Differences in human and monkey sensitivity to acoustic cues underlying voicing contrasts

Area of Science:

• Psychoacoustics
• Comparative Psychology
• Speech Perception

Background:

• Voice-onset time (VOT) is crucial for distinguishing voiced and voiceless stop consonants.
• Understanding species-specific auditory sensitivity aids in developing accurate animal models for human speech perception.

Purpose of the Study:

• To compare human and monkey sensitivity to acoustic cues in voice-onset time (VOT).
• To investigate the role of specific VOT cues in speech sound discrimination.
• To assess the validity of using monkeys as models for human speech perception.

Main Methods:

• Difference limens (DLs) for VOT contrasts were measured using a standard AX procedure and operant conditioning.
• Stimuli included prevoiced (/ba/), unaspirated (/ba/), and aspirated (/pa/) stop consonants across a VOT continuum.
• Acoustic cues, such as aspiration noise and F1 transition, were manipulated to assess their impact on discrimination.

Main Results:

• Humans demonstrated significantly lower DLs (higher sensitivity) than monkeys across all tested VOT contrasts.
• Species differences were more pronounced for phonemically nondistinctive cues (voicing lead) than for distinctive cues (voicing lag).
• Removal of aspiration noise in /pa/ substantially impaired discrimination for both humans and monkeys.

Conclusions:

• Quantitative differences in auditory sensitivity exist between humans and monkeys regarding VOT perception.
• Monkeys exhibit limitations in discriminating certain VOT cues, questioning their direct applicability as models for all aspects of human speech perception.
• Acoustic cue salience, particularly aspiration noise, plays a critical role in speech sound discrimination for both species.