Novel illusions from hearing different sounds in the two ears – and how they happen in the brain

Lina Reiss – reiss@ohsu.edu

Instagram: @reiss.lina
Oregon Health and Science University, 3181 SW Sam Jackson Park Road, Portland, Oregon, 97239, United States

Stephen David
Bluesky: @stephenvdavid.bsky.social
Oregon Health and Science University

Michela Mondesir
Oregon Health and Science University

Miles Carter
Instagram: @meelos22
Oregon Health and Science University and University of Pittsburgh.

Popular version of 1aAB2 – Neural correlates of binaural fusion
Presented at the 190th ASA Meeting
Read the abstract at https://eppro01.ativ.me/web/planner.php?id=ASASPRING2026

–The research described in this Acoustics Lay Language Paper may not have yet been peer reviewed–

If you listen to two different sounds that are similar in pitch across the ears, something strange happens. The two sounds blend perceptually to create an illusion of a new sound, similar to what happens with different colors across the eyes.

For example, if you listen here with stereo headphones to the vowels “ah” as in hot and “ee” as in heed, spoken by two different talkers with different voice pitch – a male talker and a female talker – you will hear two vowels.

Figure 1. Perception when two different vowels are played to the two ears at different pitch. Play different pitch example. Note: Stereo headphones are necessary to experience the illusion

But if these same vowels are spoken by the same talker, you will experience something called binaural fusion (Reiss and Molis, 2021). Instead of hearing two different vowels, you will hear a single new vowel. This new vowel will be a blend of the two original vowels, something in between like “eh” as in head.

Figure 2. Perception when two different vowels are played to the two ears at the same pitch. Play same pitch example. Note: Stereo headphones are necessary to experience the illusion

This illusion is not confined to steady sounds, but also happens for sounds that are fluctuating, such as a tone that is fluctuating in one ear and steady in the other ear. This makes localization of the fluctuating tone difficult.

While we know that people experience binaural fusion, we don’t know what happens in the brain so that some sounds fuse while others are heard as distinct. It’s hard to measure detailed brain activity in humans, so we are now studying what happens in the brain of animals, in this case ferrets, when they experience the same illusion. The first thing we had to do was demonstrate that ferrets perceive these illusions the same way as humans. For vowels, ferrets were first trained to indicate when they heard the vowel “eh”, and to ignore the vowels “ah” and “ee”. When “ah” and “ee” were played to the two ears at the same pitch, the ferrets responded that they heard “eh”. Similarly, for fluctuating tones, ferrets were trained to indicate the side where they heard the fluctuating tone, and they experienced the same difficulties as human listeners.

As a next step, recordings from cells in the brain will reveal how brain activity leads to these illusory phenomena. Binaural fusion and the converse, binaural fission, are important to understand because together they underlie how the brain groups components of sound that belong to one source, such as a single talker, and separates those that belong to different sources, such as other talkers (Bregman, 1990; Bronkhorst, 2000).

It is shown that people with hearing loss, including those with cochlear implants, often experience excessive binaural fusion, and fuse voices of different pitch together (Reiss et al., 2014; 2017; 2018). Excessive binaural fusion explains a large portion of difficulties with understanding speech in noisy environments (Oh et al., 2022; 2023). Understanding how brain circuits encode binaural fusion and fission will show us how to train or rewire the brain to help people with hearing loss and other auditory processing disorders.

In the meantime, think about how you can come up with other new illusory sounds by combining two different sounds of the same pitch!

Works cited
Bregman, A. S. (1990). Auditory Scene Analysis (MIT Press, Cambridge, MA).
Bronkhorst, A. W. (2000). The cocktail party phenomenon: A review of research on speech intelligibility in multiple-talker conditions. Acta Acustica united with Acustica, 86(1), 117-128.
Oh, Y., Hartling, C. L., Srinivasan, N. K., Diedesch, A. C., Gallun, F. J., & Reiss, L. A. J. (2022). Factors underlying masking release by voice-gender differences and spatial separation cues in multi-talker listening environments in listeners with and without hearing loss. Frontiers in neuroscience, 16, 1059639.
Oh, Y., Srinivasan, N.K., Hartling, C.L., Gallun, F.J., and Reiss, L.A.J. (2023). Differential effects of binaural pitch fusion range on the benefits of voice gender differences in a ‘cocktail party’ environment for bimodal and bilateral cochlear implant users. Ear Hear. 44(2), 318–329.
Reiss, L. A., Fowler, J. R., Hartling, C. L., and Oh, Y. (2018) Binaural pitch fusion in bilateral cochlear implant users. Ear Hear. 39(2), 390-397.
Reiss, L.A., Ito, R.A., Eggleston, J.L., and Wozny, D.R. (2014). Abnormal binaural spectral integration in cochlear implant users. J. Assoc. Res. Otolaryngol., 15(2), 235–248.
Reiss, L.A.J., and Molis, M.R.. (2021) An Alternative Explanation for Difficulties with Speech in Background Talkers: Abnormal Fusion of Vowels across Fundamental Frequency and Ears. J. Assoc. Res. Otolaryngol., 22(4): 443-461.
Reiss, L.A., Shayman, C.S., Walker, E.P., Bennett, K.O., Fowler, J.R., Hartling, C.L., Glickman, B., Lasarev, M.R., and Oh, Y. (2017). Binaural pitch fusion: Comparison of normal-hearing and hearing-impaired listeners. J.Acoust. Soc. Am., 141(3), 1909–1920.