Some may call it a figment of a person's imagination while others term it as delirium, but now scientists carrying out brain scan research, have found a compelling explanation for how the human mind conjures up hallucinations.
They say that what we see is driven as much by what we expect to see as by the patterns of light and colours picked up by our eyes.
This top-down, not bottom-up, picture of the way vision works could explain bereavement hallucinations. Ten per cent of grieving people believe they have caught sight of the dead person.
These hallucinations relate to higher cognitive functions of the brain, some of which reside in the frontal cortex, the area of decision-making.
In brain scan studies reported in the journal Science, Dr Christopher Summerfield of Columbia University and colleagues in France and New York asked volunteers to differentiate between houses and faces.
Signals in the frontal cortex became active whenever subjects expected to see a face, irrespective of what the actual stimulus was. These frontal regions appear to be active earlier than the parts of the brain that process vision.
This backs a current theory, ‘predictive coding’, which suggests the brain has an expectation of what it will see, then compares this template with information from the eyes to determine if it is indeed seeing a face or something else.
When this process goes awry, hallucinations can occur, leading to incidences of 'paredolia', where healthy people report seeing faces in the clouds, or on the Moon.