Binaural hearing is the perception of sound by stimulation in two ears – two sets of sound signals are being sent to the brain where they are interpreted. This gives significant gains compared to monaural hearing when just one ear is being stimulated.
A wealth of scientific research backs this up. One research article summarises it as follows:
“Binaural hearing allows listeners with normal hearing to understand speech better in silence and noisy conditions and is an essential requirement for spatial hearing and sound localization. Other benefits of binaural hearing are more natural hearing, reduced listening effort and an improved quality of life”
(E. Offeciers (Belgium), C. Morera (Spain), J. Muller (Germany), A. Huarte (Spain), J. Shllop (USA) & L. Cavelle (Spain), International consensus on bilateral cochlear implants and bimodal stimulation, 2004).
These benefits happen for a number of reasons. In summary:
- The brain is receiving twice as much input at any given time – there is a summative effect. Hearing via one ear alone requires a great deal more concentration and is tiring for the listener. Hearing via two ears greatly reduces this burden.
- The timing and intensity of sound reaching each of a pair of ears is slightly different unless the sole source of sound is directly in front or behind the listener. The brain uses the differences in these signals to determine the direction from which a sound is coming. There is good evidence that bilateral implantation allows children to judge whether sounds come from the left or the right (i.e. a binary choice). The evidence for localisation where sound is coming from more than two directions is not yet established. However, the ability to localise sounds from the left or the right is very important in many situations. One has only to think of traffic or identifying where a teacher is speaking from.
- This comparison of sound signals by the brain is also vitally important for understanding speech in noise. The brain is able to use the differences in signal-to-noise ratio (how loud the voice you’re trying to listen to is compared with all other noise) between the ears to identify what’s not wanted and suppress it. This is referred to as ‘the squelch effect’ and is important for understanding speech in noisy situations.
A small binaural squelch effect has been identified in adults and one of six papers that had been published in the peer-reviewed literature by the end of 2007, found significant binaural squelch in bilaterally-implanted children.
- A further important advantage of hearing via two ears, particularly in children, is the ‘head shadow effect’. If a person only has hearing in one ear and a person is speaking to them on their deaf side, the head acts as a barrier to that sound and makes it very difficult to hear. The head-shadow effect is probably the main benefit from bilateral implantation. On average, children who received two implants in sequential surgeries show advantages when the noise is moved to the side of the second implant (so that the child’s head shields the first implanted ear from the noise). The advantage is smaller, and in several studies has not been significant, when the noise was moved to the side of the first implanted ear.
- While it cannot be claimed that bilateral implantation confers normal binaural hearing on recipients, the body of medical evidence continues to grow and shows that bilateral cochlear implants restore aspects of binaural hearing and provide some of the binaural advantages experienced by normal-hearing subjects.
Med El have an excellent document that describes in simple terms how some of these benefits come about. It is here (Med El Bilateral Document) and well worth a read.