Interaction of spatial source separation, fundamental frequency, and vowel pairing in a sequential informational masking paradigm in Mongolian gerbils.

Abstract

Informational masking (IM) defines the compromised ability to perceive and analyze signals from a single source in a clutter of other sounds even if there is no interference between these signals’ excitation patterns in the inner ear. IM is affected by the similarity between target and masker and the variation of stimulus features from trial to trial, that is, stimulus uncertainty, both modulating discrimination thresholds. We applied a sequential IM paradigm measuring Mongolian gerbils’ sensitivity to detect level increments between constant-level standard (reference) and deviant (target) vowels with a level increase in a background of level-varying distracting (masker) vowels. Different combinations of vowels (/I/, /i/, /æ/, /ε/) and fundamental frequencies (101 Hz, 127 Hz) as well as sound source position (colocated, 90° separated) were presented, and the effect of target and masker similarity on IM in a condition of high stimulus uncertainty was determined. We observed a release from IM, that is, lower level increment thresholds, by differences in vowel type, fundamental frequency, or spatial separation between standard/deviant and distractor vowels only. The effects of vowel type and fundamental frequency interacted, such as the release from IM by fundamental frequency was stronger for similar than for different vowel types. The spatial separation of vowels did not interact with vowel type and fundamental frequency but offered an additional release from IM. If two of the cues supported stream segregation, the release from IM was nearly complete.