Cochlear response to auditory inputs

You do not hear with your ears...You hear with your brain. You hear in the center of your head and it has nothing to do with your ears. The ears are simply the way you 'encode' (and process) the signal.

Cochlear Response to Auditory Inputs

Hearing instruments don't change your hearing; hearing instruments CHANGE THE SOUND. That is, whatever residual hearing the patient has can be more efficient in utilizing that signal.


We live in an information age. We live at a time when the critical survival skill is communicative efficiency. How well can we interact with the world communicatively? How efficient are we in doing that? It's THE SURVIVAL SKILL. If we cannot communicate successfully, the only alternatives are belligerent denial or meek withdrawal.


People survive and succeed in this world communicatively, by virtue of being able to process in the center of their head, not with either ear alone. Our job is to find a way to make that happen for them. The smarter we get, the better they do.


Do you know the first thing that “goes away” when one or the other partner gets a hearing loss, the first thing that disappears?


Trivial conversation! The non-message conversations, the little jokes, the little asides, the little comments, the little things, the double entendre, the sexuality, all that they had going for them, stops! It becomes too hard. It's just not worth it. Nothing gets said that doesn't have to be said.


Those with hearing loss, when not wearing hearing instruments, say “HUH, what'd you say, I'm sorry?” If that behavior happens long enough, you end up on the receiving end of 'telegraphic communications' which are subjects, verbs, and objects, only. Anything more than that is just too much effort!


Do you know who the first people are that begin to decide it isn't worth it, they don’t make a conscious decision, but begin NOT to communicate, unless it's really important?--THE LITTLE CHILDREN. They just back away. They don't think about doing it.


Human Cochlea

The Cochlea in humans (primates) is (among other things) an “organic battery”. It is an “electric machine", and cochlear fluids are not different (in function) from the electrolytes in the battery of your car.


If you change the ion concentration of the electrolytes in your car battery, the battery is less efficient, and therefore, it doesn't work as well. That's called the loss of electrical efficiency. If you change the ion concentration of the fluids in the cochlea of humans, its electrolytes don't work as well. The electrical potential is not as great, and, therefore, the inner ear is not as efficient.


Perhaps you lose the efficiency because outer rows of hair cells have been damaged. Maybe they're damaged as a result of exposure to extreme pressure waves from loud sound. Maybe they're damaged because disease has created a high fever that causes the outer hair cells to be damaged, destroyed, or made less efficient. Or, perhaps the, electrical potential of the electrolytes are changed (ototoxic drugs).



We call a loss of efficiency in (the organic battery of) your ear, sensorineural hearing loss. That's how we define it. When the efficiency of the system goes down, and there is a pure tone sensitivity shift, it defines hearing loss.


An ototoxic drug example might be a commonly prescribed medication for patients with congestive heart disease. Furosemide (Lasix) (or one of the other powerful diuretics).


A diuretic, in addition to reducing the fluid pressure levels in the body, leaches potassium out of the system. Potassium depletion can cause significant problems with muscles (including the heart) as well as producing a loss of efficiency in the inner ear function relating to electrolyte balance. Even with potassium replacement medications, the critical electrolyte balance is seldom attained.


These patients may acquire a hearing loss. That means, if you put a hearing instrument on a patient and that patient is taking Lasix every day, you CANNOT wait two years to retest. It means you ought to be testing any patient who is on Lasix, every 6 months--minimum!


There are three rows of outer hair cells and one row of inner hair cells. They are critically important for the following reason, it takes all four rows to hear from the top of the audiogram (from -10dB HL), down to the bottom. You've got to have all four rows of hair cells.


The outer hair cells are responsible from the top of the audiogram, down to about 60dB HL. The inner hair cells are responsible from about 60dB HL to the bottom of the audiogram.



For example, if you lose the outer most row of outer hair cells, you lose about 20dB of sensitivity. If you lose the first two rows, the outer and middle rows of outer hair cells, you lose about 40dB of sensitivity. If you lose all three outer hair cell rows, you lose sensitivity down to about 60dB.


Each inner hair cell has its own nerve fiber. It is responsible for frequency specificity all the way around the 2 3/4 turns of the cochlea, from the very highest frequencies (down at the bottom, next to the oval window membrane), clear up to the very lowest frequencies (at the apex - bordering the helicotrema).



The drawing showing the three outer and one inner cell represents a discrete place along the cochlear turns and, therefore, a single frequency. If you go to any place else up in this 2 3/4 turns of that cochlea, we get a different place, and, therefore, a different frequency.


There are three places in the auditory system in which you have a set of cells that are responsible for the ability to perceive a single frequency. They are:1. Cochlea2. Cochlear Nucleus3. Primary auditory area in the temporal lobe


Yes, there are two other places responsible for every frequency you hear.How do you understand what you hear? How do you make meaning out of it? How do you incorporate it with something else and decode the totality of the message?


We know that the 8th Cranial Nerve comes out of the inner ear and enters the brainstem. When the 8th nerve goes into the brain stem, it divides (bifurcates). Part of it goes to the dorsal, and part goes to the ventral, cochlea nucleus. In that cochlea nucleus there is a set of cells that are (ALSO) responsible for the processing of the same frequencies as are arrayed in the cochlea.



There are cells in the

cochlea nuclei that are responsible for the same frequency resolution as occurs in the cochlea.

They are called isomorphic representations.


There is also the primary auditory area. It is down inside the Sylvian fissure on the temporal lobe (wrapped around it is the auditory association area).The low frequencies are, in fact, decoded in the anterior part of the primary auditory area and the high frequencies are decoded in the posterior part.



The brain can only store information on its outside surface. How much outside surface is there? What happens when you fill the available outside edge? The brain makes more outside surface. It makes more outside surface by folding in upon itself forming gyri (gyruses), thus producing more surface in the same space.


The brain is incredible in its ability to be, both plastic and to decode, but it is NOT elastic. It is enormously plastic however, in its capacity. If you dent it, it stays dented. If you crush it, it stays crushed. And, it always crushes from the outside in (remember that the brain stores information on its outside surface).


If this primary auditory area of the brain is where damage is localized, you are going to have significant problems. It can happen from the “outside in” with a head injury or a brain trauma. You could be born with it, a birth injury.


If the blood vessels in the brain erupt, this causes swelling. Swelling pushes the surface against the skull, and pushing the surface against the skull is exactly like pushing the skull into the surface. The surface of the brain “crushes” the primary auditory area—creating central hearing loss.


Regardless of where lesions may be, the critical thing is to improve word recognition and therefore, communicative skill. To improve word recognition we are looking for binaural summation. It is, truly, NOT about ears—summation occurs within the brain!


We will discuss more regarding monaural vs. binaural auditory stimulation and its effects upon the brain tomorrow.


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Cochlear response to auditory inputs