Why some people hear voices: testing the inner-speech theory

A long-standing but hard-to-prove idea about how the brain fails to recognize its own inner speech, finally put to the test.

This study tested a long-standing but hard-to-prove theory about why people with schizophrenia sometimes hear voices that aren't there. The idea is that the brain normally dampens its response to your own inner speech (the silent voice in your head), and a malfunction in that dampening might cause inner thoughts to be misperceived as external voices. The researchers had three groups, people with schizophrenia spectrum disorders who currently hear voices, people with the same diagnosis who don't, and healthy controls, silently imagine a syllable at a precise moment while simultaneously hearing an actual spoken syllable that either matched or didn't match what they were imagining, all while measuring brain activity via EEG. Healthy controls showed the expected pattern: their brains dampened the response to the real sound when their inner speech matched it, as if recognizing "I already produced this." People who currently experienced hallucinations showed the opposite effect entirely, their brains actually amplified the response when inner and outer speech matched, instead of suppressing it. People with the same diagnosis but no current hallucinations showed yet another distinct pattern. The more severe a person's hallucinations were, the more pronounced this reversed brain signal became. This gives real, measurable, brain-based evidence for the idea that auditory hallucinations may stem from a specific breakdown in how the brain distinguishes self-generated thoughts from the outside world, rather than being a vague or unexplainable symptom.