When a dog hears his owner – whether it's an affectionate call, a curt command, or an unpleasant reprimand – do the words merely go in one fuzzy ear and out the other, or are they really listening? And if they are listening, how much do they comprehend?
According to research published Wednesday in the journal Current Biology, the brain of man's best friend is specialized to process multiple components of human speech, with some regions specializing in the verbal contents of speech and others specializing in the familiarity with and emotional tone of the speaker.
Functionally specialized brains exist among many animal species, including humans. Beginning in the mid-19th century, scientists began noticing that our brains seem to have specialized regions that correlate with distinct cognitive processes. For instance, the parts that scientists associate with producing and understanding the meanings of syllables and words are, for most people, located in areas on the left side, whereas the regions that correspond with processing the prosodic aspects of speech – the tone, the stress, the rhythm – tend to be on the right. Our brains, say scientists, are lateralized, an evolutionary adaptation that allows us to perform separate mental tasks in parallel.
Dogs seem to have this, too. In a study published last year, scientists discovered that dogs appear to show similar biases in auditory processing when listening to the vocalizations of other dogs. Like humans, dogs seem to have multiple mental categories for incoming sounds, at least sounds coming from fellow dogs.
But what about when dogs are listening to humans? University of Sussex doctoral candidate Victoria Ratcliffe and psychologist David Reby of the University of Sussex wanted to find out if these hemispheric biases exist for human vocalizations, too.
"We didn't know how this information was processed," Ms. Ratcliffe told the Monitor, "if they got one signal, one form of information from it, or if they're able to divide this information up."
So Ratcliffe and Dr. Reby performed a number of audio trials on 250 different dogs from more than 60 breeds. In one test, verbal information – everyday commands that their owners had used in the past – was spoken in an accent unknown to the dog. In another experiment, the speech was stripped of any intonation, similar to a computer-generated voice.
So far, so good. But how does this tell you what's going through the dog's mind? It turns out that, for dogs as well as for humans, input coming into the left ear is transmitted to the right hemisphere, and vice versa. Dogs perceive sound that requires more processing from one hemisphere as arriving in the opposite ear, and they tend to turn their heads to where they think the sound is coming from. A head turn to the left, then, suggests that the information is being processed in the right hemisphere. A turn to the right indicates that the left hemisphere is working harder.
The researchers found that these biases were indeed present for the domesticated dogs. For example, when researchers played audio of the familiar command "come on, then" in a monotone voice to emphasize the actual phonemic information from the words, hearing the words seemed to activate the left hemispheres of the dogs' brains. And the opposite proved to be the case when those syllables in the phrase were switched around.
"They're not just responding when they hear one syllable, they need the whole combination in the right order to process it in certain ways," says Ratcliffe.
With the knowledge that dogs can break down and process the components of our speech in a way similar to how humans do it, Ratcliffe's team wants to find out if this is an example convergent evolution, where, during the domestication process, humans happened to choose canines that already had the trait for processing verbal information this way. The other possibility is that dogs gained this ability only after being exposed to speech for an extended period, indicating that other mammals might demonstrate hemispheric biases when listening to human speech.
While a better understanding of how animals take in our verbal expressions may improve our communication with dogs or possibly other animals, Ratcliffe says we might also be able to learn a bit about ourselves and our ancestral traits.
"It will give us an idea of where these specializations have come from in humans," says Ratcliffe.