Orientation map of normal visual cortex. The colours represent the orientations of visual stimuli to which neurons respond. Following monocular deprivation, the map becomes less sharp. (© 2000 Macmillam Magazines.)

Sleep has fascinated humanity for centuries. Scientists have asked why we need to sleep, while mystics have thought that our dreams can tell us what the future holds. The answers to the mysteries of the dream world might still be out of reach, but researchers are coming closer to unravelling some of the functions of a good night's sleep.

It seems that while we are sleeping, our neurons might be busy remodelling the connections and circuits of the brain. Frank et al. have now shown that in young cats, plasticity in the visual cortex is enhanced by sleep at the critical period for visual system development.

During this critical period, the visual system of the cat is particularly sensitive to monocular deprivation. If visual input through one eye is blocked for a few hours, subsequent recordings from visual cortex show a reduction in responses to input through the deprived eye, and a decrease in the number of neurons showing binocular responsiveness.

Frank et al. investigated the effects of sleep on this form of plasticity. If cats were simply left in a dark room for six hours after the end of the period of monocular deprivation — and, cats being cats, they spent most of this time asleep — the changes in visual cortex responsiveness were much more marked than if the measurements were taken immediately after the deprivation. However, if the cats were put in a dark room but kept awake — by a combination of movement of the cage floor and 'meowing' sounds being played whenever they started to drop off — the enhancement of plasticity was prevented. It seems that sleep, rather than time elapsed since the end of the deprived period or the absence of light, was responsible for the increase in the effect. In fact, six hours of sleep was at least as effective at increasing the changes in cortical responsiveness as an additional six hours of monocular deprivation.

Optical imaging confirmed the results. Frank et al. used optical imaging to map cortical responses to oriented stimuli presented to either the deprived eye or the control eye. Once again, responses to stimuli presented to the deprived eye were weaker and less selective, with the cats in the group that had slept showing the greatest effect.

Although rapid eye movement (REM) sleep has long been thought to be important for neuronal development in the young brain, Frank et al. found that the degree of enhancement of plasticity in the cats correlated with the amount of non-REM sleep during the six hours after monocular deprivation. This might tie in with the sharp increase in non-REM sleep that occurs in cats at the beginning of the critical period.

So it seems that our suspicions that sleep is essential for consolidation of memories and development or remodelling of neural pathways might be correct. Our dreams might still be a mystery, but at least we are starting to understand why sleep is so essential.