Man’s best listener? Dog brains segment speech similar to humans

The study suggests that efficient speech processing is not necessarily a consequence of humans' unique language abilities, say scientists.

Instead, Hungarian researchers found regular exposure to speech alone can reshape how the brain functions — even in a species of mammal evolutionarily distant from humans.

Study co-author Attila Andics, of Eotvos Lorand University in Hungary, said: "Words are made up of two main types of speech sounds: vowels and consonants.

"Although vowels are louder and more noticeable, consonants usually form the skeleton of words.

"In a continuous speech stream, it is easier to detect individual words when we focus on consonants.

"And from infancy onward, this is exactly what the human brain tends to do.

"This phenomenon is known as the consonant bias.

"But does the emergence of consonant bias — that is, becoming tuned to speech — require the kind of complex language abilities that are unique to humans? Does it require a brain that is inherently wired differently?

"Or could tuning to speech arise through simple pattern learning, based on principles also present in other animal species that cannot speak?"

To find out, the research team compared the brain responses of 20 humans and 20 companion dogs to speech.

Study co-first author Boglárka Morvai said: "We reasoned that if the preference for consonants — the acoustically less salient type of speech sound — requires uniquely human language abilities to develop, then other species should not show such a preference.

"Not even dogs, despite spending much of their lives surrounded by human speech.

"If, on the other hand, tuning to speech is based on simple forms of pattern learning from repeatedly heard speech, then a consonant bias should also emerge in the dog brain — even though such a bias has not previously been found in primate species much more closely related to humans."

During the experiment, the human and dog participants listened to continuous, unsegmented speech streams.

Consonants and vowels alternated throughout the streams, with no pauses or other acoustic cues separating them.

The streams differed in whether they contained recurring patterns and, if so, what kind.

There were three types. In the first one, recurring consonant patterns formed consecutive three-syllable words — or, more precisely, word skeletons, because the vowels occurring between the consonants could vary freely.

In the second type, the situation was reversed: recurring vowel patterns formed the skeleton of the words.

In the third, control type contained no recurring patterns at all, so the syllables did not form words but followed one another in a random order.

To measure brain activity, the researchers used noninvasive EEG, recording the brain's electrical activity through electrodes placed on the scalp.

They also relied on a special neural phenomenon called neural synchronization.

The research team explained that brain activity can take on the rhythm that we detect in the sounds we hear.

And the more reliably we detect that rhythm, the more rhythmic the brain response becomes.

The researchers expected that if a species' brain has a preference for consonants, it should follow the rhythm of the words more easily when consonants form the structure of those words.

The results, published in the journal Science, revealed "striking similarities" between the two species.

Study co-first author psychologist Kinga Tóth said: "In the consonant-structured speech streams, the dog brains, just like the human brains, successfully synchronized with the rhythm of the words.

"In both species, this synchronization was stronger than in either the vowel-structured or the control streams.

"This is the first evidence that a consonant bias can also emerge in the brain of a species other than humans."

She said the timing of the brain responses provided further evidence for a consonant bias in dogs.

Tóth added: "The study also revealed that this tuning to the characteristics of speech does not emerge only in dogs that were already living in a speech-rich environment during the first months of life, when they are especially sensitive to social experiences.

"Dogs that had spent their first three months living on the street or in a shelter, rather than as family pets, showed a similarly strong consonant bias.

"This provides further evidence that consonant bias does not require pre-wired language-processing mechanisms that emerge during a specific early developmental period.

"Instead, this neural preference may also emerge through relatively simple forms of pattern learning."

The results also suggest that the human brain can process continuous speech at a finer resolution.

For us, each speech sound is a separate unit, whereas for dogs, syllables form the smallest units of processing.

Andics added: "Taken together, these findings suggest that the brain's tuning to speech — and specifically the emergence of consonant bias — does not require a pre-wired brain and human-like language abilities.

"Instead, regular exposure to speech may be enough to change how the brain functions and to give rise to processing characteristics previously observed only in humans — even in a species evolutionarily distant from us."