Ears And Eyes – A Common Architecture
Both our sense of hearing and our sense of vision allow us to perceive crucial but very different information about our surroundings. Our eyes relay the location and movement of objects in our visual field, while our ears convey information about the actions of objects both in and outside of our visual field.
Despite the fact that our eyes are designed to sense light, and our ears are equipped to detect sound, there are some remarkable parallels in the way the two organs encode the physical phenomena going on around us. Both sound and light undergo multiple transformations on their journey through the respective organs before being absorbed by the body’s neural pathways.
From the very outset, our ears and eyes gather and shape sound and light respectively. Just as the curved cornea of the eye angles light onto the pupil, so too the ear cartilage and ear canal concentrate sound onto the eardrum.
In the eye, further focus is achieved by the lens, which concentrates light-rays onto the photosensitive retina lining the back of the eye. In the ear, the eardrum and the chain of bones behind the eardrum concentrate and amplify sound waves before passing them on to the vibration sensitive cochlea in the inner ear.
The eye’s retina contains two main types of photoreceptors, known as cones and rods. Cones are sensitive to bright light and different wavelengths of light, permitting colour differentiation in our central vision, while the rods are sensitive to dim light and movement in our peripheral vision.
In a similar fashion, the ear’s cochlea contains two different types of sensory receptors known as inner and outer hair cells. Each type of hair cell contains numerous microscopic hair-like filaments called cilia. These cilia bend in response to sound waves travelling though the fluids of the inner ear. Although the inner hair cells transmit the majority of information about sound to the brain, it is the movements of the outer hair cells which fine tune and amplify/dampen inner hair cell responses.
The differential tuning of the eye’s photoreceptors enables us to detect colour and brightness, while the complementary roles of the cochlea’s inner and outer hair cells relay information about the pitch and volume of sound.
Neither the eye, nor the ear are by any means passive structures. In fact they both respond to brain commands to alter our focus depending on our changing needs. The eye’s iris regulates how much light enters the pupil, and the eardrum has tiny muscles to dampen loud sounds. Both of these fine tuning mechanisms respond at a subconscious level in a fraction of a second. Without such regulatory capacities, the delicate receptors in the retina and cochlea would be vulnerable to damage from physical extremes.
Two points of reference is also a design feature common to both organs. Having two ears and two eyes allows for fine differences at each location to be compared. Comparative analysis of light arriving at each eye permits depth perception, while differences in sound waves at each ear assist with sound localization and noise suppression.
Harnessing information about sound and light is not a mutually exclusive function of the ears and eyes, as these two subsystems cross-reference information for certain critical tasks. Signals from the inner ear’s balance receptors are shared with the visual system to ensure focus is maintained on objects as we move. Similarly, sudden loud sounds elicit a blink response to protect the eyes from whatever is causing the impact.