Abstract
Human speech possesses a rich hierarchical structure that allows for meaning to be altered by words spaced far apart in time. Conversely, the sequential structure of nonhuman communication is thought to follow non-hierarchical Markovian dynamics operating over only short distances. Here, we show that human speech and birdsong share a similar sequential structure indicative of both hierarchical and Markovian organization. We analyze the sequential dynamics of song from multiple songbird species and speech from multiple languages by modeling the information content of signals as a function of the sequential distance between vocal elements. Across short sequence-distances, an exponential decay dominates the information in speech and birdsong, consistent with underlying Markovian processes. At longer sequence-distances, the decay in information follows a power law, consistent with underlying hierarchical processes. Thus, the sequential organization of acoustic elements in two learned vocal communication signals (speech and birdsong) shows functionally equivalent dynamics, governed by similar processes.
Highlights
Human speech possesses a rich hierarchical structure that allows for meaning to be altered by words spaced far apart in time
Markovian, or some combination of these two processes better explain sequential dependencies in vocal communication signals, we measured the sequential dependencies between vocal elements in birdsong and human speech
Markovian processes are not unexpected. They explain a variety of complex sequential dynamics observed in prior studies, including long-range organization[20], music-like structure[19], renewal processes[17,18], and multiple timescales of organization[23,29]
Summary
Human speech possesses a rich hierarchical structure that allows for meaning to be altered by words spaced far apart in time. Similar power-law relationships exist for the long-range dependencies between characters in texts[6,7], and are thought to reflect the general hierarchical organization of natural language, where higher levels of abstraction (e.g., semantic meaning, syntax, and words) govern organization in lower-level components (e.g., parts of speech, words, and characters)[2,3,6,7]. Organized dynamics, proposed as fundamental to sequential motor behaviors[27], could provide an alternate (or additional) structure for nonhuman vocal communication signals. Evidence supporting this hypothesis remains scarce[1,16]. Our results indicate that these two learned vocal communication signals are governed by similar underlying processes
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