# Brain mapping advances understanding of human speech and hallucinations in schizophrenia
Voice experiments in people with epilepsy have helped trace the circuit of electrical signals in the brain that allow its hearing center to sort out background sounds from their own voices.
Such auditory corollary discharge signals start and end in two subregions of the brain’s top folded surface, or cortex, a new study shows. One large part of the cortex, the motor cortex, is known to control the body’s voluntary muscle movements, including those involved in speech, while another large section, the auditory cortex, is known to control hearing.
In terms of evolution, the ability of animals and humans to tell one’s own calls or voices from those of others is thought to have enabled threat perception and enhanced survival. The back-and-forth, milliseconds-long electrical signals that let the brain downplay background sounds are present, for instance, as crickets rapidly tell apart their own mating chirps from the chirps of others, as songbirds sing mating songs, and as bats use reverberations of sound to negotiate their environments.
In humans, disruptions to this system are also thought to be hallmarks of auditory hallucinations, or “hearing voices,” in people with schizophrenia who cannot distinguish “real” voices from outside sounds, say the study authors. Disturbances in auditory corollary discharge signals are also thought to be involved in stuttering.
Amirhossein Khalilian-Gourtani et al, A Corollary Discharge Circuit in Human Speech, Proceedings of the National Academy of Sciences (2024). On bioRxiv: DOI: 10.1101/2022.09.12.507590