I recently visited Dr. Judith Ford and Dr. Daniel Mathalon’s lab at the University of California, San Francisco. During the month that I was there, I absorbed some much needed sunlight and struggled up some incredibly steep hills. More importantly, I also learned a lot about the studies they have been working on, and was able to work with some voice-hearers using some of the cognitive tests we’ve been working on in Durham. I won’t go into more detail about these tests now, as I plan to write a more detailed post about them in the future.
Their lab specialises in using electroencephalography (EEG) to measure brain activity during speaking and listening. Their previous work has established that a particular component of activity (the N1 response, a negative spike in activity that takes place about 100ms after a stimulus) is observable after listening to speech, but not when you are speaking yourself. Interestingly, their work suggests that this difference may not appear in some people diagnosed with schizophrenia.
These findings are consistent with theories of voice-hearing which suggest that an internal ‘forward model’ is disrupted. These theories suggest that the parts of the brain that produce speech, located in the frontal lobes, usually send a copy of the speech motor ‘plan’ to the speech perception areas of the brain, to ‘warn’ them that the incoming sound has been produced by the speaker. In effect, this is one part of the brain saying to another “don’t worry, that’s just me”. This results in a suppression of activity in the speech perception areas – which is reflected in the lack of N1 response to self-produced speech.
The theory goes that the suppression of the N1 response is one way that the brain labels an event as ‘self-generated’, and that if this does not happen for some reason, internal mental events (such as inner speech, here conceptualised as a motor act) may be mislabelled as non-self-generated. If true, this may help to explain aspects of the voice-hearing experience such as the feeling of ‘alien-ness’ that is often attached to the voices.
Of course, one problem with the research is that it focuses on talking out loud, not inner speech. You can imagine, though, how hard it is to study inner speech in an empirical context. Here at Hearing the Voice, these kinds of method are one thing that we want to try to develop further to aid our understanding of the mechanisms underlying voice hearing at both a cognitive and neural level.