Brad May, PhD Johns Hopkins University
When the ear cannot function normally, the brain changes. Brain deafness contributes to poor speech comprehension, problems listening in noise, abnormal loudness sensitivity, and tinnitus.
Basic mechanisms Shared coding in the brain Selective sound filters Effects of noise Clinical implications Hearing loss and increased noise sensitivity Loudness recruitment Tinnitus
Communication is the transmission of information. Sounds convey information through dynamic changes in frequency (pitch) and level (loudness).
Like a computer, the brain uses a digital code (action potentials). Information is represented by which nerve cells are active.
Collections of neurons in 13 major processing centers separate the ear from auditory cortex. Each center responds best to a different type of information.
Nerve cells encode the loudness of a sound by the rate of their action potentials. The full range of loudness is represented by multiple cells with complementary sensitivity.
Nerve cells also must work together to code the multiple frequency components of complex sounds. Some cells respond best to low frequency components, while others respond best to middle or high frequency components.
The ability to respond to some frequencies while ignoring others is called frequency tuning. The first stage of tuning is determined by the passive vibrations of the cochlear membranes.
The second stage of tuning is determined by amplification of cochlear vibrations. Outer hair cells are the motors that drive the amplification.
The third stage of tuning is determined by neural circuits in the brain. Excitatory connections (+) pass activity from one cell to another.
Inhibitory connections (-) turn off activity in the target cell. Inhibitory cells surround excitatory cells, turning off the response of the target cell at all but a narrow range of frequencies.
Inhibitory connections (-) also influence loudness tuning. Spontaneous activity decreases, threshold increases, and rate changes are less steep.
Filter bandwidth doesn t matter when the auditory signal contains a single narrow frequency component. Simple tonal signals rarely convey information.
Frequency components convey information in communication sounds. Narrowly tuned cells can separate individual frequency components.
Noise can disrupt frequency coding because it contains all frequencies at the same time. Narrowly tuned cells can respond to narrow signals in noise because most of the noise energy falls outside their range of sensitive frequencies.
Background noise interferes with hearing because it constrains the range of discharge rates that are available for sound coding.
The response to noise can be reduced by turning down the sensitivity of the ear. This adjustment is produced by nerve cells that project back from the brain to the cochlear amplifier (outer hair cells).
Sensorineural hearing loss usually involves loss of outer hair cells. Without outer hair cells, the cochlear amplifier is turned off.
Without the cochlear amplifier the ear is: Less sensitive Less tuned No longer controlled by feedback from the brain
When the ear transmits less sound activity to the brain, the brain turns off inhibition.
Without inhibition, the brain becomes more sensitive to sound. The loss of neural frequency tuning makes it harder to listen to meaningful sounds in noisy situations.
The increased steepness of loudness tuning may make quieter sounds uncomfortably loud.
Nerve cells in the brain may become more active in the absence of sound. Brain hyperactivity is thought to be the underlying cause of tinnitus.
Assistive devices It s not only about amplification Auditory filtering, loudness compression Advances in digital signal processing Better batteries Brain deafness It s not only about ear deafness Effects of activity on brain circuitry Preventing changes in the brain may be easier than correcting them