Today, the Guardian released an interesting podcast on the experience of hearing voices - specifically, experiencing auditory verbal hallucinations (AVP), a voice speaking to or about the person that others cannot hear. While AVP is known to be a symptom of schizophrenia, it can occur in many other disorders and situations.
In the writeup about the podcast, they cite survey results that up to 10% of the population reporting hearing voices no one else can hear. My initial inclination was that this information couldn't possibly be correct - auditory hallucinations have always been framed in my (relatively minimal) clinical coursework as a sign of serious psychosis, the term psychologists use to describe disorders in which the sufferer loses touch with reality.
But as I thought about it, it did make sense. The human brain is built to pay special attention to the human voice; after all, humans are a social species and being able to focus attention when others are speaking probably has a special evolutionary significance. In fact, this experience is the reason why Deke Sharon and Dylan Bell, authors of A Cappella Arranging, recommend that a cappella pieces should be shorter than other music (more like 2 to 4 minutes): because we tune in more strongly to the voice, even when it's trying to imitate instruments, a cappella music is more cognitively taxing. So it makes sense that we would tune in to anything that sounds like a voice, even if that voice is inside our head.
And as they share in the podcast, many of us (about 33%) have experienced the sensation of hearing voices when in very light stages of sleep, during which time we can experience dream-like images. I've certainly had the experience of falling asleep and being woken up after being startled by a dream-like sound or image.
The podcast discusses a lot of the new research on this topic, including insights from neuroscience and a new treatment known as Avatar Therapy.