METHODS OF DETECTION HEARING

KURSK STATE MEDICAL UNIVERSITYDEPARTMENT OF NORMAL PHYSIOLOGY

Student: Guilherme L.PaschoaliniGroup 29 – 2 year

KURSK - 2014

ABSOLUTE THRESHOLD OF HEARING

The absolute threshold of hearing (ATH) is the minimum sound level of a pure tone that an average ear with normal hearing can hear with no other sound present.

This is also known as the auditory threshold

The human range is commonly given as 20 to 20,000 Hz, though there is considerable variation between individuals, especially at high frequencies, and a gradual decline with age is considered normal.

Several animal species are able to hear frequencies well beyond the human range. Some dolphins and bats, for example, can hear frequencies in excess of 100 kHz.

MEASUREMENT OF HEARING RANGE A basic measure of hearing is afforded by

an AUDIOGRAM, WEBER TEST, RINNE TEST, TYMPANOMETRY and ACOUSTIC REFLEX TEST

Behavioural hearing tests or physiological tests can be used to find hearing thresholds of humans and other animals. Through these, the perception of a physical stimulus (sound) and our psychological response to the sound is measured

AUDIOGRAM An audiogram is a graph that shows the audible threshold for

standardized frequencies as measured by an audiometer,  and this allows different frequencies to be presented to the subject, usually over calibrated headphones, at any specified level. It is necessary for the examination that the test person is pressing a button in the moment he can perceive the signal

The Y axis represents intensity measured in decibels and the X axis represents frequency measured in Hertz.

 The threshold of hearing is plotted relative to a standardised curve that represents 'normal' hearing, in dB

Example of an audiogram of a normal

hearing person

 Example for a person who is hearing impaired

WEBER TESTE The Weber test is a quick screening test for hearing. It can

detect unilateral (one-sided) conductive hearing loss (middle ear hearing loss) and unilateral sensorineural hearing loss (inner ear hearing loss).

In the Weber test a vibrating tuning fork is placed in the middle of the forehead, above the upper lip under the nose over the teeth, or on top of the head equidistant from the patient's ears on top of thin skin in contact with the bone. The patient is asked to report in which ear the sound is heard louder.

A normal weber test has a patient reporting the sound heard equally in both sides. In an affected patient, if the defective ear hears the

Weber tuning fork louder, the finding indicates a conductive hearing loss in the defective ear.

RINNE TEST A Rinne test should always be accompanied by a Weber test to also

detect sensorineural hearing loss and thus confirm the nature of hearing loss.

primarily for evaluating loss of hearing in one ear (unilateral hearing loss). It compares perception of sounds transmitted by air conduction to those transmitted by bone conduction through the mastoid Thus, one can quickly screen for the presence of conductive hearing loss

The Rinne test is performed by placing a low frequency (512 Hz) vibrating tuning fork against the patient's mastoid bone and asking the patient to tell you when the sound is no longer heard. Once they signal they can't hear it, quickly position the still vibrating tuning fork 1–2 cm from the auditory canal, and again ask the patient to tell you if they are able to hear the tuning fork.

TYMPANOMETRY AND ACOUSTIC REFLEX TEST

Tympanometry is an examination used to test the condition of the middle ear and mobility of the eardrum (tympanic membrane) and the conduction bones by creating variations of air pressure in the ear canal. Tympanometry is an objective test of middle-ear function. It is not a hearing test, but rather a measure of energy transmission through the middle ear

A tone of 226 Hz is generated by the tympanometer into the ear canal, where the sound strikes the tympanic membrane, causing vibration of the middle ear, which in turn results in the conscious perception of hearing. Some of this sound is reflected back and picked up by the instrument. Most middle ear problems result in stiffening of the middle ear, which causes more of the sound to be reflected back.

Admittance is how energy is transmitted through the middle ear. The instrument measures the reflected sound and expresses it as an admittance or compliance, plotting the results on a chart known as a tympanogram.

Normally, the air pressure in the ear canal is the same as ambient pressure. Also, under normal conditions, the air pressure in the middle ear is approximately the same as ambient pressure. In a healthy individual, the maximum sound is transmitted through the middle ear when the ambient air pressure in the ear canal is equal to the pressure in the middle ear.

The acoustic reflex (also known as the stapedius reflex, attenuation reflex, or auditory reflex) is an involuntary muscle contraction that occurs in the middle ear of mammals in response to high-intensity sound stimuli

When presented with a high-intensity sound stimulus, the stapedius and tensor tympani muscles of the ossicles contract. The stapedius stiffens the ossicular chain by pulling the stapes (stirrup) of the middle ear away from the oval window of the cochlea and the tensor tympani muscle stiffens the ossicular chain by loading the eardrum when it pulls the malleus (hammer) in toward the middle ear

PSYCHOPHYSICAL METHODS FOR MEASURING THRESHOLDS

There are several different psychophysical methods which can be used for the measurement of absolute threshold. Firstly, the stimulus is defined, and the manner by which the person should respond is clearly specified. The sound is then presented to the listener and the level of the stimulus is manipulated in a predetermined pattern.

Some procedures use a series of trials, with each trial using the ‘single-interval “yes”/”no” paradigm’. This means that sound may be present or absent in the single interval, and the listener has to say whether he thought the stimulus was there. When the interval does not contain a stimulus, it is called a "catch trial"

PSYCHOPHYSICAL METHODS FOR MEASURING THRESHOLDS

Classical methods - Three methods are traditionally used for testing a subject's perception of a stimulus: the method of limits, the method of constant stimuli, and the method of adjustment

Method of limits - There are several series of descending and ascending runs. The trial starts with the descending run, where a stimulus is presented at a level well above the expected threshold. When the subject responds correctly to the stimulus, the level of intensity of the sound is decreased by a specific amount and presented again. The same pattern is repeated until the subject stops responding to the stimuli, at which point the descending run is finished.

Method of constant stimuli - In the method of constant stimuli, the tester sets the level of stimuli and presents them at completely random order. The stimuli are presented many times at each level and the threshold is defined as the stimulus level at which the subject scored 50% correct. “Catch” trials may be included in this method.

Method of adjustment - Method of adjustment shares some features with the method of limits, but differs in others. There are descending and ascending runs and the listener knows that the stimulus is always present. However, unlike in the method of limits, here the stimulus is controlled by the listener. The subject reduces the level of the tone until it cannot be detected anymore, or increases until it can be heard again.

Adaptive methods - Unlike the classical methods, where the pattern for changing the stimuli is preset, in adaptive methods the subject's response to the previous stimuli determines the level at which a subsequent stimulus is presented

Staircase’ methods (up-down methods) - The simple ‘1-down-1-up’ method consists of series of descending and ascending trials runs and turning points (reversals). The stimulus level is increased if the subject does not respond and decreased when a response occurs.Similarly, as in the method of limits, the stimuli are adjusted in predetermined steps.

Bekesy's tracking method - Bekesy's method contains some aspects of classical methods and staircase methods. The level of the stimulus is automatically varied at a fixed rate. The subject is asked to press a button when the stimulus is detectable.