Imagine two metronomes: one ticking with each beat equally spaced apart and the other clicking with a messy, inconsistent rhythm. Most people would find that the second metronome sounds out of place. But would an animal be able to tell the same?
A recent study shows that the ability to recognize rhythm may be intrinsic to not only humans, but also other vocal learning species. The study suggests that male zebra finches could serve as an ideal animal model for understanding rhythm perception, which may ultimately provide insights into related speech and movement disorders in people.
Zebra finches, like humans, can recognize a frequently repeated rhythm over time, according to the study published in August in the Proceedings of the National Academy of Sciences. Aniruddh Patel, a cognitive psychologist at Tufts University and co-author of the study, explained how this phenomenon works in humans.
“If you feel a beat in music, it is very natural to — without even meaning to or planning to — implicitly start to predict when the next beat is going to come,” Patel said. “That’s actually what allows you to move and dance to music.”
The researchers predicted that vocal learning species are more flexible than vocal non-learners in distinguishing rhythmic patterns. In vocal learners, areas of the brain that perceive rhythm and control movement are strongly tied.
“Species [that] have learned their songs have to listen and mimic, so they have these tight connections between hearing and complex movement[s] that are learned,” Patel said.
Mimi Kao, a neurobiologist at Tufts and co-author of the study, said that songbirds share similar traits to humans in pathways for learning and processing vocalizations.
“[The] key is that songbirds have specialized areas in their brains for learning songs and producing them,” Kao said. “Like humans, they have high level[s] of cortical control over their vocalizations, which is something that I think so far has not been found in monkeys.”
In the experiments, the researchers first trained the male zebra finches to recognize easy songs so that the finches could learn to distinguish between “isochronous” and “arrhythmic” patterns. Isochronous sounds have equal time separating them, whereas arrhythmic sounds are projected at irregular intervals. An isochronous sound would be like the ticking of a metronome, while arrhythmic sounds can be heard in unpredictable patterns.
In the training phase, the finches were rewarded when they pecked a switch after hearing an isochronous pattern. If the stimulus was arrhythmic, the finches were not supposed to react at all. If the birds pecked at the switch after hearing an arrhythmic sound, they received a timeout as punishment, indicating that the answer was incorrect.
After two training phases with different sets of songs, seven out of 10 male zebra finches were able to tell the difference between isochronous and arrhythmic patterns with an 84% accuracy during the initial trainings. After the finches became familiar with the apparatus, the researchers introduced new isochronous and arrhythmic stimuli over a wide range of new tempi to see if the finches could still pick out the isochronous sounds. They found that the finches were still able to distinguish the differences.
The most challenging part of working with birds, according to Kao, was conditioning them to behave in adherence with the experimental design and getting them motivated to perform specific tasks.
“Sometimes, I think there’s a lot of animal cognition where the animals know more than we think they know,” Kao said. “We as humans don’t really always know what’s the best way … to ask the animals to reveal what they can perceive.”
Kao also observed various personalities in the finches that she worked with.
“There might be a period when they can’t initiate another trial and some birds would just physically move away … and other birds would wait,” Kao said. “There are definitely individual differences: ‘Are you a fidgety animal who moves away or the one who can sit there patiently?’”
Enikő Ladányi, a linguist and cognitive scientist at Vanderbilt University, echoed that finding an animal model to map the mechanisms involved in speaking can help with research on speech and language disorders, including dyslexia, stuttering or developmental language disorder.
“These results and the increasing literature on tight links between rhythm and spoken language processing suggest that the impairment of the same neurological mechanism might underlie both rhythm and speech or language impairments,” Ladányi said. “An animal model that shares crucial features with human auditory processing could highly facilitate our understanding of these mechanisms, [which] can then help to identify, treat or even prevent these disorders more efficiently.”
Jennifer Zuk, a speech language pathologist at Boston University, suggested that the study's findings can be applied to learning more about rhythm in early brain development.
“To date, the neural mechanisms underlying the development of typical versus atypical rhythm processing abilities in humans has yet to be specified,” Zuk wrote in an email to the Daily. “[The study] carries promise for the potential to uncover the neural basis of rhythm perception relevant to humans by studying the learning process in zebra finches.”
