Effect of dichotic listening on pitch perception and roughness sensation

Background. Dichotic listening, in which different sounds are simultaneously presented to each ear, can evoke perceptual phenomena that are not noted during monaural or diotic listening. A well-known example is the scale illusion, in which listeners perceive coherent melodic streams that vary from the actual auditory input. This illusion underscores the interaction between pitch proximity-based grouping and laterality cues. However, the original scale illusion is complex, which makes it challenging to isolate specific perceptual mechanisms. Previously, we introduced simplified tonal sequences that allowed for a clearer evaluation of how pitch proximity and laterality interact. This study explores two questions: (1) to what extent pitch proximity-based grouping dominates over laterality in dichotic listening and (2) whether dichotic presentation decreases the perception of roughness, an unpleasant auditory sensation caused by overlapping frequencies.Methods. This study included 20 adults with normal hearing (eight men and 12 women; mean age: 35.6 years). In Experiment 1, participants listened to pairs of tone sequences (dichotic and diotic) and chose the one in which an ascending scale was perceived more clearly. In Experiment 2, the participants determined the tone pairs that sounded rougher. The two dichotic tone sequences were named 2S and 2J, respectively. The 2S consisted of the C major scale with ascending tones presented to the right ear, whereas the pitch of the simultaneously presented tones to the left ear was two whole tones lower. Furthermore, 2J was created by swapping every other tone between the right and left ears in 2S. The diotic versions included one in which the tones from the right and left channels of 2S were mixed in both ears (DO) and one in which the tones of the right channel of 2S were heard from both ears (S).Results. In Experiment 1, the participants most clearly perceived the diotic version S. Regarding dichotic tonal sequences, no considerable differences were observed in perception between 2S and 2J. In the diotic version of the DO, pitch perception varied across individuals. In Experiment 2, the DO was rated as substantially rougher than in all other conditions, including 2S and 2J.Conclusion. Our results showed two perceptual benefits of dichotic listening in individuals with normal hearing. First, dichotic listening supports the perception of coherent melodic patterns, with pitch proximity dominating the location of the sound input. Second, the dichotic presentation considerably reduced roughness, likely owing to the separation of frequency processing in the cochlea of each ear. Despite being a preliminary investigation, this study underscores the potential of dichotic listening to improve auditory clarity and comfort.