Classification and discrimination of single, complex, and interpolated speechlike stimuli

Perceptual processing of single and complex (multi-formant) CV-like and VC-like sounds, as well as interpolated natural speech-based syllables is examined in forced-choice classification and ABX discrimination tasks. The sounds have short and rapid plosive-like vocalic transitions, and are preceded or followed by an /a/-like or /u/-like stationary part. Twelve seven-item /b/-/d/ continua were created with interstimulus step sizes based on the difference limens in endpoint frequency (for the formant stimuli). It was expected that the processing strategy would depend on the stimulus complexity, i.e., that the number of discriminable or identifiable stimuli would decrease with increasing stimulus complexity. It is found that resolution varies with stimulus complexity and methodological paradigm, but that it is not controlled by speech specific properties. Perception of the single formant stimuli can approach the limits of the auditory system, because these stimuli are processed in an analytical listening mode. Complex stimuli are perceived less analytically, presumably because the additional formants partially mask the varying cue, and because the `speechlike' character of the sound hinders within-category discrimination. Our results suggest that listeners process the stimuli in a temporary memory: internal representations for each of the stimuli of the continua under test are created, which enables them to distinguish the stimuli within the /b/ and /d/ categories. Although the interpolated speech-based sounds are perceived more categorically than the formant stimuli, the data give no clear evidence that these stimuli are processed by a long-term phoneme-labelling mechanism.