Dolphin brains have two areas linked to hearing

Scientists have for the first time mapped the sensory and motor systems in the brains of dolphins and found that at least two areas of the dolphin brain are associated with the auditory system, unlike most mammals that primarily process sound in a single area.
"Dolphins are incredibly intelligent, social animals and yet very little is known about how their brains function, so they have remained relatively mysterious," said Gregory Berns, a neuroscientist at Emory University.
"We now have the first picture of the entire dolphin brain and all of the white matter connections inside of it," said Berns, lead author of the study published in the journal Proceedings of the Royal Society B.

The researchers applied a novel technique of diffusion tensor imaging (DTI) on the preserved brains of two dolphins.
The study focused on the dolphin auditory system, since dolphins - along with several other animals, such as bats - use echolocation to sense their environments.
"We found that there are probably multiple areas in the dolphin brain associated with auditory information, and the neural pathways look similar to those of a bat," Berns said.

"This is surprising because dolphins and bats are far apart on the evolutionary tree. They diverged tens of millions of years ago but their brains may have evolved similar mechanisms for using sound not just to hear, but to also create mental images," Berns said.

The study used the brains of a common dolphin and a pantropical dolphin from the Emory collection.
The researchers used a special DTI technique for post-mortem brains developed by study co-authors Sean Foxley, Saad Jbabdi and Karla Miller at the University of Oxford.
In a living, human brain, a DTI scan takes about 20 minutes. Scanning a post-mortem brain takes much longer, however, since they contain less water.
The dolphin brains posed a particular challenge since they are large - about the size of footballs - and had been preserved for years. They retained only small amounts of the water normally found in healthy tissue.

The data from the DTI scans allowed the researchers to map out the white matter pathways, essentially the wiring diagram for the dolphin brain, in high detail.
The results show that the dolphin auditory nerve enters the brain stem region and connects both to the temporal lobe (the auditory region of many terrestrial mammals) and to another part of the brain near the apex known as the primary visual region.
The researchers hypothesise that dolphins have more than one neural area associated with sound because they are using sound for different purposes.