Auditory hallucinations are perceptions of sounds absent of any external stimulus while in a conscious state1. The severity of auditory hallucinations varies among individuals affected, ranging from very mild causing individuals to not be burdened by them to severe enough to disturb daily functioning2. Auditory hallucinations are the most common type of hallucination seen in individuals2. They appear in a variety of individuals from those with neurological and psychological disorders such as schizophrenia or mania to healthy individuals1. Substances such as illicit drugs and even prescribed medication and alcohol can also cause it1. They are organized into different categories using a variety of classifications such as their content, where the apparent source is and the neurophysiological lesions or disease state that it is caused by1. Auditory hallucinations are often classified into two categories based on their content depending on whether the content is verbal or non verbalauditory_hallucin.png. Verbal auditory hallucinations are often found in individuals suffering from schizophrenia. Some examples of nonverbal auditory hallucinations include hearing a variety of sounds including the ringing of the ears known as tinnitus and musical hallucinations where the individual hears songs inside their head.

Verbal Auditory Hallucinations


Verbal auditory hallucinations (VAHs) also known as auditory verbal hallucinations (AVH) are auditory hallucinations that contain a verbal aspect1. VAHs may consist of human and or non-human voices1. The content of VAHs may vary in the types of voices they contain, their sound intensity, intelligibility, the languages it contains and their severity. There are many models that have been proposed to explain this perceptual phenomenon. Two of the most developed models are the theory of misattributed inner speech and the right hemisphere model3. The theory of misattributed inner speech proposes the speech production area normally produces efferent copies, which inhibit speech perception areas during the generation of inner speech. This model states that in individuals who experience VAHs, there is a dysfunction in this inhibition during the generation of inner speech, which results in the perception of auditory stimuli and its misattribution to an external source. This is the most commonly suggested cause for Gedankenlautwerden, which is the hallucination where a patient hears their thoughts out loud. The second theory, the right hemisphere model helps explain VAHs that seem to contain content that appears to be very different from inner speech such as non-human voices. This model states that AVH is caused by a lack of inhibition in language areas in the right hemisphere. The right hemisphere is not usually associated with language but it does have limited capacity for language production and the increased activation of this area may result in the production of strange auditory verbal hallucinations.

Neurobiological correlates


Neuroimaging studies have been conducted comparing the activity of the brain during periods when patients were experiencing hallucinations and when they weren’t4. These studies have found a variety of areas that were activated differently during VAHs which include areas such as the inferior frontal gyri (IFG), the parahippocampal gyrus, the anterior cingulate cortex and the superior and middle temporal gyri.

Many of these studies have reported that that areas associated with language are activated during auditory hallucinations5. The recent study by Diederen et al. aimed to identify which regions of the brain were activated right before the commencement of the hallucination to determine if they could reveal the origin of auditory hallucinations. They studied individuals with psychotic disorders who were experiencing auditory hallucinations and used 3-Tesla functional magnetic resonance imaging to look at which brain areas were activated differently compared to the control subjects within the six seconds preceding the auditory hallucination. They observed significant deactivation of the parahippocampal gyrus preceding the hallucination. This study is important because deactivation of this area could lead to insufficient activation of language areas during auditory hallucinations.

Another recent study was conducted by Raij et al. focused on narrowing down which of these previously studied brain areas was specifically involved with the experience of subjective reality during VAHs6. Using an fMRI scanner they examined 11 subjects who thought they would experience several intermittent voices 10-60 seconds in duration during a 30-minute noisy fMRI scanning. They either had schizophrenia or a closely related schizoaffective disorder6. Individual averages of the ratings of the subjective reality of the hallucinations ranged from 9-866.They observed brain activation similar to previous studies but they found that among the several brain areas activated, activation of the bilateral IFG correlated most strongly with the subjective reality of the hallucination. Speech comprehension and production has shown to involve the activation of the IFG6. It has also been shown to be involved in the imagination of the speech of others. Raij et al. believe that inappropriate activity within the IFG could result in the deficient controllability of the VAHs, which could be associated with their subjective reality.

rTMS Treatment


Studies have been looking at the effectiveness of low-frequency repetitive transcranial magnetic stimulation (rTMS) in treating medication-resistant VAHs7. They have particularly targeted the left temporo-parietal area. The results on efficacy proved inconclusive, possibly due to small sample sizes. Also recent fMRI studies have shown that the activation of the left temporo-parietal area is not as important as once thought in the experience of VAHs. A recent study by Slotema et al. aimed to determine the efficacy of rTMS directed at the left temporo-parietal area in a large population of medication-resistant patients with VAH compared to a control population. They also went on to test the efficacy of rTMS if it was targeted to each individual’s maximally activated areas during hallucinations, which were found using individual fMRI scans during VAH. They monitored the severity psychotic symptoms during treatment as well as at a 3-month follow up and saw that the effects of rTMS whether it was targeted at the maximally active areas during AVH or the temporo-parietal area were comparable to that of the control treatment. They concluded that low-frequency rTMS treatment was not a more effective treatment for medication-resistant VAH than the control.

Nonverbal Auditory Hallucinations


Nonverbal auditory hallucinations are auditory hallucinations that contain no voices1. They may consist of many different types of sounds such as music, machine sounds and snoring as well as sounds found in tinnitus such as ringing of the ears, hissing, humming, buzzing and sizzling to name a few.

Musical Hallucinations


Musical hallucinations are a type of nonverbal auditory hallucination1. They consist of songs and tunes just like ordinary music but without an external source. This first causes individuals who experience musical hallucinations to look for an external source such as a radio and when they are unable to, they realize that the source of the music is within them8. Musical hallucinations vary greatly between individuals as well as within individuals as well8. They can be soft or loud, simple or complex, and constant or intermittent. They usually consist of music from their childhood or other familiar songs but they can sometimes contain unfamiliar tunes as well. Some patients feel that they can exert a little control over the content of the music but most feel absolutely powerless when experiencing these hallucinations. This can be troubling to most people especially when the music is so loud and constant that it makes it difficult or even impossible to hear voices and sounds in their environment.
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Musical hallucination (Artwork by Marten Blom. copyright 2011)


Musical hallucinations typically appear abruptly but can also be prompted by external auditory stimuli. It was once thought that this type of hallucination was rare but due to recent interest developing and knowledge about this phenomena is increasing more new cases are being reported every year.

Musical hallucinations can be perceived as originating from one ear or from both and from the external environment or from within the head. They are found in individuals with and without pathology. When it occurs with pathology it is called symptomatic musical hallucinations and when it occurs absent any pathology it is called idiopathic musical hallucinations.

Pathophysiology


Areas of the brain associated with the perception of real music have been shown to be activated similarly during musical hallucinations using positron emission tomography and function magnetic resonance imaging. It seems that musical networks in the brain are activated abnormally due to a loss of normal inhibition. This can often occur from a lack of sufficient stimulation from external auditory stimuli causing spontaneous activity in associated areas. This could explain why musical hallucinations are most common in deaf individuals. Along with those with hearing impairment, older individuals and females seem to have a greater risk for developing musical hallucinations.

Treatment


Like other hallucinations the primary treatment provided are anticonvulsants, antidepressants, or antipsychotic drugs but these are not always effective treatments. To date, there is no reliable treatment available for the treatment of hallucinations.

Subjective Tinnitus


Tinnitus is the perception of sound without an external stimulus9. It consists of nonverbal auditory percepts usually in the form of ringing, buzzing, humming, hissing, etc. There are two types of tinnitus, subjective and objective. Objective tinnitus, which is rare, actually involves hearing real sounds made within the body and can be heard by anyone so it would not be considered a hallucination, but subjective tinnitus is when sounds are heard without either an external or internal auditory stimulus. Tinnitus is quite common with a reported prevalence of about 10% in the adult population. The prevalence of this symptom increases with age and is more common in men than women. The severity of tinnitus can vary, often increasing in severity over time. It is commonly seen in individuals with hearing impairment but it could also be a symptom of vascular or neurologic disorders. Just like other auditory hallucinations the sound intensity can vary from being quiet to being loud enough to make it difficult or impossible to hear voices and sounds in the patients’ environment. Sounds are reported as being perceived by one ear (unilateral), or from both ears (bilateral). Subjective tinnitus is most commonly caused by otologic causes (pathologies of the ear) but it can also have neurologic, infectious, and drug-related causes as well.

Treatment


After determining to make sure the tinnitus is not caused by any severe medical condition, treatment focuses on symptom relief. Drugs are usually prescribed to treat tinnitus but there aren’t any options that seem to improve the symptoms significantly compared to a placebo. Behavioral therapy and cognitive behavioral therapy has shown to have some success in helping patients deal with tinnitus.


References

1. Blom, J. D., & Sommer, I. E. (2010). Auditory Hallucinations Nomenclature and Classification . Cognitive and Behavioural Neurology , 23 (1), 55-62.
2. Diederen, K. M., & Sommer, I. E. (2012). Auditory Verbal Hallucinations. In J. D. Blom, & I. E. Sommer, Hallucinations Research and Practice (pp. 109-124). New York: Springer.
3. Sommer, I. E., Selten, J.-P., Diederen, K. M., & Blom, J. D. (2010). Dissecting Auditory Verbal Hallucinations into Two Components: Audibility (Gedankenlautwerden) and Alienation (Thought Insertion) . Psychopathology , 43, 137-140.
4. Allen, P., Laroi, F., McGuire, P., & Aleman, A. (2008). The Hallucinating Brain: A review of structural and functional neuroimaging studies of hallucinations. Neuroscience and Biobehavioral Reviews , 32, 175-191.
5. Diederen, K. M., Neggers, S. F., Daalman, K., Blom, J. D., Goekoop, R., Kahn, R. S., et al. (2010). Deactivation of the Parahippocampal Gyrus Preceding Auditory Hallucinations in Schizophrenia. The American Journal of Psychology , 167 (4), 427-435.
6. Raij, T. T., Valkonen-Korhonen, M., Holi, M., Therman, S., Lehtonen, J., & Hari, R. (2009). Reality of auditory verbal hallucinations. Brain , 132, 2994-3001.
Roberts, L. E., Eggermont, J. J., Caspary, D. M., Shore, S. E., Melcher, J. R., & Kaltenbach, J. A. (2010). Ringing Ears: The Neuroscience of Tinnitus. The Journal of Neuroscience , 30 (45), 4972–14979.
7. Slotema, C. W., Blom, J. D., de Weijer, A. D., Diederen, K. M., Goekoop, R., Looijestijn, J., et al. (2011). Can Low-Frequency Repetitive Transcranial Magnetic Stimulation Really Relieve Medication-Resistant Auditory Verbal Hallucinations? Negative Results from a Large Randomized Controlled Trial . Biological Psychiatry , 69, 450-456.
8. Sacks, O. W., & Blom, J. D. (2012). Musical Hallucinations. In J. D. Blom, & I. E. Sommer, Hallucinations Research and Practice (pp. 133-142). New York: Springer.
9. Chan, Y. (2009). Tinnitus: Etiology, Classification, Characteristics, and Treatment. Discovery Medicine , 8 (42), 133-136.