Effects of Adding Monaural and Binaural Noise to a Dichotic Listening Task.

Abstract

Background:  The Dichotic Digits Test (DDT) evaluates central auditory nervous system (CANS) dysfunction. The DDT is widely used in audiology clinics worldwide, because it is clinically efficient and has good sensitivity and specificity for CANS lesions. However, the DDT shows a strong ceiling effect, which can mitigate its ability to detect subtle CANS dysfunction.

Purpose:  This study examines the effects of adding monaural and binaural speech-spectrum noise to the DDT in an effort to make the test more taxing to the CANS and thereby reduce the observed ceiling effect.

Research design:  This was an experimental repeated measures study.

Study sample:  The participants were 20 adults aged 18 to 50 years with bilaterally symmetric speech-reception thresholds and pure-tone thresholds (250-8000 Hz) of 25 dB HL or better.

Data collection and analysis:  Each participant was administered one standard DDT test list (no noise added) and DDT test lists with binaural, monaural right, and monaural left noise added. For each of the noise-added conditions, lists were administered at two different signal-to-noise ratios, for a grand total of seven DDT test lists per participant, presented in randomized order. Monaural and binaural noise effects on DDT scoring indices (Right and Left Ear Percent Correct Scores, Combined Total Percent Correct Scores, and Dichotic Difference Scores), as well as noise effects on the right ear advantage for speech, were examined. Mixed model analyses of variance were used to examine fixed effects and interactions of Noise Condition and Ear.

Results:  Adding noise to the standard DDT systematically reduced Right and Left Ear Percent Correct Scores and Combined Total Percent Correct Scores. Statistically significant differences on all indices were found between monaural and binaural noise-added conditions, suggesting a possible advantage for binaural listening in noise.

Conclusions:  These findings suggest that adding noise to tests of dichotic listening increases the difficulty of the task, and that further investigation of dichotic listening patterns in noise could potentially lead to more sensitive clinical evaluations of CANS integrity and function.