8 Hearing and Balance

The objectives for this lecture were to understand the function of the inner ear in both hearing and balance, to describe how one sense can be re-routed to detect another (“sensory substitution”), and to critically evaluate synaptic plasticity in sensation and perception.

The vestibular system

The vestibular system provides sensory information about how the head is moving. The slide’s drawing shows a head with three axes through it and names the rotation around each one:

• roll: rotation around the axis, which runs from front to back through the face;
• pitch: rotation around the axis, which runs from ear to ear;
• yaw: rotation around the axis, which runs from the top of the head down.

The inner ear

The inner ear has two functions:

1. hearing;
2. the vestibular sense (balance and orientation).

The slide’s drawing is a cutaway of the ear in three parts: the outer ear (the visible ear and the external auditory canal), the middle ear, and the inner ear, which sits deep in the bone of the skull and is outlined on the slide. The inner ear holds the looping semicircular canals, the coiled cochlea, and the nerves that leave them.

The semicircular canals

The semicircular canals are the vestibular sensory organ. The slide’s drawing of the inner ear divides it in two. The vestibular system is the three semicircular canals (labeled superior, posterior and horizontal) and the chambers at their base; the coiled cochlea is for hearing. A nerve carries signals from both to the brain.

The semicircular canals are filled with fluid, with one canal per orientation (, , ). As the lecturer put it, there are basically just three tubes of fluid in the inner ear, one for each of the three axes we can turn around: we move, and the fluid splashes around in the corresponding tube.

Hair cells

Hair cells are the sensory cells of the vestibular system. They are located inside the semicircular canals and detect movement of the fluid.

Neurons come in different “flavors,” and the slide lines a hair cell up against two familiar ones: a bipolar neuron (one dendrite, a cell body and an axon) and a multipolar neuron with a myelinated axon. The hair cell has the same parts, laid out differently. Its hair-like projections stand in the place of the dendrites, its body (with a stiff cuticular plate along the top and a nucleus inside) is the cell body, and its base, where it passes its signal on, stands in the place of the axon. Strictly, hair cells are specialized receptor cells: a hair cell has no axon of its own and passes its signal across a synapse to a neuron of the vestibular nerve, which carries it to the brain.

How hair cells are activated

Hair cells are activated by physical movement in a salt solution. The fluid in the canals is a salt solution, and the hair-like projections are physically attached to ion channels: a fine link runs from the tip of each projection to a channel in its taller neighbor. When the fluid moves, it bends the projections. Depending on the direction of tilt, the movement leads to depolarization or hyperpolarization:

• tilted toward the tallest projection, the links pull the channels open, positive ions flow into the cell, and the hair cell depolarizes (the same direction of change as at the start of an action potential, when Na enters a neuron);
• tilted the other way, the channels close and the hair cell hyperpolarizes.

The depolarized hair cell then passes the signal on to the vestibular nerve, whose neurons fire action potentials to the brain. So when fluid sloshes over these cells, signals go out to the brain.

The vestibular pathway

The neural pathway for the vestibular sense runs:

The slide marks the medulla and the cerebellum on a photograph of a brain cut down the middle: the medulla is the lowest part of the brainstem, just above the spinal cord, and the cerebellum is the ridged structure behind it. The lecturer said this pathway doesn’t need to be memorized; it is there because it becomes relevant later in the lecture (in sensory substitution).

Off balance

Why do we feel motion sickness, airsickness and seasickness? The slide asks the question over pictures of a looping roller coaster, a sailboat on a rough sea, and a road over rolling hills. The lecturer’s note on this slide just says “splishy splashy ear bath”: the fluid in the canals gets sloshed around.

Sensory substitution

Paul Bach-y-Rita

Paul Bach-y-Rita (1934–2006) was, among other things, a factory worker, a fisherman and a masseur, and he attended medical school on a dare. He is recognized as the pioneer of sensory substitution. In his words: “We see with our brains, not with our eyes.”

Vision through the skin (1969)

Bach-y-Rita’s paper “Vision Substitution by Tactile Image Projection” appeared in Nature (vol. 221, March 8, 1969). Its first sentence: “We describe here a vision substitution system which is being developed as a practical aid for the blind and as a means of studying the processing of afferent information in the central nervous system.” (Afferent information is incoming sensory information.)

The slide’s photo shows the apparatus: a modified dental chair with a TV camera on a stand beside it and a panel of stimulators in the backrest. The camera’s image was turned into a pattern of vibrations on the person’s back, each stimulator acting as one pixel. A figure from the paper shows a 400-point representation of a woman’s face as it appeared on the monitor (a coarse, blurry grid of light and dark dots), next to a photograph of the face itself. The caption says that subjects could correctly identify vibrotactile stimulus patterns (patterns of vibration felt on the skin) of this level of complexity; it also notes that the blurry image on the monitor is easier to make out if you squint. In the lecturer’s summary, he put an image onto someone’s back with the stimulators as pixels, and it worked reasonably well.

Balance through the tongue

The vision-chair idea was later applied to balance. The device stimulates the tongue electrically to provide vestibular information: it “tingles” the direction of a person’s tilt onto the tongue. It takes some practice to use, but it can give feedback about a person’s balance from a pocket-sized device.

The slide’s photo shows the device (its label reads “BrainPort Balance Device”). A small intra-oral device (IOD) sits on the tongue; it has an electrode array on one side and an accelerometer (a sensor of tilt and movement) on the other. A cable carrying the circuitry connects it to a handheld unit that holds the CPU, battery and electronics, with a power button and a voltage control.

The electrode array is a grid of electrodes on the tongue, and where the stimulation lands on the grid tells the user how the head is tilted. With the head upright the stimulation sits in the center (position 2). Tilting the head to one side or the other moves it to the left or right edge (positions 1 and 3), tilting back moves it toward the back of the tongue (position 4), and tilting forward moves it toward the tip (position 5).

Why it works: shared pathways

The pathways for taste and for balance are different, but they share some architecture: both run through the medulla.

• Taste/touch: tongue medulla (solitary nucleus) thalamus, cortex, hypothalamus and amygdala taste/touch.
• Balance: inner ear medulla (vestibular nucleus) cerebellum and the eye and head muscles balance.

When the inner ear is damaged, the link from the inner ear to the vestibular nucleus is lost (the slide crosses it out). Through synaptic plasticity, some synapses in the shared location, the medulla, are rearranged so that information from the tongue reaches the vestibular nucleus, and from there the areas that process balance. In this sense the tongue substitutes for the semicircular canals.

“Seeing” with your tongue

The tongue can also be used to see, to a degree. The slide’s diagram (sources: Mitchel Tyler, Yuri Danilov, Wicab Inc.) follows the signal from the tongue to the brain. Electronic aids send impulses to a sensitive body part like the tongue, and the brain interprets them, enabling a blind person to “see” objects.

1. A camera on the forehead captures the image of a test symbol (an X).
2. The video output is sent to a processor.
3. The processor translates the camera’s output into a pattern of electronic pulses, which are sent to an array of electrodes held against the tongue.
4. The array, a little over an inch square, stimulates receptor cells on the surface of the tongue.

Tactile (“touch”) receptors on the tongue send impulses to the somatosensory cortex in response to the stimulation. The impulses are then relayed to areas of the cortex that process images, the visual cortex and the visual association cortex. In the diagram’s account, the images are analyzed and sent to the prefrontal cortex, where they are recognized, and the person perceives the object (the X).

A video (linked on the slide) shows the device in use.

Hearing: the sound map

Hearing was a quick section of the lecture. The cochlea contains hair cells that respond selectively to particular frequencies of sound, and they are neatly organized by tone. The slides call this “tonographical” organization; the standard term is tonotopic. Activation of different cells by different tones builds up our sense of hearing.

The slide’s drawing shows the cochlea “uncoiled” into a straight tube, with the stapes (a middle-ear bone) sitting on the oval window at the cochlear base. Seven positions are marked along the tube. The high-frequency response is near the base (position 2); the low-frequency response is at the far end, the apex (position 7).

The auditory cortex is tonotopically organized too: the sound map is maintained from the cochlea to the cortex. The slide shows the primary auditory cortex on the temporal lobe, surrounded by secondary auditory cortex (the “belt areas”). Across the primary auditory cortex lie bands responding to 500, 1000, 2000, 4000, 8000 and 16,000 Hz; the 500 Hz end corresponds to the apex of the cochlea and the 16,000 Hz end to its base.

Because the same map is kept all the way up, a device that delivers the right frequencies to the right places along the pathway can stand in for lost hair cells. This is what a cochlear implant does.

Hearing loss and cochlear implants

There are two types of hearing loss:

1. Conductive hearing loss: damage to the parts of the ear that conduct sound into the inner ear (the outer and middle ear).
2. Sensorineural hearing loss: damage to the hair cells.

Cochlear implants work in three steps:

1. A small microphone on the headpiece (worn behind the ear) picks up sounds.
2. The sounds are converted to electrical signals by the implantable cochlear stimulator and fed into the cochlea.
3. An electrode array threaded through the round window into the cochlea stimulates the auditory nerve directly, bypassing the damaged hair cells.

So an implant helps with sensorineural hearing loss: it does the damaged hair cells’ job.

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