Although seeing things that aren’t there sounds like a classic example of a brain pathology it is, to some degree, an integral part of the way that the normal brain works. The brain creates inferences (‘best guesses’) as to what is out there in the world, and this typically corresponds with what we consciously experience but does not always reflect the true state of the world (as in the case of visual illusions). In some cases, completely new experiences may be constructed (e.g. seeing colours when listening to music in people with synaesthesia) and we will debate whether the same kinds of mechanisms can apply in these more extreme cases too.
Dr Jamie Ward, Department of Psychology, University of Sussex.