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Biology 202
2004 First Web Paper
On Serendip

Cochlear Implants: A Bionic Sensory Experience?

Lindsey Dolich

"Hearing is the soul of knowledge and information of a high order. To be cut off from hearing is to be isolated indeed" (1).

What does it mean to hear? Imagine what it may be like if your perception and recognition of sound has changed three times during your lifespan. Phases one, two and three encompass a full spectrum of hearing, with various technological aids (in phases two and three) triggering a range of psychological and physiological repercussions. An in-depth look at the relationship between the hearing organ and the auditory processing center of the brain might illuminate hearing as a integration of audition and cognition. As someone who has experienced full hearing, deafness and rehabilitated hearing via an electronic prosthesis, how do my experiences contribute to the notion of a personalized auditory experience—an awareness which draws the distinction between the sensation and interpretation of sounds?

The ear contains complex organs which allow for sound to be converted into an electronic signal, which is transmitted to the brain for interpretation. The mechanical input of sound waves is transduced in the cochlea into an electrical response. The basilar membrane in the cochlea vibrates from the movement of the surrounding perilymph, which bends the hair cells, inducing depolarization and triggering an action potential. The ganglion cells that innervate the hair cells within the organ of Corti serve as receptors, and they are responsive to particular frequencies according to topographical (tonotopic) organization. The auditory nerve connects to the brain stem (a bilateral pathway), which synapses into the cochlear nucleus. Here, the information is separated into the ventral cochlear nucleus (time-sensitive localization) and the dorsal cochlear nucleus (quality) (2). The auditory pathway projects into the cerebral cortex, specifically the primary auditory complex located on the dorsal surface of the temporal lobe (3). Furthermore, these auditory nuclei project into other parts of the brain that constitute a neural net—a schema that allows for the functional organization of language, music, memory and knowledge (4).

Hearing loss may be caused by the destruction/degeneration of hair cells in the cochlea (sensorineural), or by damage to/malformation of the apparatus that transmits sound energy (conductive) (5). Hearing aids are one corrective device used to amplify sound; however, the cochlear implant is a fairly new innovation that targets sensorineural hearing loss by bypassing the damaged cochlea. This instrument entails an external microphone built into a speech processor which acts as acts a spectrum analyzer, deconstructing complex sounds into certain frequencies. These electrical signals are then carried to a transmitter held to the head which conveys the coded information through the skin to a receiver implanted in the bone (5). The simulator relays the signal down an electrode that is wound through the cochlea, activating specific frequency locations that coincide with the tonotopic organization of the auditory nerves. The implant "mimics" a sound by stimulating the corresponding neurons, producing the "sensation" of hearing. Subsequently, the cochlear implant is a controversial device that raises ethical questions—what does it mean to replace our "natural" senses with an artificial sensory experience through electronics?

As a unilateral cochlear implant recipient, I have come to think of my CI as an extension of myself—without it I feel helpless and vulnerable. In my personal experience as a two-year user of this fairly new medical device, the cochlear implant may be relevant in the exploration of the cognitive aspect of the auditory process. I was born with full, normal hearing—however, between the ages of three and four, a congenital birth defect (Large Vestibular Aqueduct Syndrome) resulted in a bilateral sensorineural hearing loss that left me profoundly deaf. Hearing aids boosted what little hearing I had until the age of 18. As time passed, I began to notice that I was struggling more than I used to with my hearing aids. My observations were confirmed: my discrimination (the ability to make sense of what I heard) had been declining as a result of reduced stimulation to my auditory nerve cells. This phenomenon is common to those who are born hearing, later suffering hearing loss. The neural pathways of my auditory memory did not disappear, but they failed to sustain my previous recognition of sounds and words. Embarking on the third phase, I hoped to make use of my vestigial sensory ability by getting a cochlear implant, which would directly trigger and invigorate the ganglion cells connected to the auditory nerve. Ironically, the implantation meant destroying any residual hearing function left in my hair cells. Having to undergo a third adjustment to my hearing, relearning sound artificially stimulated was incredibly different and frustrating.

The auditory neural net in my brain continues to be reorganized and reshaped to this day, in order to adjust to an entirely different sensation of sound. An electrical perception translates into an altered recognition/interpretation of sound, in the sense that my familiarity "stemming from a contact between an external event and an internal reception of a previous experience of that event" is rendered inadequate. My success from the implant is likely because I am able to draw not on the audition (the activities of the hearing organ proper, the actual stimulus) enabled at birth, but the experience of auditory processing that encompasses cognition. By cognition, I refer to Reiner Plomp's definition as "the top-down processing stressing the significance of concepts, expectations, and memory as a context for stimulus perception" (6). However, this definition raises certain tensions as I often felt that I was starting from scratch, having to practice auditory memory, language production and processing and interpretation. I continue to think of hearing as an active experience, or an acquired knowledge in which I file away every new sound I hear, attaching labels such as "train whistle," "bird song" or "shhhh" rather than remembering what I heard naturally 17 years ago. Indeed, this suggests that hearing does not rely on the simple stimulating of specific neurons contained in the auditory cortex; rather, hearing is a holistic "exercise" that involves conscious and unconscious extrapolations that construct sound as a subjective perception. Our interpretation of sounds must mediate automatic and voluntary processing (4). I often confuse one sound for another—for example, my brain may automatically mistake a train for music, but upon visual cues I will voluntarily recognize otherwise.

My cochlear implant experience as well as those of thousands of others validates the plasticity of the brain. I no longer rely on 90 percent of my eyesight for information; rather I have come to recognize specific voices, music, sounds. I still lack the ability to localize and pick out sounds from a noisy environment as a result of unilateral hearing. Do I need more time to develop this ability or is this a function inherent to my natural audition?

With the exponential progression of scientific advancement, fully implantable implants are on the horizon, in contention with hair cell regeneration research. Edmond Alexander raises the fascinating issue of the "coming merging of mind and machine," in which the biological authenticity of the human brain may be undermined by the artificial, reverse engineering of the brain which will be enhanced and expanded (7). If I still have a brain that contains a bionic device substituting for my hearing, does that mean that my behavior is still "human?" To what degree can we make the distinction between human and machine? Will the brain still equal behavior if electronic devices are responsible for our sensory experiences?

1)Helen Keller Quotations

2)Auditory Transduction

3) Kandel, Schwartz and Jessel. The Principles of Neural Science. MgGraw Hill Companies, 2000

4) McAdams, Stephen and Bigand, Emmanuel. Thinking in Sound. Oxford: Clarendon Press, 1993.

5)Sound From Silence, The Development of Cochlear Implants ; overview of cochlear implants

6) Reinier, Plomp. The Intelligent Ear: On the Nature of Sound Perception. New Jersey: Lawrence Erlbaum Associates, Publishers, 2002.

7) Edmond Alexander. "The Coming Merging of Mind and Machine." Scientific American Inc, 1999.

Other Helpful Sources:

8)Turned On ; another personal account from a Cochlear Implant recipient

9)Introduction to Cochlear Implants

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