Excess Vowel Fusion in Hearing-Impaired Listeners Across Fundamental Frequency and Spatial Separation in the Free Field: An Alternative Mechanism for Difficulties With Speech in Noise

Binaural fusion occurs when signals arriving separately at the two ears are perceived as one sound. Previously, it was shown that normal-hearing (NH) or hearing-impaired (HI) listeners fused and perceptually averaged dichotic vowels presented through headphones. HI listeners even fused vowels with different fundamental frequencies (F 0 s), indicating a new alternative mechanism underlying difficulties with speech segregation. To extend these findings to a more real-world listening context, the current study investigated if vowel fusion and averaging would also occur in the free field. Synthesized vowels/with F 0 s of 106.9, 151.2, or 201.8 Hz and were presented simultaneously via loudspeakers located on the left and right at ±60° relative to the listener’s head. Listeners indicated which single vowel or two vowels they heard. Similar trends of vowel fusion were observed as in the previous study. For ΔF 0 =0, both groups showed high rates of vowel fusion. For ΔF 0 >0, NH listeners showed decreased fusion, but fusion remained high for HI listeners. Compared to headphone presentation, overall vowel fusion occurred less frequently under free field presentation, with different patterns of perceptual averaging. Results suggest that cues available in the free field may help reduce vowel fusion, especially in NH listeners and for vowels with the same F0. However, HI listeners still experience excessive vowel fusion in the free field, even with ΔF 0 as large as 6–11 semitones. These findings reinforce the importance of reducing both the blending of multiple speech streams and excessive fusion to address difficulties with understanding speech in noise.