Recognition of synthesized vowel sequences in steady-state and sinusoidally amplitude-modulated noises

Modulation masking is known to impact speech intelligibility, but it is not clear whether the mechanism underlying this phenomenon is an invariant, bottom-up process, or if it is subjected to factors such as perceptual segregation and stimulus uncertainty thereby showing a top-down component. In the main experiment of the current study (Exp. II), listeners' ability to recognize sequences of synthesized vowels (i.e., the target) in sinusoidally amplitude-modulated noises (i.e., the masker) was evaluated. The target and masker were designed to be perceptually distinct to limit the top-down component of modulation masking. The duration of each vowel was either 25 or 100 ms, the rate at which the vowels were presented was either 1 or 6 Hz, and the masker modulation rate was varied between 0.5 and 16 Hz. The selective performance degradation when the target and masker modulation spectra overlap, as would be expected from modulation masking, was not observed. In addition, these results were able to be adequately captured using a model of energetic masking without any modulation processing stages and fitted only using the vowel-recognition performance in steady-state maskers, as obtained from Exp. I. Results suggest that speech modulation masking might not be mediated through an early-sensory mechanism.