Am I Not Just Hearing What Surrounds Me?

Our brains and spinal columns are washed by fluid circulated by ciliary hairs, and cilia coordinate the embryonic development of our organs. The light receptors in our eyes are modified cilia, the tips of their hairs no longer moving but welcoming light on their protruding arms. News of odors travels to our nerves via cilia that grab aromatic molecules. Our kidneys use cilia to sense, without our conscious awareness, urine flow and to regulate the growth of the kidneys’ network of tubes. We also hear with cilia. As a sound wave flows through the inner ear, its motions deflect these bundles. This movement causes the cells to signal to the nervous system. Physical motion is thus alchemized by cilia into bodily sensation. Outwardly, complex animals seem to have little in common with the cells that swarm through pond scum and ocean water. When we perceive sound or light or aroma, we experience deep kinship, a shared cellular heritage. The cilia in my ears, mounted atop hair cells, are arrayed along a membrane sandwiched between coiled tubes of fluid. These coils, one for each ear, form the cochleas.

To Keep Your Balance, You Must Keep Moving

Each is the size of a fat pea, and they are lodged in the skull just beyond the eardrums. The cochlear membrane is narrow and stiff at the end closest to the eardrum, but wide and floppy at the apex of the coil. Low sounds stimulate the wide part. Every frequency within the range of human hearing thus has a place along the membrane’s gradient of sound sensitivity, as if we had coiled up piano keyboards in our inner ears. Complex patterns of sound, like music or speech, stimulate waves at multiple places along the membrane’s length. Vibrations are picked up by hair cells on the inner part of the membrane, the edge closest to the center of the cochlea’s coil. These signal via the cochlear nerve to the brain. Vigorous sounds have enough energy to buck the cochlear membrane and stimulate inner hair cells. But quieter sounds are too weak. Alone, they cannot trigger nerve impulses. Hair cells on the outer part of the membrane give these softer sound waves a boost so that the inner hair cells can perceive them. Outer hair cells are three times more numerous than those on the inner part of the membrane, underscoring their importance.

Starting All Over Again

When a sound wave of the right frequency hits the outer hair cells, a protein leaps into action, pumping the cells up and down. The up and down motion of the outer hair cells amplifies the wave, turning an anemic shiver into a surge. The magnified wave triggers the waiting inner hair cells. The teamwork of outer and inner hair cells allows us to perceive sound across a millionfold difference in energy levels, from a snowflake falling into a drift in the quiet woods to the clap of thunder echoing in a canyon. What I see on the audiologist’s screen is the activity of my outer hair cells. Normally the cells would pulse with the same frequency as the incoming waves. But the test I’m undergoing throws them into confusion. The two incoming tones are precisely calibrated to hit the membrane very close together and, like two people shaking a rug at slightly different rates, the activated outer hair cells cause the membrane to judder with the weird collision of these two drivers. The third spike on the screen was the squeal of my outer hair cells. At the end of the test, my audiologist clicks at her laptop and the spiking lines disappear, replaced with a graph that shows how my hair cells performed. At low sound frequencies, the cells did fine in both ears. In my right ear, those tuned to higher frequencies have stopped bouncing or have slowed their motion.

Ignore The Signs Of Fatigue

In my left ear, it is those focused on the midranges that have quieted. These inactive cells are not resting or asleep, they’re defunct. Unlike birds that can regrow damaged hair cells, human inner ear cells get one life only. The crystal ball, my audiologist calls this test. For someone in their fifties, my results are unexceptional. In future years, more hair cells will bow out, especially in the higher frequencies. Most of us are born with hale outer hair cells, full of vim all up and down the cochlear membrane. Such is the cost of living in a body richly endowed with sense organs. Our every sensory experience is mediated by cells. Aging is a cellular process. And so to experience the passage of time in an animal body is to experience sensory diminishment. Their body consists of a sac topped by tentacles. Nerves weave through the body in a net, with no brain or complex sense organs. This uncomplicated design, made from a handful of cell types, allows Hydra to regularly purge and replace any defective cells. They live without any signs of aging. We can blame advancing deafness and the other diminishments of age on Faustian forebears. They exchanged ageless bodies for richly sensual lives. I mourn the progressive loss of my hearing. The voices and music of people, birds, and trees give me connection, meaning, and joy. But alongside the sadness, I try to accept and enjoy evolution’s bequest. These diverse voices exist only because our bodies are complex and therefore ephemeral. Our hearing cells and organs not only lock us into a trajectory of aging. They also bias sensory experience. It is not the case that in my youth I had perfect hearing and now I’ve lost some of this transparent connection to the world. Even before my hair cells started dying off, what I heard was highly mediated. Everything that I hear is an imperfect rendering. The inner and outer worlds converse and entangle in my ears. Sound is sound, surely? Am I not just hearing what surrounds me, connected to the world by open ears? This is an illusion. What we perceive is a translation of the world and every translator has special talents, errors, and opinions. Sitting in the clinic, gazing at spikes on a graph, I’m seeing the chatter of my cochlear hair cells. Along every step of the path from external sound to internal perception, our body edits and distorts.