Two dolphins who died more than 10 years ago after becoming stranded on a beach in North Carolina have just given a team of scientists some remarkable new information about how dolphins process sound.
The team has discovered that while we humans have just one pathway from the inner ear to the brain, dolphins have at least two. (A bit like Mr. Spock in Star Trek having two hearts!)
My colleague Dr. Lori Marino pioneered the study of dolphin brains using magnetic resonance imaging (MRI) and discovered, among other things, that while dolphin brains are similar to ours in some ways, they’re quite different from ours in sound processing regions Their auditory areas are much larger than their visual areas, whereas in our brains it’s the other way round. (We do much more processing from what we see, rather than what we hear.)
Their auditory areas are much larger than their visual areas, whereas in our brains it’s the other way round.This makes sense in that dolphins have highly developed echolocation abilities that enable them to “see” with sound, and with extreme precision. Need to find a fish who might be hiding under the sand? No problem. Want to keep track of someone moving very fast through muddy water? Piece of cake.
“We knew from earlier studies in Russia that the auditory pathway went to an area at the top of the brain right next to the primary visual area,” she explained, “whereas the primary auditory cortex of humans and most other mammals is located on the side of the brain in the temporal lobe.”
But marine biologists have suspected for some time that dolphins might have a second pathway, and that it might be similar to the single one we humans have.
The trouble is, standard MRI’s show structures but not how they connect to each other.
“There’s another technique, however, known as DTI (diffusion tensor imaging) that can show those connections,” Dr. Marino said. “So I called up my colleague Dr. Greg Berns, who’s done a lot of work with MRI’s, to see if he might be able to scan them using the DTI technique.”
Dr. Berns is best known for his book How Dogs Love Us – A Neuroscientist and His Adopted Dog Decode the Canine Brain, in which he writes about training his own dog and three others to lie still while he images their brains in an MRI machine and demonstrates that they are deeply sentient beings with high levels of empathy and very good at social learning.
“I warned him that the two dolphin brains have been sitting in a preservative for at least 10 years,” Dr. Marino said. “But I thought it might still work, and he thought so, too. So he put them in his scanner for a full 12 hours to give the computer a chance to capture everything possible, and then we sent all the data to some colleagues in England, who were able to convert it into 3D images.”
“They showed sound processing pathways (blue and yellow) that we’d never seen before, going from the midbrain out to the temporal lobes.”
So, as well as the auditory pathway that goes from ears to the top of brain near the visual area, we now know that there is indeed a second pathway that goes to the temporal lobe, which is where our human auditory system is set up. And while we humans just have this one pathway, we now know that dolphins have two quite separate ones.
What does this mean? “They can go back and forth between audio and visual very quickly and even integrate the two.”
“We don’t know for sure, but it strongly suggests that the auditory region at the top of the brain near their visual system (the one we knew about before) is the one they use to process echolocation signals. And the one we’ve just found is for the other kinds of sounds dolphins communicate with, such as whistles and pulsed sounds.
So, what would it feel like to be a dolphin with two separate kinds of hearing going on at the same time?
“That’s really hard to tell,” Dr. Marino said. “All we can say for sure is that they can go back and forth between audio and visual very quickly and even integrate the two, as well as process some of the different components of sound separately before putting it all together. They can also produce two different types of sounds simultaneously.”
All of which is completely outside of our own experience.
“The one thing we can say for sure is that we’ve just found more evidence that dolphin brains are even more complex than we previously thought.”