Understanding Binaural Hearing: Definition and Benefits
Have you ever wondered why we have two ears instead of just one? It’s not simply for symmetry or to keep our glasses in place. Use of both the ears to perceive the world of sound around us is defined as Binaural hearing. Just as we use two eyes to see in three dimensions, we use two ears for “dimensional hearing”.
Binaural hearing refers to the brain’s ability to integrate sound information from both ears simultaneously. When sound reaches your ears, each ear sends slightly different signals to your brain. This article aims to help in understanding how the auditory spatial cues arising from individual outer and inner ears are computed and processed at specialized subcortical centres and lead to binaural hearing.
Binaural hearing is literally opposite of monaural hearing. It allows us to (a) ‘map’ the sound in space, (b) pick out soft sounds, (c) pick out distant sound or speech and (d) separate a single voice from surrounding background noise.
Among the mammals, human is considered to be the one gifted with most developed communication skill. One of the key factors that empowers him with his communication skill is spatial hearing. Spatial hearing provides cues as to the relative number and location of sound sources and objects in the environment, helps in determining the dimension and characteristics of rooms and enclosed spaces, and contributes to the “cocktail party effect”, whereby the listeners are able to hear out against other interfering voices in crowded listening environments.
The Science Behind Binaural Hearing
A sound stimulus is distinguished from another by its characteristics of frequency, intensity and time. Thus, two sounds with equal frequency and intensity, originating directly in front of an individual with normal hearing, which arrive at the ears at the same time may be literally indistinguishable from one another. This change could be (a) Inter-aural Time Difference (ITD), (b) Inter-aural Loudness Difference (ILD), (c) Inter-aural Frequency Difference (IFD) and (d) Head Related Transfer Function (HRTF).
The maximum time lag for sound generated at one side of the head is around 0.6 millisec. A trained ear can detect a time difference as slight as 30 microsec. ITD is a major factor in localizing lower frequency sounds of <1.5kHz. That means, for many terrestrial mammals (particularly human) localization of sound source in the horizontal plane is achieved by an exquisite sensitivity to difference in fine time structure of low frequency (<1.5kHz) components between the two ears.
Though the neurones in both the major nuclei of Superior olivary complex, namely Medial Superior Olive (MSO) and lateral superior olive (LSO) are capable of extracting ITD information from binaural inputs, MSO is considered the major site for ITD analysis in mammals. Normal ears use intensity of sound as the most common clue for locating sound sources. The moment a loud sound is heard on one side, a judgment is made that the source of the sound is on that particular side. Judgement is also made about an estimated distance of the source.
Frequency Based Auditory Processing
British physicist Lord Rayleigh [5], (1907). separated the auditory spatial processing into (a) low tone processing and (b) high tone processing. Distinguishing between the two, he proposed his “Duplex Theory” and suggested that ITD is the primary cue used to localize positions of low-frequency tonal sources (< 2KHz), while ILD is used for high-frequency tonal sources (>5KHz). Anatomical and physiological studies have revealed two parallel brainstem pathways that appear to encode ITDs and ILDs separately [6]. ITDs are extracted by MSO neurons in which ITD sensitivity results from the coincident arrival of excitatory inputs from the two ears ILDs are extracted by LSO neurons via a subtraction-like process resulting from inhibitory inputs from the contralateral and excitatory inputs from the ipsilateral ear.
Binaural Summation of Loudness
Loudness of a sound depends on the number of action potentials triggered by it. It is integrated in brainstem. Each ear contributes substantially to the action potentials that reach the brainstem. The number doubles when the two ears are used instead of one ear for a sound coming from the front of the listener. This is called binaural loudness summation [7]. It is, thus, another measurable dimension associated with binaural hearing. It is common experience that if one is made to block one ear with an ear plug, the loudness of sound coming from a television in front immediately goes down which is restored as the ear is unplugged.
Sound travels in waveforms. A sound wave consists of segments of compression and rarefaction. Number of cycles of compression and rarefaction occurring in a pure tone is defined as frequency. The sound waves reach the ears either ‘in phase’ or ‘out of phase’. If the compressions of the waves of two pure tone signals arrive at the ears at the same time, they are said to have been ‘in phase’. Whereas if a compression wave arrives at one ear at the same time when a rarefaction wave arrives at the other ear, the two pure tone signals are said to have been presented ‘out of phase’. A pure tone is received by the two ears either in phase or out of phase depending on the frequency and a complex sound tone is received by the two ears in both phases.
Laboratory studies by Joris PX et al have already demonstrated the excellent, and often enhanced (relative to their auditory nerve inputs), phase-locking abilities of globular and spherical bushy cells of the cochlear nucleus that provide the inputs to the Medial Nucleus of Trapezoid Body (MNTB) and the ipsilateral inputs to the LSO [8]. Tollin et al also examined whether low-characteristic-frequency LSO and MNTB neurons exhibit phase-locked responses to pure-tone stimuli.
Between the two ears, head acts as an effective acoustic block, reflecting and diffracting the sounds whose wavelengths are smaller compared to the dimensions of head. Depending on frequency, the sound pressure presented to the ears on either side of the head differs. The difference is related to the location of the sound in the free field.
A listening environment comprises of ever changing complex sounds. It challenges us to analyse and process the slightest differences in the ever changing complex acoustic signals in order to achieve a good binaural hearing. The differences in these clues are further heightened by head movement by altering the relative intensity, time of arrival and the phase of acoustic signals at each ear. The head movement aided by reflection of sounds from pinna results in localization of sources and is described as ‘Head Related Transfer Function (HRTF)’. Whether the sound source is located in the front or at the back, is not uniquely determined by time difference. It is rather determined by the pinna which reflects the sound differently for different positions of sound source in a listening atmosphere.
A “Cone of confusion” also exists at each side of the head creating localization ambiguities for points located on the circumference of the cone. This cone is centred on the interaural axis with apex of the cone being the centre of the head. A sound source positioned at any point on the surface of the cone of confusion will have the same ITD values making sound localization difficult [10]. To determine the source location in vertical plane, front or back, the binaural cues of ITD, ILD and IFD fall short of perfection. The auditory system, in addition to binaural cues, hence also exploits HRTF.
Binaural Squelch
If two sound sources, one giving target signal and the other noise, are sited at the same place and their intensity well adjusted, the target signal will be effectively masked by the noise. When the noise source is moved to a different place, the target signal may become audible again, which indicates a release from masking in relation to spatial separation of the two sources. This effect is called binaural squelch, and is also known as binaural unmasking or Hirsh effect [12]. This phenomenon helps the listener when he is receiving diotic sounds (dichotic sounds are the sounds heard independently by each ear) in any noisy environment e.g. coffee house, party, auditorium etc. In such environment, CNS is able to suppress the interfering noises while focusing on the speech of just one person by binaural selectivity.
Binaural Redundancy
When the listener is receiving dichotic sounds, both ears send auditory information to the brain independently. Brain, acting as the central processor upon these dissimilar information arriving from each ear, perceives it much better than each ear separately. The listener gets the benefit of fusion of these information arriving from two ears separately and is able to filter out unnecessary parts of information by making them redundant. He not only perceives the sound signals louder, but is also able to understand the speech better. Binaural redundancy actually results in binaural enhancement. The effect of binaural redundancy refers to the improvement in speech reception when the same signal and noise are audible in both ears rather than in one ear alone [13].
Unilateral hearing loss or asymmetric bilateral hearing loss is literally equivalent to absence of good binaural hearing. This disadvantage puts a person under lot of strain. In any simple listening situation where the speech and noise are coming from different sources and are in obvious conflict with each other, the person loses the ability to filter out speech from noise. The level of disability caused by hearing loss is mainly determined by the hearing in the better hearing ear as assessed by pure-tone audiometry. Rehabilitation is required in these cases where a disability loss in the better ear is above 40dB hearing level (HL). In ideal situation with no cost issues, the patients should be habilitated by binaural fitting of hearing aids expecting them to extract the benefit of accurate localization, adequate segregating of competing sounds and better speech intelligibility.
Although, in these patients and in the recipients of cochlear implants, significant asymmetry may continue to appear across the frequency range over a long period of time. Fitting of binaural hearing aids should be such that aim to optimise the delivery of acoustic information and also to preserve the spatial cues. Similarly, bilateral cochlear implants fitted to deserving candidates should also aim to fulfil the above two objectives as well as provide improved use of signal to noise ratio at the two ears.
An essential task for the central auditory pathways is to parse the auditory messages sent by the two cochleae into auditory objects, the segregation and localisation of which constitute an important means of separating target signals from noise and competing sources. When hearing losses are too asymmetric, the patients face a situation in which the monaural exploitation of sound messages significantly lessens their performance compared to what it should be in a binaural situation. Rehabilitation procedures must aim at restoring as many binaural advantages as possible. These advantages encompass binaural redundancy, head shadow effect and binaural release from masking, the principles and requirements of which make up the topic of this short review.
Key Benefits of Binaural Hearing
Here are some of the most important benefits of binaural hearing:
- Improved Sound Localization: One of the most important functions of binaural hearing is helping you determine where sounds are coming from. When a sound occurs, it typically reaches one ear slightly before the other.
- Better Speech Understanding in Noisy Environments: We’ve all experienced the “cocktail party effect” - the ability to focus on a single conversation in a noisy room. With two ears working together, your brain can focus on sounds coming from a specific direction while filtering out distractions from other directions.
- More Natural Sound Quality: Binaural hearing provides a richer, more natural listening experience. When you hear with both ears, sounds have more depth, clarity, and fullness.
- Reduced Listening Effort and Fatigue: Using both ears requires less mental energy than straining to hear with just one.
- Summation of intensity of sound: With binaural hearing, there is summation of intensity of sound, hence hearing requires less loudness.
- Increased hearing range: A voice that is barely heard at 10 ft with one ear can be heard upto 40 ft with two ears.
- Less tiring and more pleasant listening: Binaural hearing is less tiring and listening is more pleasant.
The Role of Binaural Hearing Aids
When hearing loss occurs in one or both ears, the benefits of binaural hearing can be compromised. In cases of bilateral hearing loss, where hearing loss affects both ears, relying on a single hearing aid fails to fully restore this natural binaural hearing process. The listening experience may feel unbalanced or unnatural.
For individuals experiencing hearing difficulties, binaural amplification (the use of two hearing aids rather than just one) is typically recommended to fully benefit from these auditory advantages. This ensures not only better hearing but also enhanced quality of life through improved communication and reduced cognitive effort in challenging listening environments.
Binaural hearing aids address this issue by working in tandem. They offer a more balanced and natural listening experience. By harnessing the capabilities of both ears, binaural hearing aids enhance sound localization, improve speech comprehension in noisy environments, and contribute to a more comprehensive and enjoyable auditory experience.
Hearing loss often occurs in both ears, a condition known as bilateral hearing loss. In these cases, wearing two hearing aids can be beneficial as it aligns with the natural binaural hearing process.
At Alta View Audiology, we often recommend binaural hearing aids-hearing aids for both ears-even when hearing loss affects one ear more than the other.
Benefits of Binaural Hearing Aids:
- Two hearing aids work together to improve speech clarity, especially in challenging listening environments.
- Wearing two hearing aids provides a more natural, balanced sound experience.
- When hearing loss goes untreated, the auditory pathways in your brain can weaken from lack of stimulation-a condition called auditory deprivation.
Just as stereo sound is more enjoyable than mono, hearing with two aids usually provides a richer, more natural sound experience. Listening with one ear can be more tiring than listening with two. With two hearing aids, the listening effort is shared between both ears, reducing fatigue. Using a hearing aid in only one ear could potentially lead to auditory deprivation in the non-aided ear. Over time, the unaided ear might lose its ability to understand speech.
Neural Mechanisms Underlying Binaural Processing: The neural underpinnings of binaural hearing involve complex interactions within the auditory pathway. Neurons in the medial superior olive (MSO) and lateral superior olive (LSO) within the brainstem are particularly sensitive to ITDs and ILDs. They effectively compare the timing and intensity of the sounds arriving at each ear, providing the fundamental processing necessary for binaural hearing [5]. Further refinement occurs in the inferior colliculus and auditory cortex, where the integration of these binaural cues allows for higher-order auditory processing. This cortical interaction is crucial not only for spatial awareness but also for the sophisticated processing required for effective communication and environmental awareness [6,7].
| Feature | Hearing with One Ear (Monaural) | Hearing with Two Ears (Binaural) |
|---|---|---|
| Sound Localization | Limited ability to determine sound direction | Accurate sound localization in three dimensions |
| Speech Understanding in Noise | Difficulty separating speech from background noise | Improved ability to focus on specific sounds |
| Sound Quality | Flat and unnatural sound perception | Richer, more natural sound experience |
| Listening Effort | Increased mental strain and fatigue | Reduced effort and fatigue due to balanced input |
| Auditory Deprivation | Risk of weakening auditory pathways in the unaided ear | Maintains neural connections and speech comprehension |
Addressing Common Concerns About Binaural Hearing Aids
While the benefits of binaural hearing aids are clear, some patients still have concerns about using two devices. Let's address some common questions:
Cost Considerations
It's true that two hearing aids cost more than one. However, when you consider the significant improvements in quality of life and the potential long-term benefits to your hearing and cognitive health, many find that the investment is well worth it.
Comfort and Visibility
Modern hearing aids are designed to be incredibly comfortable and discreet. Many of our patients report forgetting they're even wearing their devices after a short adjustment period. We offer a variety of styles, including completely-in-canal (CIC) options for those concerned about visibility.
Battery Life
While using two hearing aids does mean you'll need to charge or replace batteries for two devices instead of one, many modern rechargeable hearing aids offer excellent battery life. For example, the Phonak Lumity and Resound Nexia offer all-day power on a single charge.
Embracing Better Hearing with Binaural Aids
From improved sound localization and speech understanding to reduced listening fatigue and enhanced overall auditory experience, using two hearing aids to address hearing loss can significantly improve your quality of life.
Conclusion
Binaural hearing is integral to our ability to effectively engage with the auditory world around us. By providing precise localization, improved speech understanding in noisy environments, and better overall sound clarity, the benefits of hearing with two ears are profound. Ensuring optimal binaural hearing capability-whether naturally or through appropriate audiological intervention-is crucial for maintaining effective communication and overall auditory well-being.