Mammals other than humans can distinguish between different speech patterns. Neuroscientists in Barcelona report that rats, like humans (newborn and adult) and Tamarin monkeys, can extract regular patterns in language from speech (prosodic) cues. The report appears in the January issue of the Journal of Experimental Psychology: Animal Behavior Processes, which is published by the American Psychological Association.
This study of 16 rats per each of four conditions showed that they were able to pick up enough cues from the rhythm and intonation of human speech to tell spoken Dutch from spoken Japanese. After the researchers trained rats to press a lever when hearing a synthesized five-second sentence in Dutch or Japanese, they tested the rats’ response to the alternative language. Rats rewarded for responding to Japanese did not respond to Dutch and vice versa. They pressed the lever only for the language to which they’d been exposed. What’s more, the rats generalized the ability to differentiate to new Dutch and new Japanese sentences they had not heard before.
This special ability to detect the features that distinguish one type of speech from another – enabled by a test using two very different spoken languages – has now been documented in three different mammalian species: Humans (both newborn and adult), Tamarin monkeys, and now rats. Scientists study Tamarin monkeys because they can use the same kinds of experiments that they use for infants, allowing for direct comparison. The rats were the first non-primate mammal studied; research on non-mammalian species (such as songbirds) may shed light on whether this ability is unique to mammals.
The rats’ linguistic sophistication was limited. When experimenters used different humans to speak each sentence, the rats found it much hard to tell the languages apart. Humans, even in early infancy, can overcome this problem – and only get better at it by learning a lexicon and syntax, phonology (letter sounds), word segments, and semantic information (what words mean).