Understanding Binaural Hearing: Definition, Mechanisms, and Importance
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 is literally opposite of monaural hearing.
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 recording setup.
Key Aspects of Binaural Hearing
Binaural hearing allows us to:
- ‘Map’ the sound in space.
- Pick out soft sounds.
- Pick out distant sound or speech.
- 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].
How Binaural Hearing Works
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. The main difference between the two ears is that they are not in the same place [1]. This change could be:
- Inter-aural Time Difference (ITD)
- Inter-aural Loudness Difference (ILD)
- Inter-aural Frequency Difference (IFD)
- Head Related Transfer Function (HRTF)
These differences enable the brain to process spatial information about sound sources.
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 Wave Phases
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’.
- In Phase: If the compressions of the waves of two pure tone signals arrive at the ears at the same time.
- Out of Phase: If a compression wave arrives at one ear at the same time when a rarefaction wave arrives at the other ear.
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.
Cone of Confusion.
Binaural Squelch and Redundancy
i) 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.
ii) 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].
Impairment and Rehabilitation
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.
5 Must Know Facts For Your Next Test
- Binaural hearing allows humans to localize sound sources in the horizontal and vertical planes, as well as determine the distance of a sound source.
- The brain uses interaural time differences (ITDs) and interaural level differences (ILDs) to determine the location of a sound source.
- Binaural hearing is essential for tasks such as speech recognition in noisy environments, sound source segregation, and spatial awareness.
- Damage or impairment to the auditory system can lead to difficulties with binaural hearing, affecting an individual's ability to localize sounds and perceive the auditory environment.
- Binaural hearing is also crucial for the perception of depth and distance in the auditory domain, contributing to a more immersive and realistic auditory experience.
Review Questions
Explain how the brain uses binaural cues to localize sound sources.
The brain utilizes two primary binaural cues to determine the location of a sound source: interaural time differences (ITDs) and interaural level differences (ILDs). ITDs refer to the slight delay in the arrival of a sound at one ear compared to the other, which the brain uses to infer the horizontal position of the sound source. ILDs, on the other hand, refer to the difference in sound intensity or volume between the two ears, which the brain uses to determine the vertical position and distance of the sound source. By integrating these binaural cues, the brain can accurately localize the position of a sound in the auditory environment.
Describe the importance of binaural hearing for everyday tasks and experiences.
Binaural hearing plays a crucial role in various everyday tasks and experiences. It allows individuals to better understand speech in noisy environments by separating the desired speech signal from background noise, a process known as sound source segregation. Binaural hearing also contributes to a more immersive and realistic perception of the auditory environment, enabling individuals to better judge the distance and direction of sound sources, which is essential for spatial awareness and navigation. Furthermore, binaural hearing is crucial for tasks that require the integration of auditory and visual information, such as localizing the source of a sound or following a conversation in a crowded room.
Analyze the potential consequences of impaired binaural hearing and how it may impact an individual's quality of life.
Impaired binaural hearing can have significant consequences for an individual's quality of life. Difficulties with sound localization can lead to challenges in navigating the environment, increased risk of accidents, and reduced spatial awareness. Additionally, the inability to effectively segregate sound sources can make it difficult to understand speech in noisy settings, such as crowded social gatherings or busy workplaces, leading to social isolation and communication challenges. Impaired binaural hearing may also contribute to difficulties with sound source identification, which can impact an individual's ability to respond appropriately to important auditory cues, such as alarms or warning signals.
Benefits of Binaural Hearing
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.