Uncertainty in location, level and fundamental frequency results in informational masking in a vowel discrimination task for young and elderly subjects.

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/.