Do you ever wonder whether Fido really understands what you're saying?
He might be catching on to more than you think -- and your intonation when saying certain words is just as important to him as the words themselves, according to a small study published in the journal Science on Tuesday.
Dogs use the left hemisphere of the brain to process words, similar to humans, said Attila Andics, a research fellow at Eötvös Loránd University in Hungary and lead author of the study.
Meanwhile, the study also suggests that dogs use a right hemisphere brain region to process intonation independently of words, which means they may separate what you say from how you say it.
Man's best friend's brain
"It was surprising that dogs, like people, have a clear left hemisphere dominance for processing meaningful words, and that they combine word meaning and intonation to arrive at a unified representation of meaning," Andics said.
"What makes dogs special is that they pay attention to human social signals, including speech," he added. "This study is the first step to understanding how dogs interpret human speech, and these results can also help to make communication and cooperation between dogs and humans even more efficient."
The study involved 13 dogs -- golden retrievers, border collies, a German shepherd and a Chinese crested -- who were trained to lay down and remain still for more than seven minutes while in a brain-scanning MRI machine.
The researchers used the machine to record and measure neural activity in the dogs' brains while they listened to a woman trainer, whom they were familiar with, recite various words in various intonations.
For instance, positive or meaningful words such as "well done," "good boy," and "clever" were said in both a praising intonation and a neutral intonation. Neutral or meaningless words, such as "even if," "although," and "however," also were said in both intonations.
The brain scans revealed that parts of the left hemisphere reacted the most to the meaningful words. In general, the brain's left hemisphere is linked to language and speech processing in most humans.
Meanwhile, parts of the right hemisphere reacted to intonation, suggesting that the dogs processed the meaning of words separately from the tone in which they were spoken, according to the brain scans.
Only when a praise word was spoken to the dogs in a praising tone of voice did the brain's reward center light up like a Christmas tree in the brain scans. The reward center is the part of the brain that responds to pleasurable stimuli, like food treats or being petted.
"Our finding supports those who think that some nice words can also work as a reward for dogs," Andics said.
I say 'Woof,' you say ...
In 2014, the same research team conducted a similar MRI study, published in the journal Current Biology, in which brain scans were taken of both 11 dogs and 22 humans while they listened to dog sounds, human sounds and random non-vocal sounds.
In that study, similar patterns were found in dog and human brains, but a left dorsal auditory region of the dog brains responded stronger than the human brains to the dog sounds.
Some scientists reference the old and new research as evidence that dogs have language-processing neural capacities that were previously thought to be uniquely human. Other scientists, however, disagree.
"Unless there is a secret dog lexicon, this all points to an attention-amplifying effect of left hemisphere bias of auditory processing," said Gregory Berns, distinguished professor of neuroeconomics and director of the Center for Neuropolicy at Emory University, who was not involved in the study.
In other words, just because the left hemisphere of both dog and human brains respond to words similarly, doesn't necessarily mean that dogs comprehend words just like we do.
To explain the new study's results, maybe "the praise words in the current study are simply alerting and attracting the dog's attention, potentially amplifying the left hemisphere bias that is there for all of the words," said Berns, who has used MRI machines to scan dog brains in his own work.
"If the dog wasn't on task to stay in the MRI she would probably cock her head to one side," he added. "I'm convinced dogs understand some things, but their semantic space is not constructed like ours."
Wagging, no matter what
Other scientists agree that the new study findings don't necessarily show that dogs think just like we do.
"The study doesn't claim that dogs understand words in the sense that we understand each other's words. It shows that dogs' brains process the sounds of a person's voice in two ways," said Clive Wynne, psychology professor and director of the Canine Science Collaboratory at Arizona State University, who was not involved in the new study.
"It reminds me how, as a kid, for fun, we would insult our family dog, Benji, in a happy tone of voice. And he would wag his tail enthusiastically. We used to think that was very cute and it seems to show that the content of the word is not important -- only the tone of voice with which it is said," Wynne said. "This study shows that my childish self was wrong: The dog's brain really can discriminate meaningful from meaningless words."
Scientists are still scratching their heads about whether other non-human animals can process words and intonation in the same way that dogs do, Andics said. "We don't know, it has never been tested."
However, the researchers pose in the new study that what makes language uniquely human may not be the brain's capacity to process words and meaning. Rather, it might be the human invention of words and our ability to give words meaning.
The study "provides interesting new evidence that dogs are tracking several of the same linguistic features that humans do, and that dogs seem to understand the intent behind some of our utterances," said Laurie Santos, psychology professor at Yale University, who has studied the cognitive capacities of animals but was not involved in the new research.
"I think it shows the way that domestication has shaped dogs to pay attention to yet another important human social cue," she said.