The auditory system is made up of a series of complex pathways that lead from the outer ear all the way up to the brain.
The auditory system has three main components: the outer ear, the middle ear, and the inner ear. The outer ear includes the pinna, a cartilaginous structure on the side of the head, and the outer ear canal. The middle ear is composed of the tympanic membrane, also known as the eardrum, and the ossicular chain, or the three bones behind the tympanic membrane. Those three bones (the malleus, the incus, and the stapes) are the smallest bones in the human body. The inner ear is composed of the cochlea, the organ of hearing. The cochlea is a small snail shell shaped structure that includes hundreds of thousands of “hair cells” that send signals to the brain.
The pinna funnels sound waves into the ear canal. Those sound waves travel down to the eardrum, which causes the eardrum to vibrate. The fluid in the inner ear moves with the vibrations from the ossicular chain, stimulating the hair cells, which then send the signals to the brain through the auditory nerve.
Hearing loss can occur anywhere in the auditory pathway. If the cause of the hearing loss is the outer and/or middle ear, it is called conductive hearing loss. If the cause is within the inner ear, it is called sensorineural hearing loss. A combination of the two is called mixed hearing loss. To determine what type of hearing loss a person may have, an audiometric evaluation is essential.
What Happens During A Hearing Test?
An audiometric evaluation may include a variety of different tests chosen at the discretion of the audiologist. A comprehensive audiometric evaluation for an adult typically includes, at a minimum: case history, air and bone conduction threshold testing, word recognition tests, and tympanometry.
First, an audiologist should complete a comprehensive case history with the patient. The audiologist should know if the patient has a family history of hearing loss or other auditory conditions. It is important to know if the patient has any preexisting conditions that may impact the auditory system. For example: has the patient had any previous head, neck, or ear surgeries? Do they have a history of chronic middle ear infections? Do they have any ringing, buzzing, or hissing sounds in their ears (tinnitus)? An audiologist should inquire about the individual’s perceived hearing status. Do they feel as if they have difficulty hearing? In which situations do they experience the most difficulty? The audiologist needs to know if the patient has had extensive noise exposure, and if so, whether or not they regularly use hearing protection. A patient should provide a list of medications they are taking so the audiologist will be able to identify any ototoxic medications, or medications that are toxic to the ears. The audiologist should ask follow up questions to the patient’s responses when necessary.
Air and bone conduction audiometry both involve a patient listening to a series of pure tones while the audiologist discovers the individual’s threshold, or the softest they can hear at different frequencies. The human cochlea is able to hear between 20 Hz (very low pitch) and 20,000 Hz (very high pitch). Most hearing tests will evaluate the frequencies that make up spoken language: 250 Hz – 8000 Hz. An audiologist may choose to test a wider range of frequencies at their discretion. Air conduction audiometry is performed with either over the ear headphones, or insert earphones. When the patient hears a tone, they should provide a response to the audiologist to let them know that they detected a sound. A response may include: raising a hand, pressing a button, or saying “yes.”
Similarly, bone conduction audiometry includes finding an individual’s thresholds across a variety of frequencies. Rather than using a pair of headphones or insert earphones, bone conduction testing is completed with a bone conductor. The instrument looks similar to a headband, with the bone conductor on one end and a rest on the other. The bone conductor traditionally rests on the bone behind the ear, called the mastoid process of the temporal bone. A bone conductor allows for the audiologist to test the inner ear directly, bypassing the outer and middle ear.
There are two different types of speech testing commonly used in comprehensive audiometric evaluations. The first is speech recognition threshold testing (SRT). The purpose of SRT is to determine the quietest volume in which a person can accurately repeat words. The second is word recognition testing (WRS). WRS is typically performed at 30 decibels (dB) to 40 dB above the SRT. The purpose is to see how well a person is able to understand and repeat speech at an appropriate volume.
Lastly, tympanometry is routinely included in most comprehensive audiometric evaluations. Tympanometry involves the process of placing a probe tip in the patient’s ear canal and allowing the equipment (a tympanometer) to adjust the pressure in the ear canal. This allows for the assessment of the tympanic membrane and middle ear space behind the eardrum.
Can You Cheat On A Hearing Test?
There are both subjective and objective tests that can be completed during an audiometric evaluation. Subjective tests require the individual to give feedback to determine the results. Objective tests do not require feedback. Subjective tests are influenced by a person’s perspective, while objective tests are not. This means that an individual may be able to cheat during subjective tests, but not during objective tests.
Subjective tests in audiology include: pure tone air conduction audiometry, pure tone bone conduction audiometry, speech recognition tests, and word recognition tests. As described above, air and bone conduction testing require the patient to indicate when they hear the tones being played. Individuals who feign hearing loss may not indicate that they heard the tone until it is presented at a very loud level. Similarly, an individual feigning hearing loss may not repeat words at an appropriate level when participating in SRT and WRS tests.
Objective tests include: tympanometry, acoustic reflexes, otoacoustic emissions, and auditory brainstem response tests.
As described above, tympanometry is an objective test that allows an audiologist to assess a person’s eardrum and middle ear space. Due to the objective nature of this test, tympanometry can be performed on patients of any age, including newborns.
The middle ear space behind the eardrum contains three small bones called the ossicular chain, with muscles and tendons attached. When an individual is exposed to an extremely loud sound, the signal travels to the brain. The brain then sends a signal to a muscle attached to one of the bones in the ossicular chain to contract. The contraction is an attempt to prevent the loud sound from traveling further into the inner ear where the hair cells can be damaged. This series of events is known as the acoustic reflex. The acoustic reflex can be tested by presenting loud tones at specific levels by using a probe placed in the patient’s ear canal. The acoustic reflex test is another objective test that does not require a patient’s feedback to evaluate.
The inner ear contains hundreds of thousands of hair cells. When specific frequencies are presented at specific levels, healthy hair cells will send a response back that can be measured by a small microphone. This is called an otoacoustic emission. This objective test is used commonly to evaluate the health of the inner ear. Otoacoustic emission testing is commonly used in hospitals to screen newborns for hearing loss.
As previously mentioned, the auditory system is made up of complex pathways that lead from the outer ear to the brain. Sound must travel through the ear canal, passing through the eardrum, moving the ossicular chain to stimulate the inner ear, travelling through the auditory nerve to the brainstem, and finally to the auditory cortex in the brain. Auditory brainstem response (ABR) testing is used to evaluate this pathway. During this test, a patient must be relaxed, asleep, or sedated. A series of electrodes are placed on the head and insert earphones are placed in the ear canal. Brainwaves are then measured in response to hearing the sounds. This objective test is helpful when determining if there is a disorder occurring along the nerve. It is also often used in hospitals to screen newborns for hearing loss.
What If You Pass A Test But Still Feel Like You Can’t Hear?
If an individual feels like they may have difficulty hearing after undergoing a comprehensive audiometric evaluation, they may benefit from seeking out additional testing. There are many different pathologies that are unable to be diagnosed in an average audiometric evaluation.
Why Do Some Forms Of Hearing Loss Go Unrecognized?
There are several different conditions that may result in difficulty hearing that may not appear on a traditional hearing test. These conditions may include, but are not limited to: auditory processing disorder, auditory neuropathy spectrum disorder, functional hearing loss, and psychological conditions that impact auditory function (e.g. schizophrenia).
Auditory processing disorder (APD) is defined as, “deficits in the mechanisms that preserve, refine, analyze, modify, organize, and interpret information from the auditory periphery.”1 According to the American Speech Language Hearing Association (ASHA), symptoms of APD may include the following: difficulty localizing sound; difficulty listening in background noise; taking longer to respond; frequent requests for repetitions; inappropriate responding; difficulty following auditory directions; difficulty detecting sarcasm; difficulty paying attention; difficulty with reading or spelling.1 Audiologists should work on multidisciplinary teams when diagnosing APD to rule out any other disorder of language and communication. At minimum, a multidisciplinary team should include an audiologist, a speech language pathologist, and a psychologist.
Auditory neuropathy spectrum disorder (ANSD) is a disorder commonly missed in an average comprehensive audiometric evaluation, and it is often misunderstood by physicians, audiologists, and patients and their families. There is not a “gold standard” list of tests to diagnose ANSD. This disorder is characterized by normal functioning of the outer, middle, and inner ears, with a pathology occurring along the nerve connecting the inner ear to the brain. Audiologically, the disorder is described as present otoacoustic emissions but an absent or abnormal auditory brainstem response, or absent or abnormal acoustic reflexes.5 A person with ANSD may have anywhere from normal hearing thresholds to profound hearing loss, and thresholds may fluctuate over time. Functionally, a person with ANSD typically has difficulty understanding speech, particularly when in the presence of background noise.5
Functional hearing loss, also known as “nonorganic hearing loss” or “psychogenic hearing loss,” is a broad term for, “hearing loss evident on an audiological test, which has no corresponding organic impairment in the auditory system, or a hearing loss that seems to be more severe than can be explained by the pathology identified in the auditory system.”3 There are three main categories of functional hearing loss, depending on how conscious or unconscious the motives are. This portion of the article will focus on the functional hearing loss known as conversion disorder.
Conversion disorder is a psychological coping mechanism in which emotional distress or trauma presents as a physical disability. There is very little conscious intent for deception in conversion disorder, meaning that patients with conversion disorder are typically unaware that there is a psychological rather than physiological component to the hearing loss.3 When treating patients with functional hearing loss, confrontation should be avoided. Clinicians should reassure and validate the patient’s struggles. Psychological interventions such as cognitive behavioral therapy may be pursued when treating functional hearing loss.2
There are psychiatric disorders that can impact the function of the auditory system. Schizophrenia, for example, may cause auditory hallucinations, but also a decrease in speech understanding. Postmortem studies of patients with schizophrenia have shown a decrease in grey matter of the auditory cortex.4 Grey matter is responsible for processing information, and a decrease in grey matter of the auditory cortex may lead to difficulties processing sounds, particularly speech. The grey matter reduction can be observed in MRI studies prior to intervention, and is often exacerbated by antipsychotic medications.4
The Importance of Getting Your Hearing Checked by Audiologists
Establishing a baseline hearing status with an audiologist is essential. As a person ages, most aspects of the human body change, and this includes the auditory system. Additionally, changes in one area of the body may impact the auditory system. Having the ability to compare records is needed in order to track any disorders and diseases, and even general health. Pursing regular audiometric evaluations can help with assessing the need for treatment and intervention.
The auditory system is made up of a series of complex pathways that lead from the outer ear all the way up to the brain. Multiple tests are needed for a comprehensive audiometric evaluation. These tests can be either subjective or objective, which allows for an audiologist to see the “big picture” regarding a person’s hearing status. However, there are several conditions that impact the auditory system that may not appear during an audiometric evaluation. If a person feels as if they have difficulty hearing, but received “normal” results on a hearing test, it may be beneficial to seek out further testing.
1. American Speech-Language-Hearing Association. (n.d.). Central auditory processing disorder. ASHA. Retrieved September 20, 2022 from: www.asha.org/Practice-Portal/Clinical-Topics/Central-Auditory-Processing-Disorder/
2. Baguley, D., Cope, T., & McFerran, D. (2016). Functional auditory disorders. Handbook of Clinical Neurology, 139.
3. Hussain, S. & Hohman, M. (2022). Non-organic functional hearing loss. StatPearls Publishing, LLC. Retrieved September 21, 2022 from: https://www.ncbi.nlm.nih.gov/books/NBK580555/?report=printable
4. Javitt, D. & Sweet, R. (2015). Auditory dysfunction in schizophrenia: Integrating clinical and basic features. Nature Reviews: Neuroscience, 16.
5. Norrix, L. & Velenovsky, S. (2014). Auditory neuropathy spectrum disorder: A review. Journal of Speech, Language, and Hearing Research, 57.