Binaural hearing: Physiological and Clinical View

The main difference between the two ears is that they are not in the same place [1]. Use of both the ears to perceive the world of sound around us is defi ned 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. 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.


Introduction
The main difference between the two ears is that they are not in the same place [1]. Use of both the ears to perceive the world of sound around us is defi ned 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. 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.

Physiology
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. Similarly, sounds with equal frequency, intensity and time, originating directly at the back of an individual with normal hearing, may also be literally indistinguishable from one another.

a) Inter-aural time difference
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 fi ne 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].

b) Inter-aural loudness difference
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. LSO is considered the major site for ILD analysis in mammals [4]. 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] [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. It proves that when two ears receive a sound stimulus, the resultant loudness sensation is greater than that occurring with just one ear.

d) Head related transfer function
Between the two ears, head acts as an effective acoustic block, refl ecting 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 fi eld. This inter-aural level differences is most signifi cant for high frequencies.
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 refl ection of sounds from pinna results in localization of sources and is described as 'Head Related Transfer Function (HRTF)'. So, HRTF is the direction dependent fi ltering effect of head and pinna.
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 refl ects 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 diffi cult [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. These spectral cues for localization underpin the ability to disambiguate the so-called cone of confusion, resolving sources in front from those behind as well as determining their elevation, a task not possible using binaural cues alone [11].
Besides these effects of ITD, ILD, IFD and HRTF, we also have the capability of central nervous system (CNS) to process the slightly different auditory inputs arriving from the two ears utilizing two important phenomena, namely Binaural squelch and Binaural 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].

Mechanism of binaural hearing
The  In these patients, after years of having to process asymmetric inputs neural circuitry may have been affected by plasticity so that time and training may be needed before binaural advantages recover [14].
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 fi tted to deserving candidates should also aim to fulfi l the above two objectives as well as provide improved use of signal to noise ratio at the two ears.

Advantages of binaural hearing
1) Since the brain can focus on the conversation the listener wants to hear, binaural hearing results in better understanding of speech.
2) Better sound and speech discrimination improves speech intelligibility in diffi cult listening situations leading to improved social communication.
3) It provides better ability to localize the sound and better