therapeutic applications of the drugs. But in 1970, the Controlled Substances Act brought an end to this era of science-based open-mindedness, and greatly limited drug research for the next four decades.

Today, research on psychedelic drugs is experiencing a renaissance of sorts. A growing body of scientific studies from major universities and medical centers suggests that the substances may hold promise as therapeutic interventions for a number of mental health conditions.

'Shrooms are known to trigger hallucinations, feelings of euphoria, perceptual distortions, inability to distinguish fantasy from reality, and sometimes a mystical feelings of oneness with nature. Because of their ability to temporarily create profound changes in consciousness, and sometimes lasting changes in psychological well-being, mushrooms have been an area of particular interest among both scientists and recreational drug users.

Here's what else we know about mushrooms, what they do the human brain, and how.

Psychedelic mushrooms grow naturally all over the world.

According to most estimates, more than 180 species of fungus produce psilocybin or psilocin, the two psychoactive substances most commonly associated with psychedelic mushrooms. While not all "magic mushrooms" rely on these compounds to produce mind-altering effects, the majority of fungus now used for recreational and entheogenic purposes are fruiting bodies from the genus Psilocybe, though species in other genera also contain psilocybin or psilocin. Psilocybin mushrooms grow naturally across a variety of climates and on every continent except Antarctica.

Psilocybe cubensis before cultivation. (Photo by Getty Images)

Mycologists -- biologists who specialize in the study of fungus -- believe that psilocybin and psilocin, as well as a number of other naturally occurring compounds, serve as an evolutionary defense mechanism for these species. While many of the psilocybin mushrooms typically consumed by humans don't contain enough of these chemicals to be fatally toxic, at least to adults, they are potent enough to deter predation by many other species. A trio of goats made news a few years back when they reportedly ate psilocybin mushrooms and started acting strange, for example. The animals were sick and disoriented, according to their owner, but returned to normal after a few days. Humans who eat mushrooms may exhibit similar symptoms of physical discomfort -- along with intense psychological effects that the goats obviously couldn't articulate.

Two of the most common species of psychedelic mushrooms are Psilocybe cubensis, the most popular on the black market, and Psilocybe semilanceata, the most widespread in the wild. Both of these species grow in the United States, though they have been known to appear in different climates. The concentration of psilocybin and psilocin present in each of these species has also been found to range greatly depending on the individual mushrooms.

Mushrooms have been used by humans for their reality-altering properties for thousands of years.

This basalt mushroom sculpture was created by the Mayans during the pre-Classic period (c. 2000 BC to AD 250). (Photo by Getty Images)

Fungi have inhabited the earth for more than 400 million years, and early ritualistic use of hallucinogenic mushrooms may date as far back as 9,000 years ago. Some anthropologists have argued that mushrooms held a central place in many early cultures -- including Greece, India and Mesoamerican cultures -- and have had a profound impact on human evolution. According to one radical theory from philosopher Terence McKenna, the incorporation of psychedelics (particularly magic mushrooms) into primitive diets may have been the catalyst for significant evolutionary advances, including the development of self-awareness and language.

Anthropologists have speculated that magic mushrooms may have been the inspiration for prehistoric rock paintings in the Sahara desert, which prominently feature mushroom imagery. They may have played a role in the evolution of Christianity, as well. Philologist John Allegro, who translated the Dead Sea Scrolls, has presented evidence for worship of psychedelic mushrooms in the early Christian era.

Historically, psychedelic mushrooms have been perhaps most widely associated with the ancient Maya. Mushroom stones and motifs have been found in Mayan temple ruins, and several varieties of psilocybin, as well as hallucinogenic Amanita muscaria mushrooms, were thought to have been available to the Mayans. Mushrooms are a common trope in Mayan art, and their symbolism often connects mushrooms with a “dreamlike state” -- for instance, a man with mushrooms all over his feet. The fact that these scenes are depicted in Mayan art and even in the codices suggests that mushroom use was an important aspect of society worthy of recording. Large mushroom stones can also be found throughout areas of Guatemala that were inhabited by the Mayans. While there are different theories to explain the presence of these stones, some have suggested that they were involved in ritual consumption of mushrooms, or that there may have even been cult worship around the mushrooms.

And the taboo surrounding psychedelic mushrooms is nothing new.

Psychologist and writer Timothy Leary conducted experiments on mushrooms in the 1960s through the Harvard Psilocybin Project.

When Western Christian conquistadors swept through Mesoamerica in the 16th century, they suppressed many aspects of traditional spiritual expression, including the use of mushrooms. Historians believe mushroom cults and shamans who used psilocybin to obtain what they believed to be a deeper understanding of the world were pushed underground, not to be widely rediscovered for hundreds of years.

But mushrooms still played an important medicinal and spiritual role in a number of indigenous cultures, despite their lengthy disappearance from the Western record. Fast-forward to 1955, when psychedelic mushrooms entered the American mainstream. R. Gordan Wasson -- author, ethnomycologist and vice president of JP Morgan & Co. -- and his wife Valentina became the first known non-native-Americans to actively participate in an indigenous Mazatec mushroom ceremony in Mexico. The Wassons published a popular article about their experiences, which appeared on the cover of Life Magazine in 1957. The article inspired psychologist and psychedelic pioneer Timothy Leary to travel to Mexico and try it for himself. Leary and Richard Alpert (now Ram Dass) started the Harvard Psilocybin Project to promote research on psychedelics, which led to their dismissal from the university in 1963.

Leary and others' popularization of mushrooms led to the creation of a psychedelic underground, associated with the counterculture moment of the 1960s, and an explosion in their non-indigenous usage. It also inspired President Richard Nixon to label Leary "the most dangerous man in America."

Our understanding of mushrooms has been stunted by decades of prohibitive international and domestic law.

Nixon ushered in a new era of prohibitive law, marginalizing drug use along with research about its potential therapeutic aspects.

In 1970, Nixon passed the Controlled Substances Act as a precursor to what would soon be called the War on Drugs. Psilocybin and psilocin, as well as any "containers," i.e. mushrooms, holding these psychoactive compounds were determined to be Schedule I drugs, considered to have a high potential for abuse and no accepted medical use. A year later, with input from U.S. authorities, the United Nations Convention on Psychotropic Substances was passed, also making psilocybin and psilocin -- though not the mushrooms containing them -- Schedule I drugs.

Certain nations are relatively permissive when it comes to psychedelic mushrooms. Until 2008 in the Netherlands, the drugs were sold openly in special shops, though the law has since been changed to permit only the sale of a specific kind of psychedelic truffle. Other nations, like Brazil and Spain, operate on the language of the U.N. convention, which doesn't explicitly mention psychedelic mushrooms themselves.

In the U.S., laws regulating the growth, possession and harvesting of psychedelic mushrooms vary slightly by state. But it's safe to bet that no matter where you live, authorities will crack down hard on any and all hallucinogenic drugs, no matter how natural they are.

Which means much of the current debate focuses on recreational use and the black market trade that sustains it.

Homer Simpson goes on a psychedelic spiritual journey in "El Viaje Misterioso de Nuestro Jomer (The Mysterious Voyage of Homer)," Season 8, Episode 9 of "The Simpsons."

Psilocybin mushroom grow naturally in a variety of habitats, from grasses and gardens, to rotting wood and animal feces. But with a healthy market for illegal psychedelic fungi, growers have taken to more reliable and controllable methods of cultivation in order to maximize profit.

Indoor operations frequently involve mushroom spores being injected into prepared beds of nutrients that are most frequently stored in jars or boxes. Given the proper conditions, the fungus can be fully grown and picked in a matter of days or weeks. Mushrooms that are specifically cultivated for recreational use are often harvested before they reach full maturity, when they contain more potent concentrations of psilocybin. After picking, mushrooms are dried in order to preserve the psychoactive ingredients within. Mushrooms are then typically eaten or boiled to make a tea.

Street prices for mushrooms vary, but the Drug Enforcement Administration puts them at about $20 for an eighth of an ounce -- considered a strong personal dose for a "trip" -- and $100 to $120 for an ounce. One particularly large bust of a mushroom growing ring in Ohio in 2013 reportedly turned up 503 pounds of material containing psilocybin, at a street value of more than $800,000.

Modern research has found that psilocybin can, quite literally, expand consciousness.

In a recent brain-scanning study, British researchers found that ingesting psilocybin caused normally disconnected brain regions to communicate with each other. FMRI scans showed that the connections aren’t random -- the brain retains its organizational features, but the connections are completely different than they are in a normal brain state. This helps to explain some of the common effects of psilocybin reported by users, such as new insights and world-shattering realizations, synesthesia and nonlinear thinking.



British researchers found significant differences in functional connectivity between a normal brain (left) and a brain on psilocybin (right).

Other research showed that psilocybin dampens activity in areas of the brain associated with sensory processing. Normally, these areas pose constraints on the way we experience the world through our senses, grounding us in material reality. By reducing activity in these areas, the senses are heightened and perception seems to expand. These brain regions are also the seat of the ego and are responsible for giving us our sense of self, so by hindering their activity, users often report experiences of oneness with the universe and interconnection.

Psilocybin also carries potential longterm effects, both positive and negative.

Mushroom users commonly report experiencing enhanced visual perception, with colors appearing more vibrant and surfaces appearing to melt or breathe.

Just one experience with psilocybin can have lasting positive emotional and psychological effects. Psychedelic experiences can make individuals more open-minded, Johns Hopkins researchers found. One “trip” was enough to cause significant changes in the “openness to experience” personality domain -- which is associated with creativity, intellectual curiosity and an appreciation for art and beauty -- for over a year.

It’s important to note, however, that while the risk of overdose is extremely low (a user would have to ingest over 35 pounds of fresh mushrooms to reach fatal levels of toxicity), experimenting with the substance doesn’t come without risk. Some users report experiences of heightened fear, anxiety and paranoia while tripping -- in some cases, if the panic reaction is great enough, they may pose a threat to themselves or others. Research has also found psilocybin to produce a psychosis-like syndrome that mirrors early episodes of schizophrenia, and some experts have suggested that psilocybin may trigger or exacerbate mental health conditions like schizophrenia, mania and depression. More research is needed to determine psilocybin’s potential longterm physical and mental health impacts.

Psilocybin could also have significant therapeutic uses.

The stigma of psychedelics may be slowly shifting as more and more research finds that substances like LSD and psilocybin show promise as therapeutic tools for dealing with a range of mental health problems.

Johns Hopkins researchers found that using small amounts of psilocybin in a controlled setting could lead to life-changing positive experiences that increased longterm psychological well-being. Fourteen months after the experience, a full 94 percent of the study’s subjects ranked taking the drug in a therapeutic setting as one of the top five most meaningful experiences of their lives, and 39 percent said it was the single most meaningful experience of their lives. Friends and family members also reported seeing positive changes in the subjects, saying that the experience had made them calmer, happier and kinder. The researchers said that they ultimately hope to see whether transcendent experiences, facilitated by taking psilocybin in therapeutic settings, could help treat conditions like addiction, depression and post-traumatic stress disorder.

Psilocybin is effective in reducing anxiety among terminal cancer patients, UCLA researchers found, and has also been shown to lead to a reduction in symptoms of obsessive-compulsive disorder.

The therapeutic uses could include potential treatment for PTSD and depression.

Magic mushrooms may be particularly promising as treatment for PTSD. A 2013 study found that psilocybin could alleviate the fear response in mice, a finding which may lay the foundation for future research on fear in humans.

By helping people to literally escape destructive thoughts, mushrooms could also be a promising treatment for depression. Depression is associated with over-connectivity of the default mode network -- the brain network associated with self-consciousness, rumination and introspection -- which can lead to excessive negative self-thought. Recent brain imaging studies from Imperial College London have shown magic mushrooms to quiet down the default mode network.

As Imperial College neuropsychopharmacology professor David Nutt told CNN, “This could create a paradigm shift to help people into a different state of thinking that they can then stay in."

But we have a long way to go to fully grasp the effects and potential uses for mushrooms.

Currently, the government does not fund psychedelic research, so funding is left in the hands of private organizations like the Multidisciplinary Association for Psychedelic Studies.

While the Western resistance to psychedelic research remains a stumbling block, scientists are optimistic.