Researchers conclude that dreams originate in the brainstem

It's a question that has long fascinated and flummoxed those who study human behaviour: from whence comes the impulse to dream?

Are dreams generated from the brain's "top" - the high-flying cortical structures that allow us to reason, perceive, act and remember? Or do they come from the brain's "bottom" - the unheralded brainstem, which oversees such basic bodily functions as respiration, heart rate, salivation and temperature control?

At stake is what to make of the funny, sexual, scary and just plain bizarre mental scenarios that play themselves out in our heads while we sleep. Are our subconscious fantasies coming up for a breath of air, as Sigmund Freud believed? Is our brain consolidating lessons learned and pitching out unneeded data, as neuroscientists suggest? Or are dreams no more meaningful than a spontaneous run of erratic heartbeats, a hot flash or the frisson we feel at the sight of an attractive passer-by?

A study published last week in the journal Brain suggests that the impulse to dream may be little more than a tickle sent up from the brainstem to the brain's sensory cortex.

The full dream experience - the complex scenarios, the feelings of fear, delight or longing - may require the further input of the brain's higher-order cortical areas, the new research suggests. But even people with grievous injury to the brain's prime motivational machinery are capable of dreams, the study found.

The latest research looked for sleep-time "mentation" - thoughts, essentially - in a small group of very unusual patients. These patients - 13 in all - had suffered damage within their brains' limbic system, the seat of our basic desires and motivations - for sex, food and pleasurable sensations brought on by drugs and friendship and whatever else turns us on.

As a result of that damage, they had a neuropsychological syndrome called auto-activation deficit, or AAD. Even while fully conscious, they could sit completely idle and mute for hours if they were not prodded to action or speech by caregivers. In fact, they were more than unmotivated to do anything; when asked about their thoughts, they would frequently report that their mind was completely blank. When prompted, they could often do maths, sing a song or conjure up memories. But left on their own, these patients might have no spontaneous thoughts at all.