Humans Evolved to Hear Themselves Speak
When you tell a loud-mouth friend that they "really like the sound of their own voice," there may be more truth in that than you realize. According to a neuroscience study from the University of California (UC) Berkeley, the brain selectively hears one's own voice while dimming all surrounding sounds. In their own heads, people will silence other noises while amplifying themselves speak.
Neuroscientists from UC Berkeley, UCSF, and Johns Hopkins University, led by doctoral student Adeen Flinker, examined the electrical brain signals of hospital patients. The discovered that certain neurons lit up when they talked while others dimmed. Previous studies have made this connection concerning monkey mating, food, and danger calls, but up to this point, none have focused on humans.
"We used to think that the human auditory system is 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 Flinker. "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. Such a mosaic of responses could play an important role in how we are able to distinguish our own speech from that of others."
There is the question of why humans have evolved to more closely track their own speech. Finkler suggests that it is important for language development, and monitoring our own speech. It is also important for adjusting our speech to match the environment. In a noisy bar, people must be aware of their own speech so they know it is loud enough to hear. Conversely, it allows us to adjust our volume down in a quieter environment.
A man's ability to more closely listen to himself speak is important because it lets him know that it is really his voice and not someone else's. This is not the case for people with schizophrenia, who have trouble distinguishing their own internal voice from voices of others. This neural condition suggests a dysfunctional selective auditory mechanism. The UC Berkeley study can be helpful in understanding aspects of auditory hallucinations suffered by schizophrenics.
The researchers examined patients who were being treated for seizures, because they already had electrodes implanted in their heads. They were asked to repeat words and vowels that they heard. The scientists watched the neural electrical activity as the patients' brains had to transfer from listening mode to speaking mode. They found that some areas of the auditory cortex fired less during speech, while others fired more, demonstrating how incredibly complex and sensitive the human brain is hearing ourselves speak.
Link to published article in the Journal of Neuroscience: http://www.jneurosci.org/cgi/content/abstract/30/49/16643