I’m still taking part in a clinical study to improve speech perception by reprogramming my cochlear implant based on pitch. My left cochlear implant’s ability to discriminate speech on its own has jumped from 48% to 70%. My right implant is still a work in progress. Bilaterally, my speech perception score has jumped from 57% to 84%.
One of the things I’ve never been able to do is to discriminate which direction sound is coming from, as you need two good ears to do this. Wearing an implant on each side of my head enables me to tell if a sound is coming from my left or right.
But how accurate is this ability to discriminate directionality of sound with bionic hearing?
Let’s look at what it actually means to hear the direction a sound is coming from. It is quite a complex process. A number of factors combine to help the listener detect direction – time lag, difference in volume, wavelength, and tone of the sound.
** TIME LAG. Sound coming from one direction will reach the ear furthest away 1/500 second later than the closer ear.
** VOLUME. The ability to hear differences in volume depends on the sound frequency. It is easier to detect the direction of high frequency sounds than low frequency sounds. It is more difficult for high frequency sounds to reach the other ear, as they are blocked by the head, so the closer ear will need a higher volume. The head does not block lower frequency sounds quite so easily.
** WAVELENGTH. The human ear is less sensitive to volumes of low frequency sounds, so it is more difficult to detect sound direction. Low frequency sounds have wavelengths greater than the distance between the ears, and the head will not prevent the sound waves from reaching both ears. Higher sounds have shorter wavelengths and the head acts as a screen if the sound comes from one side.
** TONE. There is no time lag between the ears if the sound comes from above, below or in front of the face. The outer ear helps the listener to work out the tone of the sound. Height information is given by sound (especially high frequencies) reflected off the back edge of the ear lobe, the frequency changing with the angle of the source of the sound. Motorcyle riders find it difficult to tell where an ambulance is coming from, as the helmet reduces the ability of the outer ear to detect the tone of the sound.
Sound takes 1/500 of a second to travel the distance between your two ears. Try listening to the following clip and see if you can hear which ear you hear the sound in. It is best to use headphones, but will work if you sit between your computer speakers.
Click on this sound clip
You will hear three sound clips: pure tones, a voice, and part of a Madonna song. Each sound clip is repeated three times. First, the right and left audio channels are identical. Second, the left channel precedes right by 0.5 ms. Third, the right channel precedes the left by 0.5 ms.
The surprising conclusion is that it’s difficult to tell which direction the pure tones are coming from, but it’s easy to tell which direction a more complex sound, such as the Madonna song, is coming from.
So now we see that simple tones are harder to locate, we can wonder why sirens use them. A high degree of accuracy in localising sound is only possible when the sound is complex and made up of a majority of frequencies in our hearing range. The brain cannot accurately locate simple pure tones and it is surprising that alerting devices use them.
Source: Stanford University
How does this translate to bionic hearing? Studies have shown that bilateral cochlear implant users perform better than unilateral implant users in discriminating direction of sounds;
Ear Hear: April 2002 23(2):137-49. Sound-direction identification, interaural time delay discrimination, and speech intelligibility advantages in noise for a bilateral cochlear implant user. Van Hoesel R, Ramsden R, Odriscoll M.
I have inadvertently added to the body of evidence supporting this, through my testing in this particular clinical trial. One month ago, using speech as the sound stimulus, my ability to detect which direction sound comes from had been scored at 28%. Now, my ability has been scored at 70%. Wicked, don’t you think? 🙂