June 15, 2011:
Our brains turn up the volume when we speak
Neurons in different parts of our brain decide what we hear.
Our brains can filter out unwanted noise so that we can focus on what we are listening to. But when it comes to our own speech, there is a network of volume settings which can selectively silence and amplify the sounds we make and hear. These are the findings of a brain study from the University of California, Berkeley. Activity in the auditory cortex when we speak and listen is amplified in some regions of the brain and muted in others.
Neuroscientists from UC Berkeley, UCSF and Johns Hopkins University in the US tracked the electrical signals emitted from the brains of hospitalised epilepsy patients. They discovered that neurons in one part of the patients' hearing mechanism were dimmed when they talked, while neurons in other parts lit up.
A complicated picture
"We used to think that the human auditory system was mostly suppressed during speech, but we found closely knit patches of cortex with very different sensitivities to our own speech that paint a more complicated picture," said Adeen Flinker, a doctoral student in neuroscience at UC Berkeley and lead author of the study.
"We found evidence of millions of neurons firing together every time you hear a sound, right next to millions of neurons ignoring external sounds but firing together every time you speak," Flinker added.
We need to hear ourselves
"Such a mosaic of responses could play an important role in how we are able to distinguish our own speech from that of others. Whether it's learning a new language or talking to friends in a noisy bar, we need to hear what we say and change our speech dynamically according to our needs and environment," Flinker said.
The auditory cortex is a region of the brain's temporal lobe that deals with sound. In hearing, the human ear converts vibrations into electrical signals that are sent to relay stations in the brain's auditory cortex where they are refined and processed. Language is mostly processed in the left hemisphere of the brain.
Their findings offer new clues about how we hear ourselves above the noise of our surroundings and monitor what we say. Previous studies have shown a selective auditory system in monkeys that can amplify their self-produced mating, food and danger alert calls, but until this study, it was not clear how the human auditory system was wired.