Phonological perception by birds: budgerigars can perceive lexical stress

Abstract

Metrical phonology is the perceptual "strength" in language of some syllables relative to others. The ability to perceive lexical stress is important, as it can help a listener segment speech and distinguish the meaning of words and sentences. Despite this importance, there has been little comparative work on the perception of lexical stress across species. We used a go/no-go operant paradigm to train human participants and budgerigars (Melopsittacus undulatus) to distinguish trochaic (stress-initial) from iambic (stress-final) two-syllable nonsense words. Once participants learned the task, we presented both novel nonsense words, and familiar nonsense words that had certain cues removed (e.g., pitch, duration, loudness, or vowel quality) to determine which cues were most important in stress perception. Members of both species learned the task and were then able to generalize to novel exemplars, showing categorical learning rather than rote memorization. Tests using reduced stimuli showed that humans could identify stress patterns with amplitude and pitch alone, but not with only duration or vowel quality. Budgerigars required more than one cue to be present and had trouble if vowel quality or amplitude were missing as cues. The results suggest that stress patterns in human speech can be decoded by other species. Further comparative stress-perception research with more species could help to determine what species characteristics predict this ability. In addition, tests with a variety of stimuli could help to determine how much this ability depends on general pattern learning processes versus vocalization-specific cues.

Keywords: Acoustic perception; Animal phonology; Budgerigars; Comparative cognition; Evolution of language; Metrical stress; Operant conditioning.

Fig. 1

Oscillograms and spectrograms of the trochaic and iambic versions of one of the stimuli (puvo). Both the oscillograms and spectrograms were generated in Praat (see http://www.fon.hum.uva.nl/praat/) and are shown in a 1-s time window (x axis). The oscillograms display between −0.4 and +0.4 volts and the spectrograms display frequencies from 0 to 5000 Hz (y axis)

Fig. 2

Diagram explaining what the humans and budgerigars experienced on screen during each phase of the experiment

Fig. 3

Average percent response to trained S+ and trained S− stimuli as well as novel S+ category and novel S− category stimuli during the generalization test for each species. Note that responses to the novel stimuli resulted in no feedback, but the trained stimuli continued to be reinforced the same way as during pretesting. Error bars show standard error of the mean

Fig. 4

Percent correct for each manipulated stimulus category for each species. Panel a shows the manipulated stimuli with one cue removed. Panel b shows the manipulated stimuli with only one cue remaining. The line across each graph represents chance. Error bars show standard error of the mean. Stars show bars that are significantly above chance. *Two-tailed significance, ^†one-tailed significance