THE STRANGE WORLD OF HALLUCINATIONS

From seeing things to hearing voices, hallucinations distort every one of our main senses.

There is no clear distinction between perceiving and hallucinating. What we call ‘real’ perception might just be your brain picking a hallucination that best aligns with reality. Spotting one is only easy when it’s peculiar; the rest of the time you might be hoodwinked. For all you know, the image of a brain on the previous page might have been a hallucination. But what happens at the neurological level during a hallucination? What causes it? And what, exactly, counts as a hallucination?

Welcome to the strange science of hallucinations, where cognitive scientists use the latest in brain-scanning technology to pinpoint the causes and characteristics of these sometimes psychedelic occurrences. It’s not an easy field to study: hallucinations can take place under so many different circumstances – sleep deprivation, migraines and neurological disorders to name but a few – that a simple one-size-fits-all explanation simply isn’t possible.

Hallucination is often linked to psychosis. As famously depicted in the movie 'A Beautiful Mind', for instance, the mathematician John Nash suffered from delusions due to paranoid schizophrenia. But scores of mentally healthy individuals also experience unusual perceptions.

The word ‘hallucination’ was coined by medical writer Sir Thomas Browne in the 17th Century. Although instances are sprinkled throughout ancient and medieval texts, most cases went undiagnosed – in no small part due to medical ignorance: for centuries, dementia was the go-to explanation for a variety of brain disorders. Another reason for the lack of diagnosis is that people fear that visiting a doctor to describe their experiences would leave them with the unfortunate and inaccurate label of ‘mentally ill’.

But while our understanding has come a long way since the field was established in the mid-19th Century, there are still many more questions than answers. “There are not only many definitions of hallucination, there is also no central way to hallucinate,” says Dr Vaughan Bell of the Institute of Psychiatry at King’s College London. “On top of that, hallucinations have a variety of causes.” Here we explore how they can trick our five main senses, and how new research is shedding light on this curious corner of brain science.

SMELL

Olfactory hallucinations can conjure up odours both delicious and disgusting.

Although we can hallucinate scents and odours, what we ‘smell’ varies. “People will report unpleasant smells like rotting and burning flesh,” explains Dr Kathryn Eve Lewandowski, a clinical psychologist at McLean Hospital and Harvard Medical School. “These are recurring themes, but some patients describe more pleasant scents, like the smell of flowers.”

Like the other four senses, olfactory (sense of smell) hallucinations are often linked to psychosis. According to one of Lewandowski’s studies, about 15 per cent of patients with schizophrenia, schizoafective and bipolar disorders experience non-existent odours. She also found that, like taste and touch, hallucinations of smell are linked to depression, a family history of mood disorders and psychotic illness manifesting at a relatively young age.

Hallucinating a smell isn’t always bad, however. A survey of 51 people diagnosed with schizophrenia led by psychologists at Macquarie University in Sydney found that around 30 per cent of instances were pleasant experiences, and 19 per cent were neutral.

Phantosmia – literally, ‘phantom smells’ – is a condition in which olfactory hallucinations are the primary symptom. The exact cause is still unclear, but they’re associated with a variety of psychological disorders, as well as damage or abnormalities in the olfactory system. These hallucinations also result from head injuries, brain tumours, strokes, migraines and temporal lobe seizures. Strangely, they’re sometimes isolated to just one nostril, and in most cases fantastical smells will slowly disappear over time.

SIGHT

The most well-known of hallucinations, problems of visual perception are also the most common.

One paradox of hallucinations is that they occur in people who are going blind. About 10 per cent of those who experience visual hallucinations have a significant loss of vision as a result of eye disease or damage to the optic pathways of the brain’s neural circuitry. Known as Charles Bonnet syndrome after the Swiss philosopher, this odd array of symptoms includes a variety of visual hallucinations.

“Almost all patients will see simple patterns,” says clinical psychiatrist Dr Dominic Fytche of King’s College London, who adds that about 40 per cent see small figures wearing elaborate costume or military uniform. “Another 40 per cent see disembodied faces usually described as grotesque, like a gargoyle.” An estimated two-thirds of those with Charles Bonnet syndrome don’t report their condition for fear of social rejection or an inaccurate diagnosis of being mentally unstable.

Stranger yet is Anton-Babinski syndrome. This is a result of damage to the visual processing centre of the brain and causes blind individuals to deny their blindness – that is, they’re convinced that they can see.

Visual hallucinations also result from neurological disorders, including Alzheimer’s, Parkinson’s disease and the rarer Lewy Body dementia. Unlike Charles Bonnet syndrome, patients with these conditions have normal vision and the contents of their hallucinations aren’t as bizarre. For example, instead of imaginary objects, they might hallucinate a real person who looks normal. Between 20 and 40 per cent of patients with schizophrenia experience visual hallucinations and other psychotic conditions can also produce them.

Altogether, around two million people in the UK experience visual hallucinations, but it’s dif cult to pinpoint this number because it doesn’t take into account basic visual misperceptions like phosphenes – the spots you see after rubbing your eyes. A recent £1.9 million grant from the National Institute for Health Research will help Dr Fytche investigate visual hallucinations further.

Recreational drugs of er scientists an opportunity to study hallucinations at the neurological level. In the neuropsychopharmacology unit at Imperial College London, Dr Robin Carhart-Harris induces hallucinations in subjects usingpsilocybin – the active compound in magic mushrooms – and measures blood flow and activity in the cortex to produce images of the brain. “During visual hallucinations under psilocybin, the brain is less ordered and organised,” he explains. “Individuals are more internally oriented and less goal oriented.”

Exactly which parts of the brain are active is a bit of a mystery though. Carhart-Harris predicted that the visual cortex would be associated with visual hallucinations, but didn’t find any supporting evidence and confesses that there’s still little consensus on which brain regions are associated with them.

SOUND

People with certain neurological disorders suffer from auditory verbal hallucinations, while many of us also hear voices.

Famous examples of audio hallucinations appear throughout history: the German composer Robert Schumann was said to have experienced musical hallucinations and Joan of Arc claimed that the voices of saints sent from God guided her. Psychiatrist Vaughan Bell suggests that many religious revelations were simply problems of auditory perception.

So what happens in the brain during these hallucinations? It depends on both the person and what they’re hallucinating. One approachis to study the most common type: auditory verbal hallucinations (AVHs). “There are many brain areas involved in AVHs,” explains psychiatrist Dr Renaud Jardri, who studies the phenomenon in patients with schizophrenia. “We need to think in terms of networks, not just isolated regions.”

AVHs are a hallmark of schizophrenia, with about 70 per cent of patients experiencing them. They also occur in people with other neurological disorders – including Parkinson’s disease, epilepsy and dementia – and even in healthy individuals.

The neurological dif erences of AVHs in those with a mental condition and people without a disorder are unknown, but based on a study led by clinical psychologist Dr Frank Larøi of the University of Liège in Belgium, the same brain regions might be involved. However, the differences are clear to the person experiencing such a hallucination:

Healthy individuals live normal lives by being able to control their ‘voices’, whereas patients react with stress and anxiety because the hallucinations significantly disrupt everyday life.

What’s not known is the number of normal people who experience audio hallucinations, and experimental methods make it challenging to get a precise answer. The frequency, duration and intensity of audio hallucinations can also vary greatly, so it’s difficult to define what ‘counts’ as a hallucination. Indeed, one study by clinical psychologist Vanessa Beavan of the Australian College of Applied Psychology puts the percentage of people experiencing them anywhere between 0.6 and 84 per cent.

Whether there’s a diference between patients and non-patients at the developmental level is also unclear. Research suggests that healthy people develop auditory verbal hallucinations in their early teens, whereas those with psychological problems will often experience their first voice around the age of 20. “It’s not unusual for non-patients to have a history of childhood trauma or a family history of psychological disorders,” says Frank Larøi, who cautions that more research is needed.

The good news is that these hallucinations are now receiving more attention. They’re being studied across Europe in non-clinical groups as well as in patients: an international consortium met in London last year, and a team at Durham University recently launched a project to study audio hallucinations, called ‘Hearing the Voice’.

TASTE

Unusual and uncommon, phantom tastes can have many causes.

What does a gustatory, or taste, hallucination involve? “They are the perception of taste in the absence of external stimuli and usually involve a metallic or coppery flavour,” describes clinical psychologist Dr Kathryn Eve Lewandowski. She led a 2009 study examining the prevalence of these hallucinations in patients with schizophrenia, schizoaf ective disorder and bipolar disorder.

The least common kind of cognitive misfiring, research on the psychological and neurological underpinnings of taste hallucinations is so limited that scientists still hesitate to draw definite conclusions.

Lewandowski and her colleagues found that about four per cent of patients experienced them. They are also linked to temporal lobe epilepsy, but it’s not unusual for them to be triggered by brain tumours, various psychotic disorders and drug abuse.

With this type of hallucination being so mysterious, it’s unclear what proportion of the population experiences them. But a survey conducted in 2000 by medical epidemiologist Maurice Ohayon of Stanford University in California – involving over 13,000 non-institutionalised individuals from the UK, Germany and Italy – provides a hint. Ohayon found that around 1 per cent of them reported experiencing hallucinations of taste at least once a week. These people were linked with alcohol, drugs, depression and psychosis.

TOUCH

Tactile hallucinations are the most enigmatic of neural illusions.

Misperceptions of touch are difficult to define. If hallucination is perception without external stimuli, is an itch a hallucination? That question was posed in 1982 by psychiatrist German Berrios of Cambridge University, who theorised that tactile hallucinations are fundamentally different from those occurring in the other senses.

Research on tactile hallucinations is sparse and its origins date back to the 19th Century, when French psychiatrist Jean-Étienne Dominique Esquirol reported several cases. According to clinical psychologist Dr Kathryn Eve Lewandowski, at least we now know more about how they feel. “Patients with psychotic disorders experience tactile hallucinations in the form of crawling or burning sensations,” she says. “Sometimes they have feelings of electricity or a sense that their organs have been rearranged.”

Such extreme hallucinations are also felt in some forms of psychosis, such as Ekbom syndrome. “Patients usually develop the strong delusional belief that they are infested with parasites,” says Dr Renaud Jardri. These dramatic and disturbing hallucinations are linked to brain injury and dementia, and are also common side effects for people who abuse cocaine, amphetamines and crystal meth.

It’s not known which regions of the brain are active during tactile hallucinations, but it’s possible that the same cognitive mechanisms involved in a phantom touch are also active in a real touch. If true, tactile hallucinations might be due to crossed wires in the brain as perceptual information from the senses gets mixed up. Understanding hallucinations at the neurological level will not only help patients deal with problems of perception, but also unravel these strange phenomena in healthy people.

By Sam McNerney in "BBC Focus", UK, November, 2012, excerpts pp.48-51. Digitized, adapted an illustrated to be posted by Leopoldo Costa.

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