Improving Hearing Loss Humans Adapting Echolocation

ENGINEERING Improving Hearing Loss Humans Adapting Echolocation

Jennifer Jaco ‘13

n estimated 245 million people world-wide are visu- transmitting sound waves from the tympanic membrane ally impaired, and 39 million more are completely to the oval window, the middle ear functions as an acous- Ablind. Sixty-five percent of these people are over the tic transformer, amplifying the sound waves before they age of fifty (1). Fifty is becoming the new thirty in many move into the inner ear. “The pressure of the sound waves countries, as people are starting to live much longer. Despite on the oval window is about twenty times greater than that advances in technology, after blindness has set in, there on the eardrum. This pressure increases due to the differ- is nothing that can be done to reverse the damage. At this ence in size between the relatively large surface of the ear- time, particularly in the fifty and older group, people often drum and the smaller surface of the oval window” (6). From begin to feel helpless and separated from society. They no the oval window, sounds are then transmitted to the inner longer feel “normal” without their innate ability to see. This ear, which contains the cochlea (7). The cochlea is coiled problem can theoretically be solved by echolocation, defined two and a half times around a hollow central pillar, which as the ability to “hear the locations and properties of silent contains the cochlear artery, vein, and nerve. Inside the spi- objects by noticing how sound reflects off them” (2). Bats, rals of the cochlea is the Organ of Corti, which contains a dolphins, and toothed whales all use echolocation to “see” fluid, called perilymph, as well as tiny hair cells that respond objects around them while they are in motion by using ex- to sound vibrations. There are about 24,000 of these hairs tremely high frequency sound waves outside of the human embedded in the cochlea, arranged in four long rows. The hearing range. With a device that allowed the human ear to movements in the perilymph cause different hair cells to be use the principals of echolocation, people who were visu- put into motion. When the hair cells move, they send electri- ally impaired would no longer need to use a cane to detect cal signals to the auditory nerve, which is connected to the objects around them, thus preventing injuries and hassle. temporal lobe of the brain. In the brain, the electrical im- More advanced ear technology that incorporates echolo- pulses are translated into sounds that we can comprehend cation would help improve the lives of millions of people (8). As such, these hair cells are essential to hearing ability. worldwide. Rapidly changing technology allows us the abil- Damage to the hair cells limits the capacity of the hu- ity to improve lives around the world—why not push it even man ear to detect sound. Higher frequencies are harder further to revolutionize the way we perceive our world? to hear as age increases as a result of an age-related dete- rioration process called presbycusis (9). Most human ears can hear frequencies between 20 Hz and 20 kHz (10). The Anatomy and Physiology of the Ear highest frequencies that a middle aged human can hear are The ear is divided into three portions that focus and around 12 kHz to 14 kHz, a threshold which only decreases process sound: the external ear, the middle ear, and the in- with age. Other limitations of the human ear include deaf- ner ear (3). Sound funnels through the cartilege-covered ex- ness or severe hearing impairment (11). There are four main ternal ear into the external auditory canal, a short tube that categories of hearing impairment: conductive hearing loss, ends at the tympanic membrane, commonly known as the sensorineural hearing loss, mixed hearing loss, and central ear drum (4). Encyclopedia Britannica explains that the hearing loss. Conductive hearing loss is usually caused by middle ear is “a narrow, air-filled space that resembles a rectangular room with four walls, a floor, and a ceiling, and is made up of three bones [called the ossicles]: the malleus, incus, and stapes. The lateral wall of the middle-ear space is formed by the tympanic membrane, the inferior wall is a thin bone separating the cavity from the jugular vein and carotid artery below, the anterior wall is the opening of the Eusta- chian tube, which equalizes pressure between the external and middle ear, and the medial wall is a part of the bony otic capsule of the inner ear” (5). The medial wall has two open- ings: the oval window, closed by the stapes, and the round window, which is covered by a thin membrane (5). When sound travels through the external auditory canal, it vibrates the tympanic membrane. “It is the pressure from sound waves that makes the eardrum vibrate” (6). A vibration travels from the tympanic membrane through the malleus, is transferred through to Image retrieved from http://www.britannica.com/EBchecked/topic/175622/human-ear (Accessed 2 November 2011). the incus and stapes, and finally the oval window (6). By Fig. 1: Structure of the ear.

36 Dartmouth Undergraduate Journal of Science either a disease such as measles or an obstruction of the ex- quency tones, so the latter can be assumed to be nearby (15). ternal ear. In this case, hearing can either be repaired surgically or it can be successfully compensated with a hearing aid. Sensorineural hearing loss, by comparison, is more pro- Echolocation in the Animal Kingdom found than conductive hearing loss; it is caused by damage In the animal kingdom, several animals use echolo- to the hair cells or nerves in the inner ear. Generally, only the cation to compensate for their poor vision. While some loss of certain frequencies occurs, meaning a person can still animals’ visions may be inferior to that of humans, they hear some sounds. Mixed hearing loss is a combination of have managed to overcome the sound localization and conductive hearing loss and sensorineural hearing loss. This frequency ranges inadequacies that humans possess. By means a person suffers from problems in both the external using sonar and echolocation, the animal kingdom has and middle ear, as well as the inner ear (12). Lastly, in central found a solution to blindness before the medical world has. hearing loss, nerves leading to the central nervous system are Bats have the ability to hear much higher frequencies damaged. Of the four, sensorineural hearing loss is the most than the human ear can hear. Compared to the human’s common form of permanent hearing loss. This setback can range of 20 Hz to 20 kHz, a bat can hear anywhere between be caused by head trauma, illnesses, or malformation of the 1 kHz to 150 kHz, a range nearly eight times as wide (16). inner ear, as well as more common events, such as repeated While bats can detect lower frequencies, they generally ig- exposure to blaringly loud music at concerts or consistent- nore them, as the lower frequency sounds are of no use to ly listening to an iPods with the earbuds on full blast (13). echolocation (17). In order for echolocation to work, bats Another shortcoming of the human ear is the lack of emit a high-pitched, rhythmic sound from their larynx, sound localization. If a sound arrives a few microseconds ear- which reflects off objects and returns to the bat’s ears. When lier in one ear than the other, the sound is recognized to be the sound wave returns, the bat can determine where the coming from the side that hears it first. In general, this prin- object is. A bat’s brain combines normal auditory functions, ciple works better for lower frequency sounds (14). Sound a stopwatch to determine how quickly the sound returns— localization works best when both ears are working optimal- which indicates how far away the object is—and a calcula- ly. With only one functioning ear, it is difficult to determine tor to quickly compute these figures (18). Humans are able the source of a sound. Human ears also have a problem de- to locate which side sounds are on based on which ear the termining how far away a sound source is. The only tool the sound waves hit first; this is similar to how bats can deter- human ear can depend on is loudness and relative estima- mine the location of an object. If an echo hits the bat’s right tion. Low-frequency tones propagate farther than high fre- ear before the left ear, the object must be on the right. How- ever, bats have special folds in their ears that allow them to determine an object’s vertical position. Echoes that reach a bat’s ear from below will hit the fold differently than if the echoes come from above, producing a different sound (18). A bat’s ear can also determine the size of an object by the intensity of the echo—a smaller object reflects a smaller sound wave. Also, like humans, bats can sense how close an object is by the echo’s pitch; a closer object’s echo will have a higher pitch than an object moving further away (18). Like bats, dolphins and toothed whales use echolocation to locate objects when vision is obscured, such as when they are swimming in deep or murky waters. These animals have a frequency range from about 50 kHz to 200 kHz, a maxi- mum even greater than that of bats. The velocity of sound is greater in water than in air, so at a given frequency, the wavelengths of sound waves are longer (v = f λ). As a result, aquatic animals have to use frequencies five times greater than those of bats (17). "The echoes from these sounds pro- vide information about the seafloor, the shorelines, underwater obstacles, water depth, and the presence of other animals underwater… giving these animals a three-dimensional view of the world." (19). However, unlike bats and humans, dolphins and toothed whales receive these echoing waves in the oil filled channels of their lower jaws as opposed to directly in their ear drums (19). We would not be able to process such high frequencies without bursting our eardrums, so it is safe to say that hearing in the animal kingdom has surpassed human hearing capabilities, though we do have the ability to hear frequencies lower than some animals.

Image retrieved from http://www.britannica.com/EBchecked/topic/175622/human-ear (Accessed 2 November 2011). Fig. 2: Structure of the middle ear.

FALL 2011 37 (22). Cochlear implants work best in those who have tried Analysis of Technological Alternatives hearing aids with no success, those who lost hearing after Currently, the greatest advances in improving hearing developing speech, and those with severe hearing loss (22). in humans have been hearing aids and cochlear implants. A While this device does not make sounds similar to the way hearing aid is an object that can be worn in the ear (ITE) or normal ear would hear them, it still has beneficial results. behind the ear (BTE). Hearing aids amplify certain sounds so that the person wearing the aid can “listen, communicate, and participate in daily activities” (20). A hearing aid has three Proposal for Improved Design basic parts: a microphone, an amplifier, and a speaker, which Blind individuals would greatly benefit from echolo- convert sound waves to electrical signals, increase the power cation. In order to advance human hearing beyond its nor- of the signal, and transmit the signal to the ear, respectively mal range of 20 Hz to 20 kHz, some type of implant will be (20). Undamaged hair cells can detect the larger vibrations necessary. This device, potentially called a “frequency am- and convert them into neural signals that are passed along to plification device,” is a cross between a hearing aid anda the brain. However, with increased hair cell damage comes cochlear implant. Cochlear implants are beneficial because more severe hearing loss; this requires greater amplification they are embedded deep into the ear, thus promoting bet- to compensate (20). If the damage is too severe, the signal- ter hearing than a hearing aid. Cochlear implants are more ing may not work, causing the hearing aid to be ineffective. advanced than hearing aids because they have one or more Instead, a cochlear implant would be much more effective. microphones that can pick up the sound in the environment, According to the National Institute on Deafness and a speech processor that organizes and processes speech, a other Communication Disorders (NIDCD), the cochlear transmitter that transmits to an inner device, and a receiver implant “goes around the dead hair cells that can no lon- on the inside of the body. On the other hand, hearing aids, ger transmit sound, and directly stimulates the auditory particularly ITE aids, are easier to conceal, are often invisible nerve, which takes the sound signals to the brain” (21). Co- to others, and can be removed. Hearing aids amplify sound chlear implants have internal and external components; for the hard of hearing. This frequency amplification device externally, they are composed of a microphone, a speech will have the microphone, processor, and transmitter from processor, and a transmitter (22). Like the hearing aid, the cochlear implant combined with the ease of ITE hearing the microphone picks up sounds, while the speech proces- aids. Inside the ear, there would be a device that is similar to sor analyzes sounds and sends them to the transmitter. In- the receiver of the cochlear implant. The curled up tip would ternally, a cochlear implant has a receiver and electrodes, be surgically implanted into the cochlea of the patient. On the both of which are implanted surgically. “The receiver takes outside, the ITE hearing aid would come into play. It would the coded electrical signals from the transmitter and deliv- have microphones that are able to pick up frequencies above ers them to the array of electrodes that have been surgically 20 kHz–normally unheard by the human ear—as well as nor- inserted in the cochlea. The electrodes stimulate the fibers mal speech without making sounds louder. Sounds would of the auditory nerve, and sound sensations are perceived” not be amplified, but rather the device would simply expand

Image retrieved from http://www.britannica.com/EBchecked/topic/175622/human-ear (Accessed 2 November 2011). Fig. 3: Analysis of sound frequencies in the cochlea.

38 Dartmouth Undergraduate Journal of Science Image retrieved from http://commons.wikimedia.org/wiki/File:Chiroptera_ echolocation.svg (Accessed 6 November 2011). Fig. 4: Bats emit sound waves to locate their prey (E = emitted wave, R = reflected wave). our normal range of frequencies we can perceive. The device References: would also have processors to cancel out any interference. 1. World Health Organization: Deafness and Hearing Impairment (2010). This type of processor is similar to a cellular phone, which has Available at http://www.who.int/mediacentre/factsheets/fs300/en/index. background noise cancelling technology. The outer ear de- html (20 April 2011). vice would also have a transmitter that would connect to the 2. E. Schwitzgebel, Human Echolocation (2007). Available at http:// receiver on the inner ear by a magnet. The ITE device, which schwitzsplinters.blogspot.com/2007/01/human-echolocation.html (20 April 2011). would be battery operated, can be taken out and charged, 3. Children’s Hospital of Pitssburgh, Anatomy and Physiology of the Ear allowing its user to shower and sleep without any problems. (2008). Available at http://www.chp.edu/CHP/P02025 (20 April 2011). Bats, dolphins, and toothed whales emit high fre- 4. The Ear (Human Anatomy): Picture, Function, Definition, Conditions, quency sounds that the human ear cannot detect. These and More (2009). Available at http://www.webmd.com/brain/picture-of-the- ear (20 April 2011). sound waves bounce off objects and back to the animal, 5. Encyclopedia Britannica Online, Human Ear: Cochlea (2011). Available which determines where the objects are located based on at http://www.britannica.com/EBchecked/topic/175622/human-ear/65040/ the sound waves. In order for this high frequency device Cochlea?anchor=ref531841 (20 April 2011). to work, there needs to be something that emits a high fre- 6. The Middle Ear (2011). Available at http://www.hear-it.org/The-middle- ear-1 (25 October 2011). quency for the frequency amplification device to pick up. A 7. The Ear - A Magnificent Organ (2011). Available at http://www.hear-it. small device, which has the sole purpose of emitting a high org/index.dsp?page=4753 (20 April 2011). frequency sound that is undetected by normal human ears, 8. The Inner Ear (2011). Available at http://www.hear-it.org/The-inner- would be small enough that it would be put in a pocket or ear-1 (25 October 2011). 9. Presbycusis (1997). Available at http://www.nidcd.nih.gov/health/ purse, or even carried around on the belt like a pager. This hearing/presbycusis.htm (20 April 2011). can be charged like the ITE portion of the frequency am- 10. Sensitivity of Human Ear (nd). Available at http://hyperphysics.phy- plification device. After implantation, the patient could re- astr.gsu.edu/hbase/sound/earsens.html (20 April 2011). ceive assistance and training to learn to use their device. 11. G. Elert, Frequency Range of Human Hearing (2004). Available at http://hypertextbook.com/facts/2003/ChrisDAmbrose.shtml (20 April There are many advantages and benefits to this fre- 2011). quency amplification device. First, it would allow the blind 12. Hearing Loss Overview (2011). Available at http://www.deafaccess. to learn to “see” objects by hearing them rather than feelin org/hearing-loss-overview.htm (20 April 2011). them. Not only would this make moving around much more 13. Sensorineural Hearing Loss (2011). Available at http://www.asha.org/ public/hearing/Sensorineural-Hearing-Loss (20 April 2011). effortless, but it would also help the person blend into pub- 14. Encyclopedia Britannica Online, Human Ear Anatomy (2011). lic much easier. 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