Do dogs understand words or emotions?

It's an age-old debate: Do dogs understand the words their owners say to them, or are they just cuing in to the tone of voice?

It turns out it may be both: Man's best friend not only hears the meaning of human speech, but also perceives the emotion behind it, new research finds, FoxNews reports.

Although the new findings don't prove that dogs fully understand all of the emotional aspects of human speech, they do show that dogs are at least paying attention to it, said study co-author Victoria Ratcliffe, a doctoral candidate in psychology at the University of Sussex in England.

Dogs sit, heel, shake paws and snuggle in response to commands and endearments from their human companions. Though a dog lover might insist that their little Hercules understands every single word, skeptics insist that dogs just pick up on people's intonation or other nonverbal cues.

Indeed, studies have shown that dogs are attuned to people's posture, gaze and other forms of nonverbal communication, which makes it tricky to figure out just what words they understand.

And on the other hand, some research suggests people may wrongly project human understanding and motivation onto their furry friends. For instance, one study found that dogs don't truly feel guilt when they give that sad-puppy look after eating food from the table or peeing on the floor.

To figure out exactly what was going on in dogs' heads when people speak, Ratcliffe and her colleagues brought 250 pet dogs of all breeds — from a Chihuahua to a Great Dane — into their lab. The dogs were placed in a room with speakers on either side of their heads.

The sound that a dog hears in its right ear is processed mainly in the left hemisphere of the brain (and vice versa). So when a dog turns its head right as it listens to a sound, researchers can conclude its left hemisphere played a strong role in processing that sound, Ratcliffe said.

In the trial, the researchers played a clip of the owners saying the words "Come on, then!" but with the emotion electronically stripped out of their voice, and with the sound changed so the voice itself could no longer be identified, Ratcliffe said.

But other times, the dogs heard the owners say the same phrase, but with words garbled so they could not be understood. This emphasized the intonation and emotional content of the speech.

The researchers found that dogs turned their heads to the right when they heard words without the emotions, suggesting the left hemisphere was processing that speech. By contrast, the dogs turned their heads to the left when they heard the emotional words, suggesting the right hemisphere was processing that content. And when they heard pink noise, a kind of static, the dogs didn't turn their heads in either direction.

The findings were published today (Nov. 26) in the journal Current Biology.