Understanding Binaural Hearing: Definition and Function
Binaural hearing broadly defines those aspects of audition which rely on the interaction between the two ears. 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”. Many important components of normal auditory perception are driven by binaural neural processing. 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 [2].
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.
Key Components of 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:
- Inter-aural Time Difference (ITD)
- Inter-aural Loudness Difference (ILD)
- Inter-aural Frequency Difference (IFD)
- Head Related Transfer Function (HRTF)
Inter-aural Time Difference (ITD)
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 [3]. 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 [3].
Inter-aural Loudness Difference (ILD)
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.
Head Related Transfer Function (HRTF)
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.
The Benefits of Binaural Hearing
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.
When you use both ears for hearing, the term is binaural hearing, you give your brain additional auditory information to process. Better speech recognition occurs in noisy situations, and you can detect sound sources (directionality) through this mechanism. Your right ear transmits sounds to your left brain region before your left ear sends signals to your right brain region. Your brain processes sounds from both sides before the entire brain creates a unified interpretation of the heard information. Your ability to recognize words and understand speech, along with your capacity to focus on specific sounds or voices, improves through this process.
The brain's cooperative function enables selective listening. Your brain uses this method to block out unnecessary noises when you need to listen to a distant voice during meals, at restaurants, or when speaking with family members. The majority of individuals with hearing loss face difficulties hearing in noisy environments. Binaural hearing enables individuals to hear more clearly by helping to pull what you want to hear out of background noise more successfully. When hearing aids are correctly fitted to both ears, your brain achieves better sound processing accuracy.
Here are some additional advantages of binaural hearing:
- With binaural hearing, there is summation of intensity of sound, hence hearing requires less loudness.
- A voice that is barely heard at 10 ft with one ear can be heard upto 40 ft with two ears.
- Binaural hearing is less tiring and listening is more pleasant.
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].
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].
Practical Implications for Hearing Health
Understanding the advantages of binaural hearing has significant practical implications, especially in audiological practice. 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 [1,4].
Also, a person wearing two hearing aids generally needs less amplification than someone wearing only one. Using two hearing aids requires adjusting the volume down for each device.
In less common cases in which there is a total hearing loss in one ear (also known as profound unilateral hearing loss or single-sided deafness), there are medical therapies that may help to re-create some of the effects of binaural hearing. These include bone-conduction systems (also known as bone-anchored hearing aids, or BAHA devices) that can help transmit vibrations from the nonhearing ear to the functioning ear.
The BAHA (Bone-Anchored Hearing Aid) serves as a bone-conduction device that patients can use. A CROS system (Contralateral Routing of Signal) functions by using a microphone on the deaf side to transmit sound wirelessly to a hearing aid placed in the better ear.
Unilateral Hearing Loss
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.
Hearing Loss and Treatment
The condition where hearing deteriorates differently in one ear compared to the other affects numerous individuals. Patients often inquire as to whether they can limit their hearing aid treatment to their more severely damaged ear.
Most People Have Different Levels of Hearing Loss in Each Ear And patients in this situation frequently ask us, “Can’t I just treat my really bad ear for hearing loss? We sometimes see patients with hearing loss in only one ear (also known as unilateral hearing loss), but typically the factors that led to hearing loss affected both ears - just to a different degree. Sounds collected by your left ear are initially processed by the right side of the brain, while sounds collected by your right ear are initially processed by the left side of the brain. The two halves of your brain work together to organize the input into recognizable words and sounds.
Similarly, using more of your brain to focus on the sound you want to hear is tremendously important in overcoming one of the primary complaints of individuals with hearing loss: hearing amid background noise.
Every person has different hearing loss needs, which require individualized solutions. We encourage you to schedule a hearing evaluation to determine your hearing needs. schedule an appointment
Hearing Loss: Additional Considerations
Here are some frequently asked questions about hearing loss:
- Are some types of hearing loss easier to treat? Hearing loss is a puzzle that our professionals love to solve, and it is based on your individual experiences, lifestyle, and severity of impairment. There is no one-size-fits-all treatment method for hearing loss - it’s based on the sounds that you can’t hear, which vary greatly, and the sounds that you want to be able to hear.
- Are there any health downsides to not treating hearing loss? Research has established a relationship between hearing loss and dementia. There is strong evidence that hearing loss accelerates brain-tissue atrophy, particularly in areas of the brain that auditory nerves would stimulate but can’t because they aren’t receiving a signal (due to a hearing loss). These areas of the brain are also related to memory and speech. Individuals with a mild hearing loss are three times as likely to fall down as those without, and the likelihood of falls increases as the degree of hearing loss increases.
- At what age do people normally start getting hearing loss? Since hearing loss is cumulative, hearing loss begins as an infant and continues throughout life. Most individuals don’t begin to experience symptoms until their late 20s or early 30s, and by age 45 a yearly hearing check becomes of greater importance.
- How can I improve my hearing? Unfortunately, many forms of hearing loss are permanent because there is no cure.
- How can I prevent hearing loss? Protecting your hearing from noise levels greater than 85 decibels at work and during leisurely activities will greatly reduce your chances of noise-induced hearing loss.
- Is hearing loss hereditary? Though it is difficult to say what genetic factors predispose individuals to hearing loss, there seems to be a connection.
- What should I do if I get sudden hearing loss? See your physician immediately; sudden hearing loss is considered a medical emergency. Sudden hearing loss typically resolves on its own within two weeks, but it might not - meaning your hearing might be gone for good.