The effects of stimulus variability on the perceptual learning of speech and non-speech stimuli

Previous studies suggest fundamental differences between the perceptual learning of speech and non-speech stimuli. One major difference is in the way variability in the training set affects learning and its generalization to untrained stimuli: training-set variability appears to facilitate speech learning, while slowing or altogether extinguishing non-speech auditory learning. We asked whether the reason for this apparent difference is a consequence of the very different methodologies used in speech and non-speech studies. We hypothesized that speech and non-speech training would result in a similar pattern of learning if they were trained using the same training regimen. We used a 2 (random vs. blocked pre- and post-testing) × 2 (random vs. blocked training) × 2 (speech vs. non-speech discrimination task) study design, yielding 8 training groups. A further 2 groups acted as untrained controls, tested with either random or blocked stimuli. The speech task required syllable discrimination along 4 minimal-pair continua (e.g., bee-dee), and the non-speech stimuli required duration discrimination around 4 base durations (e.g., 50 ms). Training and testing required listeners to pick the odd-one-out of three stimuli, two of which were the base duration or phoneme continuum endpoint and the third varied adaptively. Training was administered in 9 sessions of 640 trials each, spread over 4-8 weeks. Significant learning was only observed following speech training, with similar learning rates and full generalization regardless of whether training used random or blocked schedules. No learning was observed for duration discrimination with either training regimen. We therefore conclude that the two stimulus classes respond differently to the same training regimen. A reasonable interpretation of the findings is that speech is perceived categorically, enabling learning in either paradigm, while the different base durations are not well-enough differentiated to allow for categorization, resulting in disruption to learning.