Why Can We Hear Our Own Thoughts?

Read this sentence silently. Did you hear it? Many people do. The words seem to pass through the mind in something like a voice, usually their own, with rhythm, emphasis and tone, yet with no sound at all. Nothing is vibrating. No air is moving. And still, for many of us, thinking feels a great deal like listening.

This inner voice is one of the most familiar features of mental life and one of the least examined. It rehearses conversations before they happen and replays them afterwards. It reads, plans, reminds, criticises and comforts. It argues with itself. It is so constant for some people that they find it hard to imagine thinking any other way. So why can we hear our own thoughts, when there is nothing there to hear?

Psychologists call the phenomenon inner speech. One of the most influential accounts of where it comes from was developed by the Soviet psychologist Lev Vygotsky in the early twentieth century. Watching young children at play, Vygotsky noticed that they often talk to themselves out loud while solving problems, narrating what they are doing and instructing themselves: "Now the red one goes here." He argued that this self-directed talk was not a sign of immaturity but a crucial tool. Speech begins as something shared between people, a way for adults to guide children. Children then turn it on themselves, using it to guide their own behaviour.

Over time, Vygotsky proposed, this private speech becomes quieter, more abbreviated and eventually silent. It moves inside. What was once a conversation with others becomes a conversation with oneself. On this view the inner voice is not an ornament of thought but social speech that has been internalised, which may explain why so much of it takes the form of dialogue, with questions and answers, objections and replies.

The body offers clues that inner speech is closer to real speech than it feels. In some experiments, sensitive electrodes placed on the lips, tongue and throat have picked up tiny muscle signals while people read or think in words, even though nothing visible moves. Brain imaging shows that inner speech draws on many of the same regions used for producing and perceiving spoken language. Thinking in words, in a sense, is speaking with the volume turned down to nothing.

But why does it sound like hearing? Part of the answer may lie in how the brain handles its own voice. When you speak aloud, the brain does something clever. Along with the commands sent to the muscles of the mouth and throat, it sends a copy of those commands to the regions that process sound. This copy, sometimes called an efference copy or corollary discharge, allows the brain to predict what you are about to hear. When the sound arrives and matches the prediction, the response to it is partly dampened. This is how you can tell your own voice from someone else's, and why your own speech does not startle you.

Some researchers think inner speech is this prediction running on its own. When you think in words, the speech system prepares an utterance and generates the matching prediction of how it will sound, but the utterance is never actually spoken. What remains is the brain's anticipation of hearing your own voice. Experiments support the idea. In a 2017 study, Thomas Whitford and colleagues found that when people silently produced a syllable in their heads, their brains responded less strongly to a real sound that matched it, exactly as if they had said it aloud and heard themselves.

Inner speech may be the sound the brain expects to hear when you speak, without the speaking.

This also helps explain why the inner voice usually sounds like you. The prediction is built from a lifetime of hearing your own speech, from the inside, through the bones of your skull as well as through the air. Yet inner speech is not limited to your own voice. People frequently report hearing a parent's warning, a teacher's phrase or a friend's way of speaking in their heads. The psychologist Charles Fernyhough, who has studied inner speech for many years, describes how it can be expanded, like full sentences, or condensed into fragments so compressed they are barely words at all, and how it often takes the form of dialogue between different perspectives.

When the system for tagging speech as self-generated goes wrong, the results can be distressing. One influential theory of auditory verbal hallucinations, experienced by many people with schizophrenia and by some people without any diagnosis, suggests that they may arise when inner speech is not recognised as one's own and is experienced as an external voice. This remains an active area of research, and hallucinations almost certainly have more than one cause. But the theory shows how much quiet work goes into the simple feeling that your thoughts belong to you.

And yet inner speech is not universal. For decades the psychologist Russell Hurlburt has used a method called descriptive experience sampling: participants carry a device that beeps at random moments, and they note exactly what was in their experience at the instant of the beep. The results have surprised many people, including researchers. On average, inner speech appears in only a minority of samples, and the variation between individuals is enormous. Some people are narrating almost constantly. Others rarely hear words at all, and their thinking instead appears as images, feelings, bodily sensations or wordless understanding.

In recent years researchers have proposed the term anendophasia for people who report little or no inner speech. Many only discover that they are unusual when they learn, often with astonishment, that other people hear a voice in their heads. Studies suggest they can think and reason perfectly well, though they may find certain verbal tasks, such as keeping a sequence of words in memory, slightly harder without an inner voice to rehearse them. Deaf people who use sign language often describe inner signing rather than inner speech, their hands moving in the mind's eye as a voice moves in the mind's ear.

That variety is a useful reminder about the privacy of experience. Each of us assumes that our own inner life is the normal one, because it is the only one we can observe directly. The person beside you on a train may be listening to a running commentary, watching a series of images, or thinking in a wordless way that neither of you has language to describe. From the outside, you would never know.

So why can we hear our own thoughts? Because the brain that learned to speak with others keeps speaking when no one is listening, and keeps predicting the sound of its own voice even when no sound will come. What we call thinking in words is the echo of a conversation that began in childhood and never quite stopped.

The next time you read silently and hear the words, notice who is speaking. It is you, and it is not quite you: a voice assembled from memory and anticipation, making no sound, yet heard.