Research on Sensory Modality in Hearing

Sensory modality or the understanding of how we use our senses to understand the world around us is a critical issue in the study of neuropsychology. One critical area of study is how the individual perceives auditory information and how the process that information into useable data. This is critical in that the study of audition applies to several areas of both psychological, and educational study, research, and practical or clinical applications. For example, understanding how people process what they hear into useable data has an influence on how clinical psychologists treat people with schizophrenia who have auditory hallucinations, and how educators approach teaching hard of hearing students who do not have the same access to auditory stimuli as the typically developing student.

In order to understand how people hear auditory stimuli, and how they process auditory stimuli one must first understand the anatomy of the human ear. According to Kolb, and Whishaw (2013), this anatomy is broken down functionally into the outer, middle, and inner ears. The primary function of the outer ear is as an intake device. This is where all auditory stimuli enter the somatosensory systems. The outer ear is comprised of the "pinna" which includes the outer area of the ear. In some cases, this area can be moved to detect patterns in auditory stimuli or to sense different types of sounds though this is more common in non-humans. Kolb, and Whishaw (2013), state that the hills and valleys, or ridges seen on the outer ear serve to amplify auditory stimuli specifically in relation to high frequency (2000-5000 Hz) sounds that cannot be heard in normal conversation. This allows for more effective processing of the information sent to the middle ear in that the quality of important auditory stimuli is improved while, unimportant information is discarded, or suppressed.

The middle ear contains several small (minute to the point of needing a microscope to view them) bones that are responsible for amplifying and processing information that is sent to the inner ear (Kolb and Whishaw, 2013). These are called the ossicles, and are comprised of the incus, the malleus, and the stapes. These three bones are located in what is termed the "oval window" and are linked to the tympanic membrane. These bones work by displacing or moving the tympanic membrane each time a new set of auditory stimuli is sent to the middle ear. The tympanic membrane is composed to two membranes, the tensor tympani and the stapedius. These essentially process auditory stimuli by stiffening and relaxing when sounds are being processed. When these muscles are, relaxed sounds are amplified, and when they are stiff, sound is suppressed (Kolb and Whishaw, 2013).

After the auditory stimuli passes the tympanic membranes it passers into the inner ear. The inner ear is comprised of the cochlea, a small membrane filled with fluid that processes auditory stimuli (Kolb and Whishaw, 2013). The cochlea is divided into three separate structures, the scala vestibule or vestibular canal, the scala media or middle canal, and the scana tympani or tympanic canal. In addition, the inner ear processes auditory stimuli into neurological activity. This is done through an area of the inner ear called the "organ of Corti". According to Kolb, and Whishaw this contains three major areas of structures including, hair cells that serve as sensory organs, supporting cells, and the ends of the auditory fibers. Processing also occurs in the basilar membrane that separates the tympanic and vestibular canals. This will move in response to sound that varies in location depending on sound frequency. Infections of the inner ear frequently cause hearing loss since bacteria, or viruses can destroy the hair cells that process auditory information into neurological stimuli (Kolb and Whishaw, 2013).

From your Ear to your Brain

The vestibularcochlear nerve is a large group of nerves that runs on both sides of the skull. These nerves are responsible for gathering auditory and visual information from the nerves and the eyes and processing it through the areas of the brain responsible for processing and interpreting auditory and visual information. Once these nerves reach the brain the separate into separate branches linked to the type of information they are responsible for processing. The bundle of nerves responsible for disseminating information on hearing throughout the brain links to a group of cells called the cochlear nuclei. One major area of the brain that these nuclei link to is another group of cells called the super oliviary nuclei (Kolb and Whishaw, 2013). This area is responsible for comparing information from both ears to determine differences in information. The second area is the inferior colliculi that is the main processing center for information going to the mid-brain. Finally, the inferior colliculi links to the medial geniculate nucleus in the human thalamus (Kolb and Whishaw, 2013).

These structures link the ear with the cerebral cortex. Each group of nuclei and the nerves that run between these nuclei serve as roads for transporting information between the outside world and the brain (Kolb and Whishaw, 2013). As the information is sent to each of these different areas of the brain, it is filtered in order to determine what is an is not important.

According to Kolb and Whishaw (2013), sound is processed in one of several different ways. For example, the brain uses pitch, frequency, and amplification to determine what is, and what not important information is. Typically, information that is at too high a frequency for humans to hear normally is discarded as being of little value. In addition, information that is not well amplified may be discarded by the brain is unimportant. Kolb and Whishaw (2014) also state that the brain processes local sounds from both ears using coincidence detectors that determine the number of times a specific sound is present and then discards infrequent sounds. Another method that is used to filter certain sounds is spectral filtering which looks for information or stimuli coming from specific sound frequencies or wavelengths (Kolb and Whishaw, 2013).

One of the primary areas of the brain responsible for processing this information is the auditory cortex. This part of the brain is responsible for processing most auditory stimuli and in determining how that information is turned into useable information. Kolb and Whishaw (2013) state that the ability of this part of the brain to process auditory information is based primarily on experience. This is one of the main reasons why infants are easily over-stimulated due to auditory stimuli, and why they have difficulty recognizing different types of auditory stimuli (Kolb and Whishaw, 2013).

One of the most critical concepts in how people interpret auditory stimuli once the information reaches the brain is the concept of top-down processing. This is the opposite of bottom-up processing, which occurs when people construct information from minimal stimuli. In top-down processing, ideas, concepts, and information are gradually reduced so that it only involves the essential information needed to deal with a context or situation (Kolb and Whishaw, 2013). Top-down processing applies in relation to the concept or idea of how we hear because when the information enters the ear there is a great deal of auditory stimuli. As the auditory stimuli is processed through the outer, middle, and inner ear this stimuli is filtered out as various types of stimuli are discarded as unimportant. This information is further reduced once it enters the brain as processing through the different structures of the auditory cortex filter out stimuli or information that does not apply in a specific context or setting (Kolb and Whishaw, 2013).

There are several different situations in which the top-down processing of information applies to the concept of hearing and auditory processing. One of the primary examples of top-down processing is the deficits that occur in hearing loss. According to Kolb and Whishaw (2013), hearing loss occurs for several different reasons any of which can result in ineffective or limited top-down processing of auditory information. In conduction hearing, loss infections or damage to the outer or middle ear prevents auditory information from reaching the next area of the ear. This effectively prevents the ear, and eventually the brain from eliminating unnecessary stimuli or neurological information meaning that the information the individual receives is not present or confused. In sensori-neural deafness, a problem within the inner ear due to infection, or injury prevents auditory stimuli from being processed to the brain. One of the most common causes of this is when infection or exposure to loud sounds destroys the hair cells that are responsible for processing information from the inner ear into the nerves leading into the brain.

The auditory hallucinations associated with schizophrenia can also create problems with top-down processing in that the stimuli being fed into the ears, and hence into the brain is false and therefore the person cannot process stimuli into useful information. Daalman et al (2012) have performed a study in which they looked the influence of semantic top down processing on processing of auditory hallucinations. The study included, 120 participants including 40 psychotic patients with auditory hallucinations, 40 non-psychotic participants with auditory hallucinations (due to stroke etc), and 40 individuals who did not have either psychosis or auditory hallucinations.

Participants were asked to complete a hearing test to ensure they had normal hearing. They were also asked to complete a semantic expectation task, and a questionnaire that focused on questions surrounding the frequency, and type of auditory hallucinations that they experienced (Daalman et al, 2012). Finally, they assessed the patient's medical records for more information on their auditory hallucinations.

The results of the study of the study predicted the opposite of the hypothesis. Daalman et al had hypothesized that psychotic individuals with auditory hallucinations were more likely to demonstrate top-down processing errors. However, the results of the study revealed that non-psychotic individuals with auditory hallucinations were more likely to suffer from top-down processing errors. This makes sense in light of the fact that this type of auditory hallucinations is more likely to result from stroke or other types of brain damage.

Conclusion

Processing auditory stimuli is a complex physical and neurological process that involves both anatomical structures (the outer, middle, and inner ears), and neurological structures (auditory cortex). One of the key issues with auditory processing is that it is a top-down process and disruptions in this process such as, in the case of hearing loss, or schizophrenia can cause incorrect or inefficient processing of auditory stimuli.

References

Daalman, K., Verkooijen, S., Derks, E. M., Aleman, A., & Sommer, I. E. The influence of semantic top-down processing in auditory verbal hallucinations. Schizophrenia research, 139(1), 82-86.

Kolb, B., & Whishaw, I. Q. An introduction to brain and behavior, fourth edition. New York: Worth.