Quick: are you right-brained or left-brained? Chances are, you answered this question immediately and definitively. If you are the creative, intuitive type, drawn to creating music, stories, images, and other forms of art, then you are right-brained. If, by contrast, you are more analytical and logical, drawn to mathematics and pattern recognition, then you are left-brained. And if you don’t already know which you are, then there is a whole industry dedicated to helping you find out, offering countless online quizzes, seminars, and other materials to help you pinpoint, strengthen, and get the most out of your inherent cognitive strengths. Indeed, Left/Right brain theory has found its way into all sorts of disciplines – particularly the world of business, where many companies hire right-brained employees for more creative roles and left-brained employees for managerial positions. Given its widespread acceptance and use, this theory must be based on the latest neuroscience, right?

Well…no, sorry.

Like the notion that we only use 10% of our brains, the idea that our skills and personality are determined by which hemisphere of our brain is most dominant is a persistent pop-psychology myth – one which, like most personality tests including the popular Meyers-Briggs, has about as much predictive power as astrology. Yet like many modern scientific myths, the Left-Right dichotomy is based on a kernel of truth. Certain brain functions are indeed concentrated in different hemispheres, but the way this lateralization works is far more subtle and complex than the simplistic model presented in pop psychology. So, exactly how much of the modern myth is true, and how much is complete fiction? Let’s find out as we dive into the fascinating science of the divided brain.

From the outside, the brain appears perfectly symmetrical. At the bottom is the hindbrain, composed of two structures: the brainstem, which controls autonomous functions like breathing and digestion; and the limbic system, which controls more complex but basic functions like memory processing, emotion, and motivation. Below the hindbrain is the cerebellum, which handles sensory processing and motor coordination; while wrapped around it is the forebrain or cerebrum, which is responsible for higher cognitive functions. The cerebrum is divided down the middle into two hemispheres, which are further divided into four lobes: frontal, parietal, temporal, and occipital. The cerebellum is also divided into two symmetrical hemispheres, while the cerebral hemispheres are connected together via a bundle of nerve fibres known as the corpus callosum.

In the 1860s, however, evidence began to emerge that the brain is not as symmetrical as it outwardly appears. In 1861, French physician and anatomist Paul Broca encountered two patients with striking verbal impairments. The first, Louis Victor Leborgne, had almost completely lost the ability to speak, only being able to pronounce a single word: temps [pronounced “tan” or “tahn”] – the French word for “time”. Strangely, his other cognitive faculties – including his ability to read, write and understand spoken language – were unimpaired. The second patient, Lazare Lelong, suffered a similar impairment, and could only pronounced five words: yes, no, three, always, and lelo – a mispronunciation of his own surname. Following these patients’ deaths, Broca autopsied their brains and discovered that both had been suffering from neurosyphilis, which had inflicted lesions in the exact same area of their brains: the third convolution of the frontal lobe of the left hemisphere. Based on these and 12 similar historic cases, in 1865 Broca published a landmark paper titled Localization of Speech in the Third Left Frontal Cultivation in which he theorized that speech production was centralized in this location, now known as Broca’s Area.

A decade later in 1874, German physician and anatomist Karl Wernicke described a similar but distinct kind of aphasia in which patients were able to pronounce words fluently, but spoke in incoherent sentences devoid of structure and meaning – what is today commonly known as word salad. Strangely, these sentences retained the cadence and syntax of ordinary speech, while the patients remained completely unaware that their speech was in any way disordered. Their ability to understand language, both spoken and written, was also often impaired. Like Broca, Wernicke discovered that this impairment was caused by damage to a very specific area of the brain: in this case a region in the left posterior frontal lobe now known as Wernicke’s area. These observations led Wernicke to divide aphasia into two basic types: Broca’s or motor aphasia and Wernicke’s or sensory aphasia – also known as receptive aphasia.

Broca and Wernicke’s discoveries led psychologists to conclude that language production and comprehension are localized entirely in the left hemisphere of the brain. However, it was not until the 1960s that the full extent of lateral specialization in the brain truly started to be appreciated. At the start of the decade, neurosurgeons began performing a radical new procedure to help sufferers of severe epilepsy.

Known as a corpus callosotomy, the procedure involved severing the nerve fibres of the corpus callosum to prevent epileptic signals spreading from one hemisphere to the other. Initially this surgery was believed to have no side effects, but the longer patients were observed, the stranger their behaviour became. They began favouring the right side of their bodies when performing everyday tasks, and seemed completely unaware of any stimulation coming from their left side. For example, if they bumped their left arm they would not notice, and if an object was placed in their left hand they would deny its existence. Intrigued by this strange behaviour, in 1962 psychologists Roger Sperry and his graduate student Michael Gazzaniga of the California Institute of Technology began a series of groundbreaking experiments to find out just what was going on inside the split-brain patients’ heads. What they discovered would change our understanding of the human brain forever.

While the corpus callosum had previously been considered a largely functionless structure, with psychologist Karl Lashley had even speculating that its served no greater purpose than to “keep the hemispheres from sagging”, Sperry and Gazzaniga quickly discovered how vital a role it plays in the functioning of the brain. Due to a quirk of vertebrate evolution, our nervous systems are contralateral, meaning that each hemisphere receives information largely from the opposite side of the body. For example, the optic nerves, which convey visual information from our eyes to our occipital lobes, cross over at a junction called the optic chiasm, meaning that information from the right eye is transmitted to the left hemisphere and vice-versa. Similarly, each hemisphere controls the opposite side of the body, such that a stroke in the left hemisphere will produce paralysis on the right side of the body and vice-versa. Ordinarily this counter-intuitive arrangement works just fine as the information is immediately transmitted to the correct hemisphere via the corpus callosum. But in split-brain patients this channel of communication no longer exists, meaning that information transmitted to a particular hemisphere stays in that hemisphere. And this is where things start to get weird.

Sperry and Gazzaniga probed the patients’ hemispheres individually by stimulating the opposite side of the body – for example, by presenting an image to the right eye to stimulate the left hemisphere. In one early experiment, they flashed a series of lights across the patients’ field of view. When asked to report when they had seen a light, the patients only reported seeing lights flashing on the right. But when asked to point whenever they saw a light, they successfully reported seeing lights on both sides. Next, Sperry and Gazzaniga projected the word HEART such that the letters HE appeared in the patient’s left-hand field of vision and ART in their right-hand field of vision. When asked to report what they saw, the patients verbally responded “ART”; but when asked to point to the word they saw using their left hand, they pointed to HE. Similarly, if an object was placed in the patient’s right hand, they were easily able to name it, though when asked to point to an image of the same object using their right hand, were unable to do so. When the sides were reversed, the patients could easily point to the object, but, much to their confusion, were unable to name it. This and similar experiments confirmed that language processing abilities are almost entirely localized in the left hemisphere, while the right hemisphere specializes in visual perception tasks such as recognizing faces and emotions and spotting differences between objects.

But as strange as these discrepancies are, the experience of living with a split brain can sometimes be even more bizarre, with patients feeling as though they literally have two separate brains – brains which are often at odds with one another. For example, patients have reported doing up their shirt buttons with one hand only to have the other hand spontaneously unbutton them, or placing items in a shopping cart with one hand only for the other to place them back on the shelf. Many patients are even able to copy two different images using each of their hands, though given the right hemisphere’s greater spacial reasoning capabilities, the left hand is generally superior at this task than the right. In rare cases this phenomenon can even take the form of “alien hand syndrome,” in which a patient’s hand appears to have a mind of its own and sometimes attempts to strangle its owner or others. This is also sometimes known as “Dr. Strangelove Syndrome” after Peter Sellers’ character in the 1964 Stanley Kubrick film who exhibits similar symptoms. Unfortunately there is no cure for the condition other than keeping the offending hand occupied with other tasks and restraining it at night to prevent injuries.

In his seminal 1974 paper summarizing his findings – which would eventually win him the 1981 Nobel Prize in Physiology and Medicine – Sperry concluded:

“…[each hemisphere is] indeed a conscious system in its own right, perceiving, thinking, remembering, reasoning, willing, and emoting, all at a characteristically human level, and … both the left and the right hemisphere may be conscious simultaneously in different, even in mutually conflicting, mental experiences that run along in parallel.”

Specifically, Sperry concluded that the left hemisphere specializes in logic, sequencing, linear thinking, mathematics, hard facts, and thinking in words. By contrast, the right hemisphere handles “softer” tasks like imagination, holistic thinking, intuition, visual-spacial processing, facial recognition, and interpreting non-verbal social cues. Indeed, according to Sperry’s experiments, this lateralization of mathematical thinking was almost complete:

“…tests for mathematical performance in the minor hemisphere with nonverbal readout and with the sensory input restricted to the left visual field or the left hand, indicate … that the capacity for calculation on the minor side is almost negligible. By manipulating marbles or dowel sticks, watching spots of light flashed to the left field and pointing with the left hand, … (split brain) patients may succeed in matching numbers or in adding one to numbers below 10, but they fail when required to add or subtract two or higher numbers and they fail also at the simplest tasks in multiplication and division.”

Later, Sperry observed that the right Hemisphere could, in fact, perform addition to sums of less than 20 – the only exception to the left Hemisphere’s total dominance over mathematical thought.

Later observations seemed to confirm Sperry’s findings, with neurologists concluding that primary acalculia, the fundamental inability to comprehend and perform mathematics, results only from damage to the left parietal region of the brain. By contrast, secondary acalculia, caused by damage to the right hemisphere, affects the brain’s ability to receive mathematical information through the senses or express it through language but not its fundamental ability to comprehend and process said information.

Less invasive studies further confirmed the task lateralization of the brain. In 1973, Drs. Robert Ornstein and David Galin of the Langley Porter Neuropsychiatric Institute in San Francisco had test subjects perform a variety of cognitive tasks while an electroencephalogram or EEG monitored the activity in each of their hemispheres. When asked to perform mental arithmetic, think about writing a letter, or perform language exercises like thinking up a list of verbs beginning with the letter R, the subjects’ left hemispheres produced the fast brainwaves characteristic of attention and activity, while their right hemispheres produced the low-frequency alpha waves characteristic of relaxation, indicating that that hemisphere was largely switched off during those tasks. When, however, the subjects were asked to produce designs using coloured blocks, remember sequences of musical notes, or draw with an Etch-a-Sketch, the reverse occurred, with the right hemisphere producing fast waves and the left hemisphere alpha waves. As Ornstein and Galin concluded:

“Our opinion is that in most ordinary activities, we simply alternate between cognitive modes, rather than integrating them. These modes complement each other but do not readily substitute for each other.”

But while Roger Sperry cautioned that:

“…experimentally observed polarity in right-left cognitive style is an idea in general with which it is very easy to run wild.”

…it was already too late. In a 1973 New York Times Magazine article discussing Ornstein and Galin’s experiments, the two scientists stated that in different people one hemisphere or the other dominates, shaping their innate talents and abilities:

“Ideally we should be able to turn on the appropriate hemisphere and turn off the other, whenever the task requires it. But in fact we cannot always do it. “Many persons are dominated by one mode or the other,” observes Dr. Ornstein. “They either have difficulty in dealing with crafts and body movements, or difficulty with language.” Culture apparently has a lot to do with this. Children from poor black neighborhoods generally learn to use their right hemisphere more than the left—they outscore whites on tests of pattern recognition from incomplete figures, for instance, but tend to do badly at verbal tasks. Other children, who have learned to verbalize everything, find this approach a hindrance when it comes to copying a tennis serve or learning a dance step. Analyzing these movements verbally just slows them down and interferes with direct learning through the right hemisphere.

“We don’t have the flexibility we could have,” says Ornstein. “We are under the illusion of having more control than we really do.” Early in life, it seems, many of us become shaped either as “left hemisphere types,” who function in a largely verbal world, or as “right‐hemisphere types,” who rely more on nonverbal means of expression. These are two basically different approaches to the world.”

Further, the article stated that:

“When the habit of always using the same side of the brain becomes too pronounced, it can narrow one’s personality, Drs. Ornstein and Galin believe. The two researchers are currently working on a test that may enable them to tell which half‐brain a person chronically favors, and whether this habit interferes with the ability to shift dominance to the other side when necessary. They plan to try it out on people who are really specialized, like Ralph Nader (a left‐hemisphere type who has no hobbies of any kind) and right‐hemisphere potters, dancers and sculptors (“preferably people who have trouble with language”). They expect to find. significant differences between the two groups. This should give them a tool with which to guide children or adults to new aspects of themselves, to open them to a full range of experiences.”

Thus a persistent pop-psychology myth was born, with all manner of publications including TIME Magazine, Harvard Business Review, and Psychology Today soon jumping on the Left-Brained / Right-Brained bandwagon. The theory was further popularized by Betty Edwards’ 1979 book Drawing on the Right Side of the Brain, in which the author presents various techniques for bypassing the “analytical” left hemisphere and allowing visual creativity to flourish. Today, Left Brained/Right-Brained theory has spawned an entire industry of online quizzes, seminars, and other material aimed at helping people determine which side of their brain is dominant, get the most out of their natural abilities, or even strengthen the non-dominant side of their brains. This notion has even found its way into the world of business, where some companies try to hire right-brained employees for more creative roles and left-brained employees for managerial positions.

But as is often the case with the human mind, things are nowhere near as clear-cut as pop psychology would have us believe. For while the findings of Sperry, Gazzinaga, Ornstein, and Galin seem to indicate that the right hemisphere plays absolutely no role in mathematical thinking and language processing, this is not, in fact, the case. Indeed, Sperry and Gazzinaga observed multiple instances of the right hemisphere playing an active role in supposedly left-hemispheric tasks. For example, one patient, when presented with a picture of his girlfriend in his left eye, was unable to speak her name, but was able to spell it out using Scrabble tiles. They also discovered that while the left hemisphere excels at making straightforward word associations, the right hemisphere is better at recognizing subtler relationships and insinuations. For example, when the left hemisphere was presented with the word foot, it was better at picking out a related term like heel from a list of words. But when the right hemisphere was presented with two additional words, cry and glass, it more easily picked out the connecting word – in this case, cut.

The right hemisphere also plays a larger than expected role in mathematical thinking. As Dr. Kara Federmeier, professor of psychology at the University of Illinois, explains:

“One problem with answering this question is that we would first have to agree on what “logical” and “creative” even mean. So let’s consider a (relatively) more well-defined case: math skills, which are often taken to be part of what the “logical” left hemisphere would be good at.

There are different kinds of math skills, ranging from being able to estimate which of two sets of things has a greater number of items, to counting, to various types of calculations. Research shows that, overall, the abilities that make up math skills arise from processing that takes place in BOTH hemispheres (especially the brain area in each hemisphere that is known as the intraparietal sulcus) and that damage to either hemisphere can cause difficulties with math. A left hemisphere advantage for math is mostly seen for tasks like counting and reciting multiplication tables, which rely heavily on memorized verbal information (thus, not exactly what we think of as “logical”!). And there are right hemisphere advantages on some math-related tasks as well, especially estimating the quantity of a set of objects. This kind of pattern, in which both hemispheres of the brain make critical contributions, holds for most types of cognitive skills. It takes two hemispheres to be logical – or to be creative.”

Indeed, according to the popular conception of the Left/Right brain divide, we would expect people suffering from right-hemisphere brain damage to become emotionless but hyper-rational calculating and decision-making machines, rather like the Vulcans from Star Trek. In reality, however, such individuals struggle to make even basic decisions or plans, as they lack the intuitive and emotional faculties to conceptualize the bigger picture and turn logic into practical action. Thus, logic and emotion do not, as is often assumed, stand in opposition to one another; both are needed to effectively function in the world.

Recent research has revealed that many cognitive functions are actually shared evenly between both hemispheres, including processing of visual and auditory stimuli, spatial manipulation, facial recognition, artistic ability, numerical estimation, and comparison. Even the left-lateralization of core language abilities as established by Broca, Wernicke, and others does not always hold true. For example, while the Broca’s and Wernicke’s areas are usually located in the left hemisphere, in 5% of right-handers and 30% of left-handers – whose dominant hand, remember, is controlled by the opposite hemisphere – they are instead located in the right hemisphere. Indeed, the lateralization of different cognitive functions varies so widely between individuals that neurosurgeons often perform a special test to pinpoint these cognitive loci prior to performing invasive brain surgery like removing tumours. Known as the intracarotid sodium amobarbital procedure or Wada-Milner test, this involves injecting a barbiturate sedative into one hemisphere or the other to disable it and asking the patient to perform various cognitive tasks.

The popular conception of brain lateralization also fails to account for the phenomenon of neuroplasticity – the brain’s remarkable ability to adapt to physical damage. Children who have had one entire hemisphere of their brains removed due to cancer or other disorders have gone on to live completely normal lives, with the brain rewiring itself to perform all its necessary functions using just one hemisphere. Such neuroplasticity has also been observed in adults who have undergone similar surgeries or suffered traumatic brain injuries – though to a somewhat lesser extent.

Alright, so the brain is more complicated than pop culture would have us think, and its functions more evenly distributed than we would expect. But surely different people favour one side of their brains over another, right? After all, how else can we account for some people being more logical and analytical while others are more creative and artistic? Unfortunately, science does not bear out this notion either. A 2013 survey carried out by Jared Nielsen and colleagues at the University of Utah analyzed the neural activity of 1,011 individuals aged between 7 and 29 while they performed various cognitive tasks in a Resting State Functional Connectivity Magnetic Resonance Imaging or RS-FCM-MRI machine. The study found that:

“Nine left- and 11 right-lateralized hubs were identified…[as] significantly lateralized connections. The left-lateralized hubs included regions from the default mode network…whereas the right-lateralized hubs included regions from the attention control network … Left- and right-lateralized hubs formed two separable networks of mutually lateralized regions. Connections involving only left- or only right-lateralized hubs showed positive correlation across subjects, but only for connections sharing a node. Lateralization of brain connections appears to be a local rather than global property of brain networks, and our data are not consistent with a whole-brain phenotype of greater “left-brained” or greater “right-brained” network strength across individuals. Small increases in lateralization with age were seen, but no differences in gender were observed…[we also found that] lateralized connections are independent from one another across individuals, and that the majority of functional lateralization occurs before age seven.”

In other words, while localization of various cognitive functions does vary among individuals, on the whole neither the left or the right hemisphere dominates in any significant way. Indeed, many skills and talents derive not from one hemisphere working more than the other, but from both working together more efficiently. For example, children considered gifted in mathematics or music tend to show greater communication between their two hemispheres, allowing them to more effectively combine both their logical/analytical and creative/intuitive faculties. Conversely, those who struggle at certain tasks do so not necessarily because one hemisphere of their brain is weaker, but often because one hemisphere developed to perform a task usually handled by the other. However, as with nearly all cognitive tasks, even the weakest skills can be gradually strengthened through practice.

Yet despite more than five decades of research debunking it, it is estimated that more that 68% of people still believe in the Left-Brained/Right-Brained myth. But why? Simple: because we humans love to think about ourselves and come up with various systems to sort ourselves and others into neat categories. Like horoscopes and many personality tests, quizzes for determining left or right-brainedness exploit a psychological phenomenon known as the Barnum or Forer Effect – the tendency of people to believe descriptions of themselves that seem personally tailored to them but which are in fact vague enough to apply to everyone. This effect, commonly used by astrologers, mediums, mind readers, and other hucksters, was most famously demonstrated by stage magician and paranormal debunker James Randi, who handed out personalized horoscopes to a classroom of students and asked them to rate the accuracy of the descriptions. Almost without exception, the students rated the horoscopes as highly accurate. Randi then asked the students to exchange horoscopes, revealing that they had all received the exact same one! The Left-Brain/Right-Brain fallacy validates and provides a seemingly rational explanation to our everyday observations that some people seem more logical and analytical and others more creative and intuitive. It also allows us to excuse and downplay our deficiencies: it’s not my fault I’m bad at math; you see, I was just born right-brained! In reality, however, our cognitive skills and abilities are affected by a whole host of factors including genetics; upbringing; mindset; and training and education – none of which can be neatly boiled down to simply being “right-brained” or “left brained.”

But while lateralization of cognitive functions may not significantly influence our personalities or innate skills, it nonetheless has a profound effect on how the brain functions. For example, while the information received and processed by each hemisphere is typically shared with the other via the corpus callosum, this cannot always happen. As Dr. Kara Federmeier explains:

“Processing within each hemisphere relies on a rich, dense network of connections. The corpus callosum that connects the hemispheres is big for a fiber tract, but it is tiny compared to the network of connections within each hemisphere. Physically, then, it doesn’t seem feasible for the hemispheres to fully share information or to operate in a fully unified fashion. Moreover, in a lot of cases, keeping things separate is (literally!) the smarter way for the hemispheres to function. Dividing up tasks and allowing the hemispheres to work semi-independently and take different approaches to the same problem seems to be a good strategy for the brain.

One of my favorite findings came from an experiment in which we used adjectives to change the meaning of the same noun. For example, the word “book” in “green book” refers to something concrete – that is, something for which it is easy to create a mental image. However, given “interesting book” people now usually think about the content of the book rather than its physical form, so the same word has become more “abstract” in meaning.

We wanted to see if those differences could be found for exactly the same word depending on what it was referring to, and whether the two hemispheres were similarly affected by concreteness. We found in this experiment…that the left hemisphere is very sensitive to the predictability of word combinations. Fewer nouns can go with “green” than with “interesting,” and brain activity elicited in response to “book” reflected this when the words were presented initially to the left hemisphere.

However, to our surprise, it was the right hemisphere that elicited imagery-related brain activity to “green book” compared to “interesting book.” Thus, although the left hemisphere is clearly important for language processing, the right hemisphere may play a special role in creating the rich sensory experience that often accompanies language comprehension … and that makes reading such a pleasure.”

In other words, even in those of us with an intact corpus callosum, our brain can sometimes behave like two independent entities – just like Roger Sperry’s split-brained patients. Even stranger, until relatively recently in human history, this condition of having “two minds” may have been literally true. In his 1976 book The Origin of Consciousness in the Breakdown of the Bicameral Mind, Yale psychologist Julian Jaynes argued that up until the Bronze Age around 3,000 years ago, humans did not possess a unified conscious mind. Rather, the two hemispheres acted as separate, independent entities, with the “master” left hemisphere coming up with ideas and “speaking” them to the “servant” right hemisphere, which obeyed and carried out its commands. According to Jaynes, this bicameral mentality accounts for the ancient conception of ideas and inspiration as originating from muses or gods. As people would have been unaware that these thoughts were coming from inside their own brains, they would have experienced them as auditory hallucinations whose origins they would then have ascribed to an outside – typically supernatural – sources. This experience would have been similar to those suffering from schizophrenia, in whom internally-generated thoughts are manifested as auditory hallucinations. Indeed, schizophrenia, along with autism and many mood disorders including depression and bipolar, are associated with significant changes in the asymmetry of cognitive functions. For example, the left hemisphere is more associated with positive emotions and the right hemisphere with negative emotions, and those with depression often suffer from an excess of left vs. right hemisphere activity. Schizophrenia, by contrast, is associated with a reduced asymmetry in activity between the two hemispheres.

Jaynes’s theory does not, however, imply that human brains were once physically divided; the neurology of the ancients was identical to our own – corpus callosum and all. Rather, the mental schema of the ancients allowed them to react to situations, generate thoughts, and perform actions without the introspective ability to reflect on said thoughts and realize their internal origin. In other words, humans lacked a meta-consciousness or ego. This schema, Jaynes argued, was a product of the simpler communal living conditions of ancient humans, which did not require an introspective, unified mind to operate in. Only when people started living in more complex societies like city states and began developing writing did the two halves of the mind merge to create the integrated, self-reflective consciousness we know and love today.

Taking Jaynes’s theory one step further, British psychiatrist and philosopher Iain McGilchrist posits that the unification and lateralization of the brain has gone too far in one direction – much to the detriment of modern Western Society. In his 2009 book The Master and His Emissary: the Divided Brain and the Making of the Western World, McGilchrist argues that not only do the two hemispheres function differently, they also conceive of the world in different ways and promote different sets of ethics and values. The left hemisphere, for example, tends to reduce complex, nuanced subjects like ethics to simple rules and measures, while the right hemisphere is better able to view the world holistically in terms of interconnected systems. According to McGilchrist, Western civilization since Ancient Greece has been increasingly dominated by left-hemisphere thought, which promotes a narrow, reductive view of the universe which has led to many of our modern global problems. But while highly influential and popular, Jaynes’ s and McGilchrist’s ideas have also attracted a great deal of criticism, with many neurologists, philosophers, and historians arguing that, like the pop-psychological notion of people being right or left-brain, these theories overly simplify and distort the more complex and subtle realities of brain lateralization and are based on shaky historical evidence. Whatever the case, what is certain is that, to paraphrase Scottish biologist J.B.S. Haldane, the human brain is not only queerer than we suppose, but likely queerer than we can suppose.

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