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Catacombs of Paris: The city of darkness finds its new raison d'être
Ancient corridors below the French capital have served as its ossuary, playground, brewery, and perhaps soon, air conditioning.
Excerpt from a 19th century map of the Paris Catacombs, showing the labyrinthine layout underground (in color) beneath the straight-lined structures on the surface (in grey).
- People have been digging up limestone and gypsum from below Paris since Roman times.
- They left behind a vast network of corridors and galleries, since reused for many purposes — most famously, the Catacombs.
- Soon, the ancient labyrinth may find a new lease of life, providing a sustainable form of air conditioning.
Ancient mining areas below Paris for limestone (red) and gypsum (green).Credit: Émile Gérards (1859–1920) / Public domain
"If you're brave enough to try, you might be able to catch a train from UnLondon to Parisn't, or No York, or Helsunki, or Lost Angeles, or Sans Francisco, or Hong Gone, or Romeless."
China Miéville's fantasy novel Un Lun Dun is set in an eerie mirror version of London. In it, he hints that other cities have similar doubles. On the list that he offhandedly rattles off, Paris stands out. Because the City of Light really does have a twisted sister. Below Paris Overground is Paris Underground, the City of Darkness.
Most people will have heard of the Catacombs of Paris: subterranean charnel houses for the bones of around six million dead Parisians. They are one of the French capital's most famous tourist attractions – and undoubtedly its grisliest.
But they constitute only a small fragment of what the locals themselves call les carrières de Paris ("the mines of Paris"), a collection of tunnels and galleries up to 300 km (185 miles) long, most of which are off-limits to the public, yet eagerly explored by so-called cataphiles.
The Grand Réseau Sud ("Great Southern Network") takes up around 200 km beneath the 5th, 6th, 14th, and 15th arrondissements (administrative districts), all south of the river Seine. Smaller networks run beneath the 12th, 13th, and 16th arrondissements. How did they get there?
Paris stone and plaster of Paris
It all starts with geology. Sediments left behind by ancient seas created large deposits of limestone in the south of the city, mostly south of the Seine; and gypsum in the north, particularly in the hills of Montmartre and Ménilmontant. Highly sought after as building materials, both have been mined since Roman times.
The limestone is also known as Lutetian limestone (Lutetia is the Latin name for ancient Paris) or simply "Paris stone." It has been used for many famous Paris landmarks, including the Louvre and the grand buildings erected during Georges-Eugène Haussmann's large-scale remodelling of the city in the mid-19th century. The stone's warm, yellowish color provides visual unity and a bright elegance to the city.
The fine-powdered gypsum of northern Paris, used for making quick-setting plaster, was so famed for its quality that "plaster of Paris" is still used as a term of distinction. However, as gypsum is very soluble in water, the underground cavities left by its extraction were extremely vulnerable to collapse.
Like living on top of a rotting tooth: subsidence starts far below the surface, but it can destroy your house.Credit : Delavanne Avocats
In previous centuries, a road would occasionally open up to swallow a chariot, or even a whole house would disappear down a sinkhole. In 1778, a catastrophic subsidence in Ménilmontant killed seven. That's why the Montmartre gypsum quarries were dynamited rather than just left as they were. The remaining gypsum caves were to be filled up with concrete.
The official body governing Paris down below is the Inspection Générale des Carrières (IGC), founded in the late 1770s by King Louis XVI. The IGC was tasked with mapping and, where needed, propping up the current and ancient (and sometimes forgotten) mining corridors and galleries hiding beneath Paris.
A delightful hiding place
Also around that time, the dead of Paris were getting in the way of the living. At the end of the 18th century, their final destination consisted of about 200 small cemeteries, scattered throughout the city — all bursting at the seams, so to speak. There was no room to bury the newly dead, and the previously departed were fouling up both the water and air around their respective churchyards.
Something radical had to happen. And it did. From 1785 until 1814, the smaller cemeteries were emptied of their bones, which were transported with full funerary pomp to their final resting place in the ancient limestone quarries at Tombe-Issoire. Three large and modern cemeteries were opened to receive the remains of subsequent generations of Parisians: Montparnasse, Père-Lachaise, and Passy.
Who says stacking skulls and bones can't be fun?Credit: Rijin via Wikimedia and licensed under CC BY-SA 4.0
The six million dead Parisians in the Catacombs, from all corners of the capital and across many centuries, together form the world's largest necropolis — their now anonymized skulls and bones methodically stacked, occasionally into whimsical patterns. The Catacombs are fashioned into a memorial to the brevity of life. The message above the entrance reads: Arrête! C'est ici l'empire de la Mort. ("Halt! This is the empire of Death.")
That has not stopped the Catacombs, accessible via a side door to a classicist building on the Avenue du Colonel Henri Rol-Tanguy, making just about every Top 20 list of things to see in Paris.
An underground economy
However, while the Catacombs certainly are the most famous part of the centuries-old network beneath Paris, and in non-pandemic times draw thousands of tourists each day, they constitute just 1.7 km (1 mile) of the 300-km (185-mile) tunneling total.
Subterranean Paris wasn't just used for mining and storing dead people. In the 17th century, Carthusian monks converted the ancient quarries under their monastery into distilleries for the green or yellow liqueur that still carries their name, chartreuse.
Because the mines generally keep a constant cool temperature of around 15° C (60° F), they were also ideal for brewing beer, as happened on a large scale from the end of the 17th century until well into the 20th century. Several caves were dug especially for establishing breweries, and not just because of the ambient temperature: going underground allowed brewers to remain close to their customers without having to pay a premium for real estate up top.
Overview of the Paris Catacombs.Credit: Inspection Générale des Carrières, 1857 / Public domain.
At the end of the 19th century, the underground breweries of the 14th arrondissement alone produced more than a million hectoliters (22 million gallons) per year. One of the most famous of Paris' underground breweries, Dumesnil, stayed in operation until the late 1960s.
In that decade, the network of corridors and galleries south of the Seine, long since abandoned by miners, became the unofficial playground for the young people of Paris. They explored the fantastical world beneath their feet, in some cases via entry points located in their very schools. Fascinated, these cataphiles ("catacomb lovers") read up on old books, explored the subterranean labyrinth, and drew up schematics that were passed around among fellow initiates as reverently as treasure maps.
As Robert Macfarlane writes in Underland, Paris-beneath-their-feet became "a place where people might slip into different identities, assume new ways of being and relating, become fluid and wild in ways that are constrained on the surface."
Some larger caves turned into notorious party zones: a 7-meter-tall gallery below the Val-de-Grâce hospital is widely known as "Salle Z." Over the last few decades, various other locations in subterranean Paris have hosted jazz and rock concerts and rave parties — like no other city, Paris really has an "underground music scene."
Hokusai's Great Wave as the backdrop to the "beach" under Paris.Credit: Reddit
Cataphiles vs. cataphobes
With popularity came increased reports of nuisance and crime — the tunnels provided easy access to telephone cables, which were stolen for the resale value of their copper.
The general public's "discovery" of the underground network led the city of Paris to officially interdict all access by non-authorized persons. That decree dates back to 1955, but the "underground police" have an understanding with seasoned cataphiles. Their main targets are so-called tourists, who by their lack of knowledge expose themselves to risk of injuries or worse, and degrade their surroundings, often leaving loads of litter in their wake.
The understanding does not extend to the IGC. Unlike in the 19th century, when weak cavities were shored up by purpose-built pillars, the policy now is to inject concrete to fill up endangered spaces — thus progressively blocking off parts of the network. That procedure has also been used to separate the Catacombs to prevent "infiltration" of the site by cataphiles.
Many subterranean streets have their own names, signs and all. This is the Rue des Bourguignons (Street of the Burgundians) below the Champs des Capucins (Capuchin Field), neither of which exists on the surface.Credit: Jean-François Gornet via Wikimedia and licensed under
The cataphiles, however, are fighting back. In a game of cat and mouse with the authorities, they are reopening blocked passages and creating chatières ("cat flaps") through which they can squeeze into chambers no longer accessible via other underground corridors.
Catacomb climate control
Alone against the unstoppable tide of concrete, the amateurs of Underground Paris would be helpless. But the fight against climate change may turn the subterranean labyrinths from a liability into an asset — and the City of Paris into an ally.
The UN's 2015 Climate Plan — concluded in Paris, by the way — requires the world to reduce greenhouse gas emissions by 75 percent by 2050. And Paris itself wants to be Europe's greenest city by 2030. More sustainable climate control of our living spaces would be a great help toward both targets. A lot of energy is spent heating houses in winter and cooling them in summer.
This is where the constant temperature of the Parisian tunnels comes in. It's not just good for brewing beer; it's a source of geothermal energy, says Fieldwork, an architectural firm based in Paris. It can be used to temper temperatures, helping to cool houses in summer and warming them in winter.
One catch for the cataphiles: it also works when the underground cavities are filled up with concrete. So perhaps one day, Paris Underground, fully filled up with concrete, will completely fall off the map, reducing the city's formerly real doppelgänger into an air conditioning unit.
Cool in summer, warm in winter: Paris Underground could become Paris A/C.Credit: Fieldwork
Strange Maps #1083
Got a strange map? Let me know at strangemaps@gmail.com.
Baby's first poop predicts risk of allergies
Meconium contains a wealth of information.
- A new study finds that the contents of an infants' first stool, known as meconium, can predict if they'll develop allergies with a high degree of accuracy.
- A metabolically diverse meconium, which indicates the initial food source for the gut microbiota, is associated with fewer allergies.
- The research hints at possible early interventions to prevent or treat allergies just after birth.
The prevalence of allergies arising in childhood has increased over the last 50 years, with 30 percent of the human population now having some kind of atopic disease such as eczema, food allergies, or asthma. The cause of this increase is still subject to debate, though it has been associated with a number of factors, including changes to the gut microbiomes of infants.
A new study by Canadian researchers published in Cell Reports Medicine may shed further light on how these allergies develop in children by examining the contents of their first diaper.
The things you do for science
The research team examined the first stool of 100 infants from the CHILD Cohort Study. The first stool of an infant is a thick, green, horrid-looking substance called meconium. It consists of various things that the infant ingests during the second half of gestation. Additionally, it provides not only a snapshot of what the infant was exposed to during that time, but it also reveals what the food sources will be for the initial gut bacteria that colonize the baby's digestive tract.
The content of the meconium was examined and found to contain such varied elements as amino acids, lipids, carbohydrates, and myriad other substances.
A graph of the comparative, summed abundance of different elements in a metabolic pathway after scaling to median abundance of each metabolite. The blue figures are those children without atopy, the yellow ones show the data for those with an atopic condition. Petersen et al.
The authors fed this information into an algorithm that used this data, along with the identities of the bacteria present as well as the baby's overall health, to predict which infants would go on to develop allergies within one year. The algorithm got it right 76 percent of the time.
A way to prevent childhood allergies?
Infants whose meconium had a less diverse metabolic niche the initial microbes to settle in the gut were at the highest risk of developing allergies a year later. Many of these elements were associated with the presence or absence of different bacterial groups in the digestive system of the child, which play an increasingly appreciated role in our overall health and development. The findings were summarized by senior co-author Dr. Brett Finlay:
"Our analysis revealed that newborns who developed allergic sensitization by one year of age had significantly less 'rich' meconium at birth, compared to those who didn't develop allergic sensitization."
The findings could be used to help understand how allergies form and even how to prevent them. Co-author Dr. Stuart Turvey commented on this possibility:
"We know that children with allergies are at the highest risk of also developing asthma. Now we have an opportunity to identify at-risk infants who could benefit from early interventions before they even begin to show signs and symptoms of allergies or asthma later in life."
A model for early childhood allergies
Petersen et al.
As shown above, the authors constructed a model of how they believe metabolites and bacterial diversity help prevent allergies. Increased diversity of metabolic products in the meconium encourage the development of "healthy" families of bacteria, like Peptostreptococcaceae, which in turn promote the development of a healthy and diverse gut microbiome. Ultimately, such diversity decreases the likelihood that a child will develop allergies.
Big think: Will AI ever achieve true understanding?
If you ask your maps app to find "restaurants that aren't McDonald's," you won't like the result.
- The Chinese Room thought experiment is designed to show how understanding something cannot be reduced to an "input-process-output" model.
- Artificial intelligence today is becoming increasingly sophisticated thanks to learning algorithms but still fails to demonstrate true understanding.
- All humans demonstrate computational habits when we first learn a new skill, until this somehow becomes understanding.
It's your first day at work, and a new colleague, Kendall, catches you over coffee.
"You watch the game last night?" she says. You're desperate to make friends, but you hate football.
"Sure, I can't believe that result," you say, vaguely, and it works. She nods happily and talks at you for a while. Every day after that, you live a lie. You listen to a football podcast on the weekend and then regurgitate whatever it is you hear. You have no idea what you're saying, but it seems to impress Kendall. You somehow manage to come across as an expert, and soon she won't stop talking football with you.
The question is: do you actually know about football, or are you imitating knowledge? And what's the difference? Welcome to philosopher John Searle's "Chinese Room."
The Chinese Room
Credit: http://america.pink/ via Wikipedia and licensed under CC0 1.0
Searle's argument was designed as a critique of what's called a "functionalist" view of mind. This is the philosophy that argues that our mind can be explained fully by what role it plays, or in other words, what it does or what "function" it has.
One form of functionalism sees the human mind as following an "input-process-output" model. We have the input of our senses, the process of our brains, and a behavioral output. Searle thought this was at best an oversimplification, and his Chinese Room thought experiment goes to show how human minds are not simply biological computers. It goes like this:
Imagine a room, and inside is John, who can't speak a word of Chinese. Outside the room, a Chinese person sends a message into the room in Chinese. Luckily, John has an "if-then" book for Chinese characters. For instance, if he gets <你好吗>, the proper reply is <我还好>. All John has to do is follow his instruction book.
The Chinese speaker outside of the room thinks they're talking to someone inside who knows Chinese. But in reality, it's just John with his fancy book.
What is understanding?
Does John understand Chinese? The Chinese Room is, by all accounts, a computational view of the mind, yet it seems that something is missing. Truly understanding something is not an "if-then" automated response. John is missing that sinking in feeling, the absorption, the bit of understanding that's so hard to express. Understanding a language doesn't work like this. Humans are not Google Translate.
And yet, this is how AIs are programmed. A computer system is programmed to provide a certain output based on a finite list of certain inputs. If I double click the mouse, I open a file. If you type a letter, your monitor displays tiny black squiggles. If we press the right buttons in order, we win at Mario Kart. Input — Process — Output.
Can imitation become so fluid or competent that it is understanding.
But AIs don't know what they're doing, and Google Translate doesn't really understand what it's saying, does it? They're just following a programmer's orders. If I say, "Will it rain tomorrow?" Siri can look up the weather. But if I ask, "Will water fall from the clouds tomorrow?" it'll be stumped. A human would not (although they might look at you oddly).
A fun way to test just how little an AI understands us is to ask your maps app to find "restaurants that aren't McDonald's." Unsurprisingly, you won't get what you want.
The Future of AI
To be fair, the field of artificial intelligence is just getting started. Yes, it's easy right now to trick our voice assistant apps, and search engines can be frustratingly unhelpful at times. But that doesn't mean AI will always be like that. It might be that the problem is only one of complexity and sophistication, rather than anything else. It might be that the "if-then" rule book just needs work. Things like "the McDonald's test" or AI's inability to respond to original questions reveal only a limitation in programming. Given that language and the list of possible questions is finite, it's quite possible that AI will be able to (at the very least) perfectly mimic a human response in the not too distant future.
Credit: Oli Scarff via Getty Images
What's more, AIs today have increasingly advanced learning capabilities. Algorithms are no longer simply input-process-output but rather allow systems to search for information and adapt anew to what they receive.
A notorious example of this occurred when a Microsoft chat bot started spouting bigotry and racism after "learning" from what it read on Twitter. (Although, this might just say more about Twitter than AI.) Or, more sinister perhaps, two Facebook chat bots were shut down after it was discovered that they were not only talking to each other but were doing so in an invented language. Did they understand what they were doing? Who's to say that, with enough learning and enough practice, an AI "Chinese Room" might not reach understanding?
Can imitation become understanding?
We've all been a "Chinese Room" at times — be it talking about sports at work, cramming for an exam, using a word we didn't entirely know the meaning of, or calculating math problems. We can all mimic understanding, but it also begs the question: can imitation become so fluid or competent that it is understanding.
The old adage "fake it, 'till you make it" has been proven true over and over. If you repeat an action enough times, it becomes easy and habitual. For instance, when you practice a language, musical instrument, or a math calculation, then after a while, it becomes second nature. Our brain changes with repetition.
So, it might just be that we all start off as Chinese Rooms when we learn something new, but this still leaves us with a pertinent question: when, how, and at what point does John actually understand Chinese? More importantly, will Siri or Alexa ever understand you?
Jonny Thomson teaches philosophy in Oxford. He runs a popular Instagram account called Mini Philosophy (@philosophyminis). His first book is Mini Philosophy: A Small Book of Big Ideas.
How cell phone data can help redesign cities
With the rise of Big Data, methods used to study the movement of stars or atoms can now reveal the movement of people. This could have important implications for cities.
- A treasure trove of mobility data from devices like smartphones has allowed the field of "city science" to blossom.
- I recently was part of team that compared mobility patterns in Brazilian and American cities.
- We found that, in many cities, low-income and high-income residents rarely travel to the same geographic locations. Such segregation has major implications for urban design.
Almost 55 percent of the world's seven billion people live in cities. And unless the COVID-19 pandemic puts a serious — and I do mean serious — dent in long-term trends, the urban fraction will climb almost to 70 percent by midcentury. Given that our project of civilization is staring down a climate crisis, the massive population shift to urban areas is something that could really use some "sciencing."
Is urbanization going to make things worse? Will it make things better? Will it lead to more human thriving or more grinding poverty and inequality? These questions need answers, and a science of cities, if there was such a thing, could provide answers.
Good news. There already is one!
The science of cities
With the rise of Big Data (for better or worse), scientists from a range of disciplines are getting an unprecedented view into the beating heart of cities and their dynamics. Of course, really smart people have been studying cities scientifically for a long time. But Big Data methods have accelerated what's possible to warp speed. As "exhibit A" for the rise of a new era of city science, let me introduce you to the field of "human mobility" and a new study just published by a team I was on.
Credit: nonnie192 / 405009778 via Adobe Stock
Human mobility is a field that's been amped up by all those location-enabled devices we carry around and the large-scale datasets of our activities, such as credit card purchases, taxi rides, and mobile phone usage. These days, all of us are leaving digital breadcrumbs of our everyday activities, particularly our movements around towns and cities. Using anonymized versions of these datasets (no names please), scientists can look for patterns in how large collections of people engage in daily travel and how these movements correlate with key social factors like income, health, and education.
There have been many studies like this in the recent past. For example, researchers looking at mobility patterns in Louisville, Kentucky found that low-income residents tended to travel further on average than affluent ones. Another study found that mobility patterns across different socioeconomic classes exhibit very similar characteristics in Boston and Singapore. And an analysis of mobility in Bogota, Colombia found that the most mobile population was neither the poorest nor the wealthiest citizens but the upper-middle class.
These were all excellent studies, but it was hard to make general conclusions from them. They seemed to point in different directions. The team I was part of wanted to get a broader, comparative view of human mobility and income. Through a partnership with Google, we were able to compare data from two countries — Brazil and the United States — of relatively equal populations but at different points on the "development spectrum." By comparing mobility patterns both within and between the two countries, we hoped to gain a better understanding of how people at different income levels moved around each day.
Mobility in Brazil vs. United States
Socioeconomic mobility "heatmaps" for selected cities in the U.S. and Brazil. The colors represent destination based on income level. Red depicts destinations traveled by low-income residents, while blue depicts destinations traveled by high-income residents. Overlapping areas are colored purple.Credit: Hugo Barbosa et al., Scientific Reports, 2021.
The results were remarkable. In a figure from our paper (shown above), it's clear that we found two distinct kinds of relationship between income and mobility in cities.
The first was a relatively sharp distinction between where people in lower and higher income brackets traveled each day. For example, in my hometown of Rochester, New York or Detroit, the places visited by the two income groups (e.g., job sites, shopping centers, doctors' offices) were relatively partitioned. In other words, people from low-income and high-income neighborhoods were not mixing very much, meaning they weren't spending time in the same geographical locations. In addition, lower income groups traveled to the city center more often, while upper income groups traveled around the outer suburbs.
The second kind of relationship was exemplified by cities like Boston and Atlanta, which didn't show this kind of partitioning. There was a much higher degree of mixing in terms of travel each day, indicating that income was less of a factor for determining where people lived or traveled.
In Brazil, however, all the cities showed the kind of income-based segregation seen in U.S. cities like Rochester and Detroit. There was a clear separation of regions visited with practically no overlap. And unlike the U.S., visits by the wealthy were strongly concentrated in the city centers, while the poor largely traversed the periphery.
Data-driven urban design
Our results have straightforward implications for city design. As we wrote in the paper, "To the extent that it is undesirable to have cities with residents whose ability to navigate and access resources is dependent on their socioeconomic status, public policy measures to mitigate this phenomenon are the need of the hour." That means we need better housing and public transportation policies.
But while our study shows there are clear links between income disparity and mobility patterns, it also shows something else important. As an astrophysicist who spent decades applying quantitative methods to stars and planets, I am amazed at how deep we can now dive into understanding cities using similar methods. We have truly entered a new era in the study of cities and all human systems. Hopefully, we'll use this new power for good.
The never-ending trip: LSD flashbacks and a psychedelic disorder that can last forever
A small percentage of people who consume psychedelics experience strange lingering effects, sometimes years after they took the drug.
- LSD flashbacks have been studied for decades, though scientists still aren't quite sure why some people experience them.
- A subset of people who take psychedelics and then experience flashbacks develop hallucinogen persisting perception disorder (HPPD), a rare condition in which people experience regular or near-constant psychedelic symptoms.
- There's currently no cure for the disorder, though some studies suggest medications may alleviate symptoms.
In February 2021, Josh was in his room and looking at his phone when he was struck by a strange feeling.
"The room looked normal, nothing was moving, but I felt as though I was under the influence of a psychedelic," he told Big Think.
As a teenager, Josh had experimented with LSD, mushrooms, and other psychedelics a couple dozen times. Now 25, he had been sober for about a year. He brushed off the incident.
But soon, Josh, which is not his real name, was struck again by the same strange feeling.
"I had no idea what was going on in my brain at that time and the anxiety and paranoia grew so intense that I became fearful I had developed everything from brain cancer to schizophrenia," he said.
The physical and psychological symptoms he began suffering were "devastating."
"The world [looked] crooked and out of focus, pictures had an eerie quality to them, things would go in and out of focus, at night while falling asleep I would experience vivid and terrifying hypnagogic hallucinations that made rest impossible."
After three weeks, Josh said his visual symptoms amplified with "unbelievable intensity."
"The floors would [breathe], paint on the walls looked wet, visual snow was so intense [that] pure black looked like it was glowing, at night I would see tracers everywhere, halos appeared around text. [...] I did not sleep, my thoughts were anxious and at times deranged, I had unbelievably intense dereliction that made the world seem fake."
LSD flashbacks
What Josh experienced is commonly called an LSD flashback. It's a mysterious phenomenon in which someone who's previously taken a hallucinogenic drug suddenly and temporarily experiences the effects of that drug days, weeks, or even years after consuming it.
Flashbacks can occur after taking a wide range of psychedelic drugs. But compared to other hallucinogens, flashbacks seem to be most common among people who have consumed LSD, according to studies.
Credit Newwup via Adobe Stock
People have reported acid flashbacks for decades. The earliest recorded case may be
Havelock Ellis' 1898 report of taking mescaline and then experiencing sustained heightened sensitization to "the more delicate phenomena of light and shade and color."
But it wasn't until the 1950s, little more than a decade after Albert Hoffman first synthesized LSD, that scientists started researching LSD and its potential long-term effects. While studies have illuminated some aspects of how psychedelics affect the brain, scientists still have much to learn about the nature of LSD flashbacks, what causes them, and how to treat them.
What's certain, however, is that a small percentage of people who consume psychedelics report bizarre and sometimes debilitating effects that emerge long after taking hallucinogens.
Symptoms of LSD flashbacks
Among the most common symptoms of LSD flashbacks are visual distortions. In a 1983 study titled " Visual Phenomenology of the LSD Flashback," the psychiatrist and LSD researcher Dr. Henry David Abraham described 16 common visual disturbances reported by people with LSD flashbacks. To name a few:
- Acquired color confusion: The color of objects changed or presented a newly discovered problem of color confusion.
- Difficulty reading: Text may appear jumbled or leave afterimages of the type against the background of the page.
- Geometric phosphenes: Phosphenes, or eigengrau, are non-specific luminous perceptions that occur when the eyes are closed and may originate from entopic (i.e., arising from within the eye itself) stimuli in normal persons. They also may be induced by gentle pressure on the closed eyelid.
- Pareidolias: This is literally an image within an image. These were described when a subject gazed into a finely reticulated design in linoleum, veneer, or a cloud formation. Besides the abstract pattern of the linoleum, subjects often would be able to see a series of concrete images as well, such as "a fish," "a face," and "a little boy."
- Macropsia: Macropsia is the perception of an object larger than it really is. A characteristic description of this phenomenon came from a subject who noticed that his hand was enormous and then of normal size a few seconds later.
- Micropsia: Micropsia is the perception of an object smaller than reality. One subject said, "My feet looked so tiny, like they were a million miles away."
The effects of LSD flashbacks aren't limited to visual distortions. In a 1970 study called "Analysis of the LSD Flashback," researchers sorted LSD flashbacks into three broad categories: perceptual, somatic (meaning of the body), and emotional.
The emotional flashback is "far more distressing" than the other two, the researchers wrote, providing a case study of a 21-year-old woman who was suffering from LSD flashbacks:
"The patient had these frightening flashbacks during the day, while walking down the street, after smoking marijuana or drinking wine, during the night, and occasionally even while asleep. In one situation she awoke during the middle of the night with a feeling of panic and began running around her house fleeing an imagined threat she could not identify or comprehend. She had taken LSD a number of times, but her last few trips were bad ones with panic and fright followed by loneliness to the point of suicidal despair when she 'came down.' The combination of bad trips and emotional flashbacks made her seek professional help because of her fear that she would harm herself."
To be sure, LSD flashbacks aren't always emotionally distressing. A 2010 survey of 600 hallucinogen users found that, of the minority of users who reported experiencing at least one flashback, only 3 percent described it as a negative experience. In fact, some people enjoyed their flashbacks. On the website Erowid, which promotes research of psychedelic drugs, one user wrote:
"After 2 years of my last acid trip, while on vacation in a very nice wilderness place I was sitting on a rock and then I experienced a clear acid high. I was looking at a very steep hill and suddenly it started moving in nice patterns, exactly as one sees patterns while on acid. It wasn't something uncomfortable. In fact it was really pleasant and there was absolutely no trace of the nasty anxiousness after effects common to LSD. It lasted approximately 2 minutes and I enjoyed it very much."
But some LSD flashbacks are neither brief nor pleasant. A subset of people who use psychedelics develop hallucinogen persisting perception disorder (HPPD), a rare and poorly understood condition in which people experience omnipresent or recurring flashbacks. While the symptoms of HPPD vary, the condition can cause intense pain, irreversible perceptual distortions, emotional and psychological distress, and even suicidal thoughts.
HPPD: The never-ending trip
HPPD is estimated to affect between one to five percent of LSD users, though the actual figure is impossible to determine without better data. The disorder was first described formally in 1986 by the American Psychiatric Association's Diagnostic & Statistical Manual of Mental Disorders, 3rd edition, revised (DSM-III-R). The current edition of the manual (DSM-5) says patients need to meet several criteria to be diagnosed with HPPD:
- Patients must reexperience perceptual symptoms they experienced while intoxicated with the hallucinogen.
- These symptoms must cause "significant distress or impairment in social, occupational, or other important areas of functioning."
- These symptoms aren't due to a separate medical condition or mental disorder.
So, what's the difference between a flashback and HPPD? Mainly frequency and duration. A 2017 review published in Frontiers in Psychiatry noted that while "a flashback is usually reported to be infrequent and episodic, HPPD is usually persisting and long-lasting."
A 2014 review published in the Israel Journal of Psychiatry and Related Sciences outlined two types of HPPD. The first, HPPD I, is the "flashback type," which is a generally short-term, non-distressing, benign and reversible state accompanied by a pleasant affect. The severity of HPPD I varies, with some people describing their mild flashbacks as annoying, while others say it's like getting "free trips."
But HPPD II is a different beast. The condition can be permanent, with perceptual distortions and other symptoms manifesting irregularly or almost constantly.
"The symptoms usually include palinopsia (afterimages effects), the occurrence of haloes, trails, akinetopsia, visual snows, etc.," according to the aforementioned 2017 review. "Sounds and other perceptions are usually not affected. Visual phenomena have been reported to be uncontrollable and disturbing. Symptomatology may be accompanied by depersonalization, derealization, anxiety, and depression."
What causes flashbacks and HPPD?
When asked what causes flashbacks and HPPD, Dr. Abraham told Popular Science, "I've spent my life studying this problem and I don't know, is the short answer."
But researchers have proposed explanations. One centers on memory. Because psychedelics can cause extremely powerful and emotional experiences, it's theoretically possible that certain environmental stimuli can remind people of those experiences, and then memory "transports" them back into that subjective mindset — similar to how a soldier with post-traumatic stress disorder might suffer an episode after hearing a loud, sudden noise.
Another hypothesis involves how LSD interacts with the brain's visual processing center. Dr. Abraham proposed that HPPD may arise due to "disinhibition of visual processing related to a loss of serotonin receptors on inhibitory interneurons," which may be caused by consuming LSD.
The basic idea is that LSD somehow changes the way the brain interprets visual stimuli. That might explain why people with HPPD have difficulty properly "disengaging" from the things they see around them. For example, a red stoplight might appear not as a discrete red circle but as a streak of red light painted across their field of vision; or a strobe light might not appear as a flickering light but a light that's constantly on.
Credit Yurok Aleksandrovich via Adobe Stock
"Such a locking of visual circuitry into an 'on' position following perception of a visual stimulus would explain such diverse complaints as trailing, color intensification, positive afterimages, phosphenes, and color confusions, each of which may represent a failure of the respective visual function to turn off the brain's response to the stimulus once the stimulus is gone," Dr. Abraham
wrote.
It's also possible that people are genetically predisposed to HPPD and that ingesting LSD is the key that unlocks the disorder. This hypothesis would help explain why people have reportedly developed HPPD after taking a single, moderate dose of LSD.
Ultimately, the exact causes of HPPD are unclear. Partially as a result, there's currently no cure for the disorder, though studies show that people with HPPD have reported improvements in symptomatology after taking benzodiazepines. There's also anecdotal evidence that fasting can alleviate the disorder.
Despite uncertainty over the causes of HPPD, researchers do have a good idea of what can trigger "flare-ups" of HPPD. Dr. Abraham's 1983 study listed the most common triggers, some of which include:
- Emergence into a dark environment
- Intention (intentionally inducing visual aberrations by, say, staring at a blank wall)
- Marijuana
- Phenothiazines
- Anxiety
- Fatigue
People with HPPD describe the condition
To get a better understanding of HPPD, Big Think posted a questionnaire to the HPPD community on Reddit. Here are some of the responses:
How did HPPD first manifest for you?
IBeatMyGlied wrote:
"First I noticed highly enhanced creativity and intense visuals when [high on] weed and I really enjoyed that part. The realization that this is not going to go away soured the whole experience tho."
"My enhanced creativity left me after about a week and what I was left with was mild visual snow. I hardly knew anything about HPPD at the time and just didn't really care about my symptoms and still thought they were just going to vanish at some point, which they didn't. I kept taking drugs simply because I was addicted and felt like life is no fun without them. My HPPD got gradually worse over time and more symptoms appeared. First, I noticed mild tracers, which got worse over time (again due to continued drug use) and then tinnitus and brain fog. But primarily my symptoms are visual."
Are your symptoms episodic or constant?
"Both constant and episodic," wrote user LotsOfShungite. "A stressful event can trigger my symptoms off into the deep end."
Halven89 wrote:
"Except the brain fog and head pressure that varies, my visual disturbances are constant. The most debilitating ones are the visual snow, especially when I'm inside except if I watch the TV since it filters some of it out. It's also VERY frustrating that I no longer can focus on objects/details (can't stare) and the astigmatism-like symptoms that I got, like blurriness, especially in the distance and ghosting (double vision) plus starbursts from strong light sources. When I'm outside, the pattern glare is really annoying, same with the excessive amount of floaters that came with this. I also see halos from light sources."
IBeatMyGlied wrote:
"My symptoms are mostly constant and only change through rather obvious outside influences, such as certain drugs (almost all drugs), stress, lack of sleep, etc. Although my HPPD is quite pronounced, I have learned to accept it and almost only notice it when I pay attention to it. I always [know] it's there and it somewhat bugs me but I get along."
What are some common misconceptions about HPPD?
IBeatMyGlied wrote:
"One of if not the biggest 'misconception' is that many people believe that HPPD does not exist. But I guess there is no way to prove to another person that it does, so this is gonna stay the case until HPPD enters the public consciousness of the psychedelic community."
Halven89 wrote:
"They usually don't understand anything about it since most haven't heard about it, which really is crazy considering how debilitating this disorder is for many. And as Dr. Abraham said: in the medical field it's highly under- and misdiagnosed. Often as psychosis."
Lopyriev via Adobe Stock
Hope for HPPD
Since experiencing his first acid flashback in February, Josh has found a few helpful strategies to minimize symptoms, including seeing a psychologist, staying sober, getting enough sleep, staying productive, and talking regularly with friends. He seemed optimistic about the future:
"The symptoms will lessen with time and sobriety, and HPPD provides an opportunity to improve yourself. That being said, because thoughts of suicide are apparently common with people that have HPPD, the medical community should take the condition seriously. Especially given how many people use psychedelics today."
While the future of HPPD research remains unclear, general psychedelic research is going through something of a renaissance. In recent years, researchers have published a growing body of studies showing how psychedelics like psilocybin, LSD, and MDMA can help treat conditions like depression, anxiety, post-traumatic stress disorder, and existential distress.
But, among people with HPPD, opinions on the utility of psychedelics vary. Josh advised caution:
"I would not recommend [hallucinogenic] drugs be taken for recreational purposes. They are tools to help us treat illnesses and should be treated as such. If someone has depression or other mental health issue, maybe psychedelics administered in a clinical setting by a doctor is appropriate, but otherwise, playing with your brain like it's a chemistry playset is asking for trouble down the road."
Mind and God: The new science of neurotheology
Studies show that religion and spirituality are positively linked to good mental health. Our research aims to figure out how and why.
- Neurotheology is a field that unites brain science and psychology with religious belief and practices.
- There are several indirect and direct mechanisms that link spirituality with improved mental health.
- Compassion and love are positive emotions that will make your brain healthier.
The field of neurotheology continues to expand from its early origins several decades ago to the present day. In its simplest definition, neurotheology refers to the field of scholarship that seeks to understand the relationship between the brain and our religious and spiritual selves. As I always like to say, it is important to consider both sides of neurotheology very broadly. Thus, the "neuro" side includes brain imaging, psychology, neurology, medicine, and even anthropology. And the "theology" side includes theology itself, but also various aspects related to religious beliefs, attitudes, practices, and experiences.
The mental health benefits of spirituality
Neurotheology also ranges from considering very esoteric concepts including questions around free will, consciousness, and the soul, to very practical concepts such as understanding how the brain functions and the relationship between spirituality and physical and mental health. This latter topic might be called "applied neurotheology." Applied neurotheology, therefore, seeks to understand the health-related aspects pertaining to our brain and our spiritual selves. In particular, we can try to understand how being religious or spiritual, or performing various spiritual practices, might be beneficial to our overall health and well-being. In our latest book, entitled Brain Weaver, we consider this important dimension of human brain health.
Even for those who are not religious, pursuing practices such as meditation and prayer — even when secularized — can be beneficial for reducing stress and anxiety.
A growing number of studies have shown how spirituality and mental health are linked. Importantly, studies have shown that those who are religious and spiritual tend to have lower rates of depression, anxiety, and suicide. This is true across the age spectrum with studies of adolescents showing that religious and spiritual pursuits are protective against mental health problems. And many adults cite religious and spiritual beliefs as important for coping with various life stressors.
Indirect mechanisms
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If there is a relationship between spirituality and positive mental health, we might question what the mechanism of action might be. I have typically divided the mechanisms into indirect and direct ones. The indirect mechanisms have to do with specific aspects of a given tradition that end up having ancillary mental health benefits. For example, going to church or other social events that are part of a religious tradition can be beneficial because social support, in and of itself, is beneficial to our mental health. The more people that we have in our social support network, the better we are at coping with various life stressors including problems with jobs, relationships, or health.
Most religions also teach people to avoid a lot of high-risk behaviors that can be very detrimental to our mental health and well-being. For example, most religions teach us to avoid alcohol and drugs, to not be promiscuous, and to try to be compassionate and charitable to others. By following these teachings, people will naturally avoid mental health problems such as substance abuse and tend toward being more optimistic and less depressed. These effects have nothing to do with being religious per se and everything to do with following a religion's advice.
Another interesting indirect mechanism of action related to religion has to do with diet and nutrition. Diet and nutrition are frequently overlooked when it comes to good mental health, even though research increasingly indicates they are essential. Many traditions ask individuals to follow certain dietary guidelines. For example, Hindus tend to have vegetarian diets, and most research to date shows that eating a more plant-based diet with a lot of low-inflammatory foods is good not only for your body but for your brain as well. In fact, we are currently performing a study with patients who have chronic concussion symptoms to determine the effect of dietary improvements on overall brain function.
Direct mechanisms
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The direct mechanisms of action have to do with specific spiritual practices and even a person's personal sense of spirituality. Much of my research over the past 30 years has been to study the brain while people engage in different practices such as meditation or prayer. We have even observed brain changes associated with unique spiritual practices such as speaking in tongues or trance states. The brain effects related to these practices are quite remarkable and diverse. It should come as no surprise since these practices affect people on many different levels, such as the way people think, feel, and experience the world around them. Thus, we should expect to observe physiological differences in the parts of the brain involved with these practices.
Meditation and prayer, for example, activate the frontal lobes as well as the language areas of the brain, and research demonstrates that this occurs not only while the practice is performed but over the long-term as well. Our study of Kirtan Kriya meditation showed improvements of about 10 to 15 percent in cognition as well as reductions in stress, anxiety, and depression. These were associated with baseline changes to the brain's frontal lobe functions, which regulate these cognitive processes and modulate emotional responses.
More recent research has been exploring the effects of these practices on larger brain networks, and perhaps more important, specific neurotransmitter systems. One of our recent studies of a spiritual retreat program showed significant changes to the areas of the brain that release dopamine and serotonin. These are areas known to be involved in both cognition and emotional health. And there are a growing number of clinical studies which have documented the value of various spiritual practices or religiously oriented therapies for helping people manage a variety of mental health conditions including depression, anxiety, and ADHD as well as neurological conditions like Alzheimer's and seizure disorders.
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Finally, a personal sense of spirituality may be protective in and of itself. When people feel connected to all of humanity, a higher power, or the entire universe, that experience gives people a sense of meaning and purpose in life and an optimistic perspective on what the future holds. A number of research studies have shown that having such faith can be beneficial to your overall physical and mental health.
Improving brain health with applied neurotheology
Applied neurotheology can teach us the value of exploring our religious and spiritual side as a way of improving our mental health and well-being. Even for those who are not religious, pursuing practices such as meditation and prayer — even when secularized — can be beneficial for reducing stress and anxiety. Connecting with the larger world — by going on a nature walk, socializing with friends and family, or trying to make your neighborhood a better place by helping others — leads to a greater sense of compassion and love, positive emotions that will make your brain healthier.
Dr. Andrew Newberg is a neuroscientist who studies the relationship between brain function and various mental states. He is a pioneer in the neurological study of religious and spiritual experiences, a field known as "neurotheology." His latest book is Brain Weaver.
