The brains of all mammals are divided into two separate, but connected hemispheres that function together. However, dolphin brain hemispheres appear capable of operating independently; as well as moving each eye separately,each half of the brain can generate slow-wave-sleep brain patterns independently of the other. Little else was known about the mammal's remarkable ability to apparently send one half of its brain to sleep while the other half remains conscious until Sam Ridgway and his colleagues in San Diego, California, embarked on a series of groundbreaking brain imaging experiments to find out how a dolphin's half-sleeping brain functions().
After months of training (learning to slide out of the water and listening to recordings of the noisy scanner), 3 dolphins were ready to have their brains imaged. Having injected the animals with materials to visualise blood flow and sites of metabolic activity, the team induced slow-wave-sleep in one brain hemisphere with diazepam before beginning the scanning process. Fortunately the animals were content to lie on the specially constructed scanner table, accompanied by a trainer, for 37 min while the scans were completed, and then they were returned to the water quickly. Reconstructing images of the animals' brains, the team could clearly see that the blood flow to the `sleeping' half was reduced significantly, and this was also accompanied by a significant reduction in the `sleeping' half's metabolic activity.
So why are dolphins capable of shutting one half of their brain down while the other remains active? Ridgway and his colleagues suggest that this could be an energy saving mechanism for deep diving dolphins, allowing the mammal to conserve precious oxygen supplies while foraging at depth. They add that sending half of the brain to sleep while the other is conscious may also allow the animal to remain aware and keep swimming while sleeping.
