Claudia*, a sailor from Lichfield in her late 30s, is not Italian. She has never been to Italy. She has no Italian family or friends. And she has no idea why a belligerent Italian couple have taken over her inner voice, duking it out in Claudia’s brain while she sits back and listens.

"I have no idea where this has come from," says Claudia, apologetically. "It’s probably offensive to Italians." The couple are like the family in the Dolmio pasta sauce adverts: flamboyant, portly, prone to waving their hands and shouting. If Claudia has a big decision to make in her life, the Italians take over.

"They passionately argue either side," Claudia says. "It’s really useful because I let them do the work, so I don’t get stressed out by it." These disagreements always take place in a kitchen, surrounded by food. Claudia hasn’t given the Italians names – yet. But they did help Claudia make a major life decision, encouraging her to quit her job as a scientist two years ago and fulfil a lifelong dream of running away to sea.

"They were chatting non-stop before I handed in my notice," Claudia sighs. "I’d wake up and they’d be arguing. I’d be driving to work and they’d be arguing. It was exhausting, to be honest." The woman was in favour of Claudia going, but her husband was wary. "He’d be saying: ‘It’s a stable job!’ And she’d go: ‘Let her enjoy life!’" The woman prevailed, and Claudia left to work on a flotilla in Greece (although she’s now back in the UK temporarily, due to Covid). She’s much happier, even if she did have to have neurolinguistic programming to get the shouting to calm down. "They’re quieter now," Claudia says with relief. "Less shouting. They just bicker."

Most of us have an inner voice: that constant presence that tells you to "Watch out" or "Buy shampoo" or "Urgh, this guy’s a creep". For many of us, this voice sounds much like our own, or at least how we think we sound. But for some people, their inner voice isn’t a straightforward monologue that reproaches, counsels and reminds. Their inner voice is a squabbling Italian couple, say, or a calm-faced interviewer with their hands folded on their lap. Or it’s a taste, feeling, sensation or colour. In some cases, there isn’t a voice at all, just silence.

"Like a tiny island, surrounded by an infinite ocean," is how Justin Hopkins describes his brain. "The tiny island is where all the conscious things seem to happen, but it’s surrounded by this infinite, inaccessible stuff." Hopkins, who is 59 and works for a social enterprise in London, doesn’t have an inner voice. There is no one in his brain to blame, shame or criticise. In his head, there is emptiness: just the still warm air before a rustling breeze.

"There’s nothing there," says Hopkins. "And I don’t think there ever has been." Of course, Hopkins has thoughts: we all do. But the inner monologue that fills our brain while the engine stands idling isn’t there. It’s been clicked off, permanently. "When I am alone and relaxed, there are no words at all," he says. "There’s great pleasure in that." He can easily while away an hour without having a single thought. Unsurprisingly, Hopkins sleeps like a baby.

What makes someone like Hopkins lack an inner voice so completely? "That’s a really good question," says the neuroscientist Dr Helene Loevenbruck of Grenoble Alpes University’s laboratory of psychology and neurocognition. "I don’t know." Loevenbruck is one of a handful of neuroscientists in the world who has studied the inner voice. She explains that it is created in a network of different areas in our brain, including the inferior parietal lobe, the inferior frontal gyrus and the superior temporal cortex.

In order to understand how the inner voice works, you need to understand how human thought translates into action. "Whenever we do any action, our brain makes a prediction of the sensory consequences of that action," says Loevenbruck. Say you want to fetch a glass of water. "Your brain sends the appropriate motor signals to your hand, but it also generates a sensory prediction of the command," she says. "Before you’ve even picked up the glass, your brain has made a prediction of what the motor command will do, which means you can correct for mistakes before you make an error. This system is very efficient, and it’s why humans can do so many actions without making errors."

The same principle applies with human speech. Every time our mouths move to form a word, our brain is simultaneously generating a predictive simulation of that speech in our brain, to correct for error. "The current understanding of inner speech is that we do the same as in overt speech – make predictions in our mind of what we will say – but we don’t actually send the motor commands to our speech muscles," Loevenbruck concludes. "This simulated auditory signal is the little voice we hear in our brain."

Loevenbruck explains that, for the most part, we hear what she terms "inner language". But not always. "You can have expanded and more condensed forms of inner speech," she says. "People may experience them as abstract representations of language, without sound … some people say their inner voice is like a radio that’s on all day long. Other people don’t have a voice at all, or they speak in abstract symbols that don’t involve language." Loevenbruck can’t explain why some people experience the inner voice differently: we are at the limits of neuroscience, already the most slippery of all the branches of human knowledge.

She explains that deaf people tend to experience the inner voice visually. "They don’t hear the inner voice, but can produce inner language by visualising hand signs, or seeing lip movements," Loevenbruck says. "It just looks like hand signing really," agrees Dr Giordon Stark, a 31-year-old researcher from Santa Cruz. Stark is deaf, and communicates using sign language.

His inner voice is a pair of hands signing words, in his brain. "The hands aren’t usually connected to anything," Stark says. "Once in a while, I see a face." If Stark needs to remind himself to buy milk, he signs the word "milk" in his brain. Stark didn’t always see his inner voice: he only learned sign language seven years ago (before then, he used oral methods of communication). "I heard my inner voice before then," he says. "It sounded like a voice that wasn’t mine, or particularly clear to me."

The broadcaster Jenni Murray lives in author Hannah Begbie’s brain. Well, not Murray exactly, but a facsimile of Murray, with the same kind but quizzical voice, and a scarf flung loosely over her shoulder. "My inner voice is a duologue, like I am in a constant state of interviewing myself," says Begbie, who is 44 and lives in London. "The interviewing always happens in a plush radio studio," she says. "There’s nice rich crushed-velvet walls. There’s a warmth and a colour to it." The duologue can be about anything, trivial or serious.

"Jenni might say: ‘When did you eventually decide to take the plunge on those shoes?’" Begbie says. "And I go: ‘Well Jenni, that’s an interesting question.’" Murray’s gentle, firm questioning has nudged her into making big life decisions: before Begbie quit her job as a literary agent, Murray helped her rehearse her reasons for doing so, in her head. "It’s a way of organising the chaos of my mind," Begbie says. She is aware it is odd. "I’ve never met Murray," Begbie says. "I know it’s ludicrous."

Former librarian Mary Worrall’s inner voice has always been a TV screen, or sometimes a slide projector, that is continuously playing inside an attic, inside her head. The attic is accessed by a spiral staircase behind her left ear, explains Worrall, who is 71 and lives in Birmingham. "There’s not a great deal of sound," she says. "It’s just images really, like a film is playing." When Worrall’s inner voice reminds her to pick up some washing powder, she doesn’t hear the words, "buy washing powder." Instead, she sees herself reaching for a box of washing powder on a TV screen in the attic.

"It’s an emotion," says Mona*, a 53-year-old CEO from Telford, of her inner voice. "The closest way I could describe it would be in terms of colour." Her inner voice doesn’t manifest itself obviously: it never chatters away. Rather, Mona has to turn her attention to it in order to perceive it. "When I’m going about my day, the inner voice isn’t talking to me in the English language," Mona says. "It’s something that sits underneath and behind what I do."

The voice becomes more insistent when she is in a situation that requires her to be emotionally deft. Mona often works with troubled children, and was recently in a situation where a teenager was being angry and outspoken. At first, Mona’s instinct was to remonstrate with her. But then Mona’s inner voice made itself felt to her in a wash of the colour grey. "I had this profound sense that this young person was in real trouble … I felt a sense of sadness and despondency, and saw a foggy cloud." Her inner voice was right: Mona later confirmed that the young person was going through a difficult time in her personal life.

Many of the people I speak to learned late in life that their inner voices were not the norm. For years, Worrall thought that other people also had attics in their brains. "I thought everyone else was like this!" she laughs. Mona only outlined the contours of her inner voice to her husband of 30 years in advance of our phone interview. "You don’t ever really realise your inner voice is different," says Mona. "It’s not something you talk about."

Unknowable, inscrutable, uniquely our own: inner voices are our lifelong confidantes and secret friends. It’s only a shame no one gets to meet them, but us. "I wish I could invite someone in," says Worrall. "It would be so nice if I could download the attic on to some sort of hard drive, so other people could look at it."

Additional reporting by Rachel Obordo. *Some names have been changed

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