Many people have experienced false perceptions. They may have heard phantom footsteps on a night, felt a touch on their shoulder when they were alone, or glimpsed a figure in an empty corner of a shadowy room. When these experiences are mistaken for reality}, they are known as hallucinations.
There is nothing mysterious about hallucinations. They arise from the same brain machinery that is used when we summon up an image from memory or create a new experience from our imagination. In all these cases, the brain produces sensory experiences in the absence of outside stimuli. These ‘false’ experiences are usually less vivid than those triggered -by external stimuli, so we know that they are not real.’ But in certain circumstances, the imagined experiences can be extremely intense, and we- lose our ability to perceive them for what they are.
Seeing what you expect to see
There are two types of hallucinations: those produced spontaneously by a normal brain and those triggered by drugs or by disturbances in brain function. Spontaneous hallucinations are created (like all imaginary experience) from memory. A hallucination can be a disc re the ‘chunk’ of memory, such as a familiar face, or it can be assembled from many ‘bits’ of experience spliced together to form something apparently new. The sensory component of the hallucination – what is actually seen, heard d, or felt – is generated by activity in the sensory cortices, just like ordinary perceptions. But, the interpretation of this activity also depends on how the sensations are processed by the parts of the brain that deal with beliefs and expectations. So, if two observers both see a black shape in the corner of a room, depending on their expectations, one might see a coat on a hook, while the other sees a ghostly figure lurking in the shadows.
Ordinary perceptions, too, are open to different interpretations by different observers. But because they are re-triggered by external stimuli, the ‘real’ sensory information tends to override the internal imagery generated by expectation and belief. Someone looking at a picture of a cat when she is expecting a picture of a dog may ‘see’ a dog if she takes only a brief glimpse. But the longer she looks at the picture, the harder it is to sustain the mistaken perception. In other words, the ‘bottom-up’ content from the senses overrides the signals flowing ‘top-down’ from the expectations in the conscious brain. Hallucinations, by contrast, contain more ‘top-down’ content and have little or no outside information to conflict with the self-generated image. For this reason, hallucinations tend, more than ordinary perceptions, to reflect what the mind expects to find.
So, people who believe in ghosts are more likely to see them than those who do not. Seeing a dark shadow in a room, they are more likely to notice it, turn their attention to it, and amplify the impression into something that seems very real. By contrast, someone who does not believe in ghosts is unlikely to notice the shadow and may not even remember seeing it. Similarly, religious believers who expect to see manifestations of gods or saints quite often do. Their visions are wishful embellishments of real perceptions, turning a pattern of currants in a bun into a portrait of Mother Theresa or a cloud formed into an image of the Virgin Mary.
The importance of belief and expectation in shaping visions and hallucinations becomes clear when you compare reports from different cultures. Visions of the Virgin Mary are often reported in Catholic countries, whereas, in the USA, where sci-fi movies are a significant influence, sightings of UFOs and aliens are far more common.
Errors of perception
Certain environmental factors predispose us to hallucinations. Many visions occur in semi-darkness because the sensory cells in the eye that detect forms in poor light lie at the periphery of our visual field. This means that when we see objects in the dark, we see them indistinctly – out of the ‘corner of the eye.’ Darkness also promotes fear, which tends to make our senses more alert. So, in scary situations, the visual cortex is more likely to pick up small stimuli that might otherwise be ignored.
We all experience false perceptions from time to time. Usually, the error is detected because it does not tally with the information. Our sense organs or because it does not fit into our belief system: But in some cases, the false perception does tally with our belief system and therefore becomes fixed. Suh’s full-blown hallucinations can be dangerous. For example, an aircraft pilot may misread the position of a needle on a dial, but because the information seems reasonable, he may not examine it closely. Such errors become more common if the pilot is tired or in a situation where information from outside is sparse (when flying through a cloud, for example), and they sometimes cause serious accidents.
Dreamscapes
The most familiar way in which our minds produce vivid images and experiences without input from the outside world is in dreams. Dreams are the result of activity in the sensory areas at the back of the brain. These become active and produce a stream of sensations, which are woven into a narrative. When we dream, the frontal areas of the brain, which in the wrong state select and direct sensory attention according to our wishes, are partially ‘turned off ‘. This means that we have very little control over the content of our dreams – we cannot direct the action in the same way we can in imaginative daydreams.
The frontal areas of the brain are also partly responsible for producing the round sense of self – the awareness of who and where we really are. Because they are underactive in sleep (except in lucid dreaming), we are not even aware that we are dreaming, heightening the reality of the experience.
Hallucination and the damaged mind
People with damaged or diseased brains can suffer severe hallucinations that cannot be distinguished from perceptions triggered by outside stimuli. For example, suppose a stroke damages the sensory areas of the brain. In that case, the reduced sensory information received from outside is constantly over ridden by ‘top-down’ processing, so imagined events appear real. Providing the sufferer’s thinking is still normal – that is if the parts of the prefrontal cortex that govern reason and logical thought are still intact, the odd, imagined material will be recognized as false.
It is when damaged sensory brain function is combined with a bizarre belief system or disordered thinking that false perceptions may be catastrophic. This occurs in people suffering from disorders such as Alzheimer’s disease, schizophrenia, and severe depression. For example, some forms of schizophrenia are marked by odd and often terrifying hallucinations that make it impossible for sufferers to function in the real world. The most common form of schizophrenic hallucination is auditory- voices, which may be threatening or dictatorial. In extreme cases, the voices may tell the person to commit suicide or murder.
Brain scans of people with schizophrenia hallucinating voices show that their audi cortex is activated in just the same way as when voices are actually heard. They also show that the voices are generated by the speech centers of the person’s brain, and sensors that pick up minute movements of the throat muscles reveal that the person even starts the process of articulating them. When normal people generate silent speech, a signal is sent to the auditory cortex to tell it that the speech comes from the ‘inside,’ so the person knows that it is imaginary. In people with schizophrenia, these signals are absent, so they cannot distinguish between imaginary internal speech and actual voices.
The Sleep-Wake Boundary
Hallucinatory phenomena are often experienced in the twilight state between waking and sleep. These experiences are described as hypnagogic (if they occur as we are falling asleep) or hypnopompic (if they occur as we are waking up). Hypnagogic hallucinations tend to be visual and often rather alluring. People often report beautiful, dynamic images such as roses unfurling their petals, fascinating landscapes with changing cloud shapes or colors, and kaleidoscopic forms that appear to dance and shimmer.
Hypnopompic phenomena are also primarily visual but seem to be experienced more fully. People are often convinced that they are awake and, therefore, find the hallucinations powerful and even frightening. Dreamers may believe that there is an intruder in the room poised to attack them or that strange creatures are lurking in the darkness. The dreamers may feel even more vulnerable because they are unable to move. This sleep paralysis is a normal feature of REM sleep and prevents us from acting out our dreams.
Drugs And Hallucinations
A wide range of drugs can generate hallucinations. Some, including LSD, peyote, mescal, and heroin, are used specifically for this purpose, while others may produce hallucinations as side effects. These include prescription medicines used to treat high blood pressure (clonidine), pain (pentazocine, fentanyl), and depression (Prozac).
The type of hallucination depends on which part of the brain is most affected by the drug. Those that target the visual cortex produce whirling colors, patterns, and altered perception of the size and shape of objects. Those that affect ‘higher’ brain areas – where sensations are interpreted – may make objects do peculiar things: a spider may suddenly start leapfrogging with the cat. Drugs that affect the auditory cortex can alter the way that sound is heard or generate phantom sounds. Some chemicals (including alcohol and steroids) produce tactile sensations such as the feeling of ‘bugs’ crawling over the skin. Amyl nitrate produces genital sensations that often induce sexual arousal. Drugs that work mainly on the limbic system, such as Ecstasy, may produce emotional hallucinations, such as a heightened sense of beauty and love, and those that excite the frontal lobes (amphetamines, cocaine) generate delusions of power and strength.
Hallucinations
- Visual hallucinations affect about 3 percent of people with sight problems.
- Sleep deprivation produces hallucinations in most people within 48 hours.
- Phantom limbs, in which amputees· experience the presence of missing limbs, are a type of tactile hallucination.
- Artists such as Poe, Coleridge, and Baudelaire used hashish and opiates to increase the richness of their visual imagery.
- One of the constituents of cannabis, dronabinol, produces hallucinations in 5 percent of people who take it.
