Evidence for this comes from an unlikely source: tickling. Most people can’t tickle themselves. Why not? Tickling should feel the same no matter who does it, but tickling ourselves involves conscious choice and action on our part, which requires neurological activity, which the brain recognizes as being internally generated, so it’s processed differently. The brain detects the tickling, but internal conscious activity flagged it up beforehand, so it’s ignored. As such, it provides a useful example of the brain’s ability to differentiate between internal and external activity. Professor Sarah-Jayne Blakemore and her colleagues at the Wellcome Department of Cognitive Neurology studied the ability of psychiatric patients to tickle themselves.28 They found that, compared with non-patients, patients who experienced hallucinations were far more sensitive to self-tickling, suggesting a compromised ability to separate internal and external stimuli.
While an interesting approach (and one not without flaws), please note that being able to tickle yourself does not automatically mean you’re psychotic. People vary tremendously. My wife’s college roommate could tickle himself, and has never had any psychiatric issues. He’s extremely tall though; maybe the nerve signals take so long to get to the brain from the tickling site it just forgets how they originated??
Neuroimaging studies have suggested further theories about how hallucinations generally come about. An extensive review of the available evidence, published by Paul Allen and his colleagues in 2008,29 suggests an intricate (but surprisingly logical) mechanism.
As you may expect, our brain’s ability to differentiate between internal and external occurrences is derived from multiple areas acting together. There are fundamental subcortical areas, predominantly the thalamus, that provide raw information from the senses. This ends up in the sensory cortex, which is an umbrella term for all the different areas involved in sensory processing (the occipital lobe for vision, auditory and olfactory processing in the temporal lobes, and so on). It’s often subdivided into primary and secondary sensory cortex; primary processes the raw features of a stimulus, secondary processes more fine detail and recognition (for example, the primary sensory cortex would recognize specific lines, edges and colors, the secondary would recognize all of this as an oncoming bus, so both are important).
Connecting to the sensory cortex are areas of the prefrontal cortex (decisions and higher functions, thinking), premotor cortex (producing and overseeing conscious movement), cerebellum (fine motor control and maintenance) and regions with similar functions. These areas are generally responsible for determining our conscious actions, providing information needed to determine which activity is internally generated, as in the tickling example. The hippocampus and amygdala also incorporate memory and emotion, so we can remember what we’re perceiving and react accordingly.
Activity between these interconnected regions maintains our ability to separate the outside world from the one inside our skull. It’s when the connections are changed by something that affects the brain that hallucinations occur. Increased activity in the secondary sensory cortex means signals generated by internal processes get stronger and affect us more. Reduced activity from the connections to the prefrontal cortex, premotor cortex, and so on, prevents the brain from recognizing information that is produced internally. These areas are also believed to be responsible for monitoring the external/internal detection system, ensuring genuine sensory information is processed as such, so compromised connections with these areas would mean more internally generated information is “perceived” as genuine.30
All of this combined causes hallucinations. If you think to yourself, “That was stupid,” when you buy an expensive new tea set and let your toddler carry it out of the store, this is usually processed as an internal observation. But if your brain wasn’t able to recognize that it came from the prefrontal cortex, the activity it produces in the language-processing areas could be recognized as something spoken. Atypical amygdala activity means the emotional associations of this wouldn’t be dampened either, so we end up “hearing” a very critical voice.
The sensory cortex processes everything and internal activity can relate to anything, so hallucinations occur in all senses. Our brains, knowing no better, incorporate all of this anomalous activity into the perception process so we end up perceiving alarming, unreal things that aren’t there. With such a widespread network of systems responsible for our awareness of what’s real and what isn’t, it is undoubtedly vulnerable to a wide variety of factors, hence hallucinations in psychosis are so common.
Delusions, a false belief in something that is demonstrably untrue, are another common feature of psychosis, and again demonstrate a compromised ability to distinguish between real and not-real. Delusions have many forms, such as grandiose delusions, where an individual believes they’re far more impressive than is accurate (believing they’re a world-leading business genius despite being a part-time shoe-store employee), or (more common) persecutory delusions, where an individual believes they’re being relentlessly persecuted (everyone they meet is part of some shadowy plot to kidnap them).
Delusions can be as varied and strange as hallucinations, but are often far more stubborn; delusions tend to be “fixed,” and highly resistant to contradictory evidence. It’s easier to convince someone the voices they’re hearing aren’t real than it is to convince a delusional person that not everyone is plotting against them. Rather than regulating internal and external activity, delusions are believed to stem from the brain’s systems for interpreting what does happen and what should happen.
The brain has to deal with a lot of information at every given moment, and to do this effectively it maintains a mental model of how the world is meant to work. Beliefs, experiences, expectations, assumptions, calculations—all of these are combined into a constantly updated general understanding of how things happen, so we know what to expect and how to react without having to figure it out again each time. As a result, we’re not constantly surprised by the world around us.
You walk along the street and a bus stops alongside you. This isn’t surprising because your mental model of the world recognizes and knows how buses operate; you know buses stop to let passengers on and off, so you ignore this occurrence. However, if a bus pulls up outside your house and doesn’t move, this would be atypical. Your brain now has new, unfamiliar information, and it needs to make sense of it in order to update and maintain the mental model of the world.
So you investigate, and it turns out the bus has broken down. But, before you discover this, a number of other theories will have occurred to you. The bus driver’s spying on you? Someone bought you a bus? Your house has been designated as a bus depot without your knowledge? The brain comes up with all these explanations, but recognizes them as very unlikely, based on the existing mental model of how things work, so they’re dismissed.