Abstract
The influence of experience with human speech sounds on speech perception in budgerigars, vocal mimics whose speech exposure can be tightly controlled in a laboratory setting, was measured. Budgerigars were divided into groups that differed in auditory exposure and then tested on a cue-trading identification paradigm with synthetic speech. Phonetic cue trading is a perceptual phenomenon observed when changes on one cue dimension are offset by changes in another cue dimension while still maintaining the same phonetic percept. The current study examined whether budgerigars would trade the cues of voice onset time (VOT) and the first formant onset frequency when identifying syllable initial stop consonants and if this would be influenced by exposure to speech sounds. There were a total of four different exposure groups: No speech exposure (completely isolated), Passive speech exposure (regular exposure to human speech), and two Speech-trained groups. After the exposure period, all budgerigars were tested for phonetic cue trading using operant conditioning procedures. Birds were trained to peck keys in response to different synthetic speech sounds that began with “d” or “t” and varied in VOT and frequency of the first formant at voicing onset. Once training performance criteria were met, budgerigars were presented with the entire intermediate series, including ambiguous sounds. Responses on these trials were used to determine which speech cues were used, if a trading relation between VOT and the onset frequency of the first formant was present, and whether speech exposure had an influence on perception. Cue trading was found in all birds and these results were largely similar to those of a group of humans. Results indicated that prior speech experience was not a requirement for cue trading by budgerigars. The results are consistent with theories that explain phonetic cue trading in terms of a rich auditory encoding of the speech signal.
Highlights
Phonetic cue trading experiments on human speech have shown that listeners are able to perceive the same phoneme under a variety of cue manipulations so that, despite changes in the acoustic signal, the same phonetic percept exists
After learning the categories based on the ends of the two series, all of the birds exhibited cue trading when exposed to the novel, intermediate stimuli of the two series that differed in F1 onset frequency
There was a shift in the category boundary as a result of the F1 onset frequency, indicating the budgerigars relied on evidence for the distinction from this other acoustic cue
Summary
Phonetic cue trading experiments on human speech have shown that listeners are able to perceive the same phoneme under a variety of cue manipulations so that, despite changes in the acoustic signal, the same phonetic percept exists. In these experiments, one cue is attenuated. Cue trading has been cited as being one of the most compelling sources of evidence for an articulation-based speech mode of perception because several early studies observed cue trading between specific spectral and temporal auditory cues occurring only when listeners engaged in phonetic classification of speech signal and not when they were identifying analogous nonspeech stimuli [2]. Phonetic trading relations were believed to be a product of phonetic categorization, specific to speech and reliant on reference to speech production, rather than a result of more general interactions in the auditory system
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