Informational masking emerges with processing of complex sounds in the central auditory system and can be affected by uncertainty emerging from trial-to-trial variation of stimulus features. Uncertainty can be non-informative but confusing and thus mask otherwise salient stimulus changes resulting in increased discrimination thresholds. With increasing age, the ability for processing of such complex sound scenes degrades. Here, 6 young and 4 old gerbils were tested behaviorally in a vowel discrimination task. Animals were trained to discriminate between sequentially presented target and reference vowels of the vowel pair/I/-/i/. Reference and target vowels were generated shifting the three formants of the reference vowel in steps towards the formants of the target vowels. Non-informative but distracting uncertainty was introduced by random changes in location, level, fundamental frequency or all three features combined. Young gerbils tested with uncertainty for the target or target and reference vowels showed similar informational masking effects for both conditions. Young and old gerbils were tested with uncertainty for the target vowels only. Old gerbils showed no threshold increase discriminating vowels without uncertainty in comparison with young gerbils. Introducing uncertainty, vowel discrimination thresholds increased for young and old gerbils and vowel discrimination thresholds increased most when presenting all three uncertainty features combined. Old gerbils were more susceptible to non-informative uncertainty and their thresholds increased more than thresholds of young gerbils. Gerbils’ vowel discrimination thresholds are compared to human performance in the same task (Eipert et al., 2019).

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. (PsycINFO Database Record (c) 2020 APA, all rights reserved).

Informational masking (IM) is defined as the compromised ability to perceive and analyze signals from a single sound source in a cacophony of sounds from other sources even if the excitation patterns produced by these signals in the auditory periphery are well separated from those produced by the sounds from the other sources. IM that causes an elevation of discrimination thresholds is affected by the similarity between target and masker and by stimulus uncertainty. Here, six young and six elderly subjects were asked to discriminate between sequentially presented reference and target vowels of the vowel pairs /I/-/i/, /æ/-/ε/, and /α/-/Λ/. Psychometric functions were collected characterizing the discrimination of target vowels from reference vowels. Target vowels differed from the reference by one of seven steps shifting the three formants of a reference vowel towards the formants of the corresponding target vowel. Stimulus statistics were varied, generating uncertainty by non-informative but potentially distracting location, level, and fundamental frequency changes or all three combined. Young subjects tested with distracting changes applied to the target vowels only, the reference vowels only, or the target and reference vowels showed similar amounts of IM for all three conditions. Elderly subjects were tested with distracting changes applied to target vowels only. Applying uncertainty only to the target vowels led to worse vowel discrimination thresholds for young and elderly subjects and thresholds increased most for the three distracting changes combined. Elderly subjects showed higher vowel discrimination thresholds than young subjects, but the increase in vowel discrimination thresholds due to IM did not differ between young and elderly subjects. The temporal fine structure processing of elderly subjects was degraded in comparison to young subjects, but it was only correlated with the discrimination threshold for vowel pair /I/-/i/.

Harmonicity and spatial location provide eminent cues for the perceptual grouping of sounds. In general, harmonicity is a strong grouping cue. In contrast, spatial cues such as interaural phase or time difference provide for strong grouping of stimulus sequences but weak grouping for simultaneously presented sounds. By studying the neuronal basis underlying the interaction of these cues in processing simultaneous sounds using van Rossum spike train distance measures, we aim at explaining the interaction observed in psychophysical experiments. Responses to interaural phase differences imposed on single components of harmonic and mistuned complex tones as well as noise delay functions were recorded as multiunit responses from the inferior colliculus of Mongolian gerbils. Results revealed a better representation of interaural phase differences if imposed on a harmonic rather than a mistuned frequency component of a complex tone. The representation of interaural phase differences was better for long integration‐time windows approximately reflecting firing rates rather than short integration‐time windows reflecting the temporal pattern of the stimulus‐driven response. We found only a weak impact of interaural phase differences if combined with mistuning of a component in a harmonic tone complex.