the humpback whale, fin whale, sperm whale, orca, bottlenose dolphins, Risso's dolphins, and beluga whales. Humans, great apes, and elephants, species all well known for their high intelligence, are the only others known to have spindle cells. Spindle neurons appear to play a central role in the development of intelligent behavior. Such a discovery may suggest a convergent evolution of these species. Brain structure Elephant brains also show a complexity similar to dolphin brains, and are also more convoluted than that of humans, and with a cortex thicker than that of cetaceans. It is generally agreed that the growth of the neocortex, both absolutely and relative to the rest of the brain, during human evolution, has been responsible for the evolution of human intelligence, however defined. While a complex neocortex usually indicates high intelligence, there are exceptions. For example, the echidna has a highly developed brain, yet is not widely considered very intelligent, though preliminary investigations into their intelligence suggest that echidnas are capable of more advanced cognitive tasks than were previously assumed. In 2014, it was shown for the first time that a species of dolphin, the long-finned pilot whale, has more neocortical neurons than any mammal studied to date including humans. Unlike terrestrial mammals, dolphin brains contain a paralimbic lobe, which may possibly be used for sensory processing. The dolphin is a voluntary breather, even during sleep, with the result that veterinary anaesthesia of dolphins would result in asphyxiation. Ridgway reports that EEGs show alternating hemispheric asymmetry in slow waves during sleep, with occasional sleep-like waves from both hemispheres. This result has been interpreted to mean that dolphins sleep only one hemisphere of their brain at a time, possibly to control their voluntary respiration system or to be vigilant for predators. This is also given as explanation for the large size of their brains. Dolphin brain stem transmission time is faster than that normally found in humans, and is approximately equivalent to the speed in rats. The dolphin's greater dependence on sound processing is evident in the structure of its brain: its neural area devoted to visual imaging is only about one-tenth that of the human brain, while the area devoted to acoustical imaging is about 10 times as large. Sensory experiments suggest a great degree of cross-modal integration in the processing of shapes between echolocative and visual areas of the brain. Unlike the case of the human