How can species as distant as dogs and humans understand each other?
A recent study, published in
Plos Biology, conducted by researchers from the University of Geneva and the Hearing Institute, part of the Pasteur Institute, reveals that the duo meets halfway in their differences in order to communicate.
From their results, the researchers hypothesize that dogs and humans have co-adapted to communicate with each other. Comparing them with other canids not domesticated by humans, such as wolves, could help determine which linguistic abilities of the dog are due to genetics and which are due to socialization.
Illustrative image by Pixabay
Having one's own speech rate has advantages for a species: it helps avoid predators or recognize members of its kind, including potential mates. But sometimes, it can be useful to adapt to a foreign tempo to understand what others are saying, whether competitively by "eavesdropping" or collaboratively, as between dogs and humans.
In this study, the researchers highlighted the adaptation of both dogs and humans to enable communication beyond species barriers.
Changing one's voice when speaking to a dog is a natural and useful process. Eloïse Déaux, a researcher in animal behavior and neuroscience at the University of Geneva, analyzed with her team hundreds of vocalizations from dogs and humans.
According to her results, dogs' vocalization pace is slower than that of humans. And when a human addresses a dog, their speech rhythm falls somewhere in the middle. "
Humans slow down their speech when speaking to their pets, which brings it closer to the dog's typical vocal rhythm and may facilitate understanding," explains Eloïse Déaux.
How can we explain this difference between humans and dogs? According to the researcher, the explanation lies not only in their anatomy but also in their brain mechanisms, more specifically in brain waves, the electrical patterns in the brain that result from synchronous neuronal activity.
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Studying the role of brain oscillations in speech perception is relatively recent, even for humans," states Anne-Lise Giraud, professor of neuroscience at the University of Geneva and director of the Hearing Institute, part of the Pasteur Institute, which initiated the project. "
Applying it to our four-legged companions is no small feat." To study brain oscillations in canids, the researchers adapted non-invasive electroencephalography (EEG) protocols. Both human and dog participants were exposed to auditory stimuli, and their brain activity was measured in response.
Brain waves are classified by their frequency: Delta waves have a frequency between 0.5 and 4 Hertz (Hz), while theta waves range from 4 to 7 Hz. They are also distinguished by their roles in cognitive mechanisms. In the human language domain, for instance, gamma waves correspond to phoneme coding (speech sounds), theta waves to syllabification, and delta waves to intonation (also known as prosody).
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We studied the correlation, or similarity, between the acoustic signal and the EEG oscillations," explains Anne-Lise Giraud. The result? "
In humans, the theta waves in their EEG align with the acoustic envelope, whereas in dogs, it's the delta waves." In order to prove that the EEG-measured oscillations assist in the perception of auditory signals, the scientists ensured that these signals were well understood by the participants.
For humans, assessing the intelligibility of a sound stimulus is straightforward: a person can report their understanding both orally and in writing. With dogs, it's different. "
We used dog-specific language consisting of commands rather than praise, to objectively measure the intelligibility of the stimuli for the dog, based on its execution of the commands," explains Eloïse Déaux. This innovative approach allows parallels to be drawn between behavior, understanding, and brain mechanisms.
Thanks to this approach, the intelligibility limits of acoustic signals for humans and dogs can be tested: the researchers modified the sentences to speed up or slow down their pace. They also played signals devoid of content or prosody. Eloïse Déaux explains her method: "
To achieve the condition without content, for example, we reverse the master's command recording. Thus, 'sit' becomes something like 'tis'. We then reverse the prosody again so it matches the original signal's prosody."
According to the scientists' results, dogs do not respond to commands with a pace that's too fast. They are thus capable of processing human language, but only if it falls within the delta band, between 1 and 3 Hz. Additionally, the loss of content also affects dogs' understanding: with just prosody, the intelligibility of commands is lower than in normal conditions.
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Thus, our study debunks the myth that dogs are solely sensitive to our intonation, our prosody. For them to truly understand what we say, phonological content is important, but unlike us, the syllable is not the building block of their understanding." reports the researcher.
Moreover, and this is critical, the results show that understanding and oscillations are closely linked. The better the theta oscillations in humans, and delta oscillations in dogs, align with the signal's envelope, the higher the level of understanding.
From their results, the researchers hypothesize that dogs and humans have co-adapted to communicate with each other. But is this behavior solely due to the special relationship between the two species? "
It would be interesting to look at other animals with which humans interact—cows, sheep, pigs... –
to see whether they have developed the same adaptive capability, and whether humans slow their speech to talk to them," the two researchers note.
Comparing dogs with other canids not domesticated by humans, such as wolves, would help determine which of the dog's linguistic abilities stem from genetics and which from socialization. The Canid Cognition Special Interest Group (SIG) of the
NCCR Evolving Language, initiated by Klaus Zuberbühler (UniNE), Martin Meyer (UZH), and Anne-Lise Giraud, may shed more light on this subject in the near future.
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A better understanding of the dog's brain mechanisms will grant us insights into humans and our evolution, since this animal is evolutionarily very distant from us. It will also help us improve the training techniques for our four-legged friends," concludes Eloïse Déaux.
References:
Dog-human vocal interactions match dogs' sensory-motor tuning,
Plos Biology, October 1
st, 2024