Throughout history, auditory hallucinations have been construed as evidence of communication with divine powers, although contemporary medical models often view them as undesirable and a sign of mental illness. In psychiatry, auditory hallucinations carry considerable weight in the diagnostic process, so there is a clear need for clinicians to have a greater understanding of the multiple facets of this phenomenon.

Auditory hallucinations are false perceptions of sound. They have been described as the experience of internal words or noises that have no real origin in the outside world and are perceived to be separate from the person's mental processes.¹ Auditory hallucinations have veridical perceptual qualities in the sense that individuals are often convinced of the objective reality of the experience. In most cases, auditory hallucinations are unintentional, intrusive, and unwanted. Affected individuals may or may not have insight into the hallucinations. A person with insight will acknowledge that the experience is abnormal and will report less interference with daily activities than a person with no insight.

This article provides an overview of the characteristic features of auditory hallucinations in psychiatric illness. The assessment and differential diagnosis of auditory hallucinations, recent theoretical frameworks, and treatment options are also briefly discussed.

Auditory hallucinations in diagnosis

Auditory hallucinations feature prominently in many psychiatric disorders. It has been estimated that approximately 75% of people with schizophrenia experience auditory hallucinations. These hallucinations are also relatively common in bipolar disorder (20% to 50%), in major depression with psychotic features (10%), and in posttraumatic stress disorder (40%).²

Not all auditory hallucinations are associated with mental illness, and studies show that 10% to 40% of people without a psychiatric illness report hallucinatory experiences in the auditory modality.^3,4 A range of organic brain disorders is also associated with hallucinations, including temporal lobe epilepsy; delirium; dementia; focal brain lesions; neuroinfections, such as viral encephalitis; and cerebral tumors.⁵ Intoxication or withdrawal from substances such as alcohol, cocaine, and amphetamines is also associated with auditory hallucinations.

Hypnagogic and hypnopompic hallucinations are especially common in healthy individuals and occur during the period of falling asleep or waking up. The frequency of these experiences in the general population may be evidence of the existence of a symptomatic continuum, which ranges from subclinical experiences of psychosis to full-blown psychotic episodes with severe, unwanted, and intrusive symptoms.⁶

The phenomenological characteristics of auditory hallucinations differ on the basis of their etiology, and this can have diagnostic implications. People without mental illness tend to report a greater proportion of positive voices, a higher level of control over the voices, less frequent hallucinatory experiences, and less interference with activities than people who have a psychiatric illness.^7,8

There is also evidence that delusion formation may distinguish psychotic disorders from nonclinical hallucinatory experiences.⁹ In other words, the development of delusions in people with auditory hallucinations significantly increases the risk of psychosis when compared with individuals who have hallucinations but not delusions.

By contrast, characteristics of auditory hallucinations that are thought to be more indicative of psychosis include^8,10:

• Higher frequency of hallucinatory experiences
• Localization of voices outside the head
• Greater linguistic complexity
• Greater emotional response
• The extent to which patients believe that other people share this experience

Because of the multiple causes of auditory hallucinations, physicians must take care to obtain detailed histories from the patient, to assess for mood and psychotic symptoms, and to obtain collateral information. Laboratory tests and brain scans can also offer further clues to the underlying cause of the hallucinations.

Clinical assessment of auditory hallucinations

Patients are usually able to describe their hallucinatory experiences. The Schedules for Clinical Assessment in Neuropsychiatry¹ provides a standard question that can be used in assessing symptoms: "Do you ever seem to hear noises or voices when there is nobody about, and no ordinary explanation seems possible?" A description of the experience in the patient's own words is required for a positive rating.

Patients will often underreport their hallucinatory experiences because of the possible implications of further treatment action. Collateral information can be helpful, as is repeated interviewing. True auditory hallucinations must be differentiated from:

• Auditory illusions (misinterpretations of real existing stimuli)
• Vivid auditory imagery (under volitional control)
• Abnormal beliefs (such as a delusion of reference, when individuals report that other people are talking about them)

To help distinguish hallucinations from abnormal beliefs, ask whether the experience is closer to a real external voice than to a thought. Patients are usually able to make this distinction.

 

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Characteristics, forms, and content of auditory hallucinations

The phenomenological complexity of auditory hallucinations was demonstrated long ago. A number of researchers have clustered the multiple phenomenological dimensions of hallucinations into a number of variables.^10-12 The Table presents a characterization of the main phenomenological variables in auditory verbal hallucinations, as described by Stephane and colleagues.¹⁰

In summary, auditory hallucinations may be experienced as coming through the ears, in the mind, on the surface of the body, or anywhere in external space. The frequency can range from low (once a month or less) to continuously all day long. Loudness also varies, from whispers to shouts. The intensity and frequency of symptoms fluctuate during the illness, but the factor that determines whether auditory hallucinations are a central feature of the clinical picture is the degree of interference with activities and mental functions.

The most common type of auditory hallucinations in psychiatric illness consists of voices. Voices may be male or female, and with intonations and accents that typically differ from those of the patient. Persons who have auditory hallucinations usually hear more than one voice, and these are sometimes recognized as belonging to someone who is familiar (such as a neighbor, family member, or TV personality) or to an imaginary character (God, the devil, an angel). Verbal hallucinations may comprise full sentences, but single words are more often reported.

Voices that comment on or discuss the individual's behavior and that refer to the patient in the third person were thought by Schneider¹³ to be first-rank symptoms and of diagnostic significance for schizophrenia. Studies show that approximately half of patients with schizophrenia experience these symptoms.¹⁴

A significant proportion of patients also experience nonverbal hallucinations, such as music, tapping, or animal sounds, although these experiences are frequently overlooked in auditory hallucinations research. Another type of hallucination includes the experience of functional hallucinations, in which the person experiences auditory hallucinations simultaneously through another real noise (eg, a person may perceive auditory hallucinations only when he hears a car engine).

The content of voices varies between individuals. Often the voices have a negative and malicious content. They might speak to the patient in a derogatory or insulting manner or give commands to perform an unacceptable behavior. The experience of negative voices causes considerable distress.¹² However, a significant proportion of voices are pleasant and positive, and some individuals report feelings of loss when the treatment causes the voices to disappear.¹⁵

CASE VIGNETTE

Joel is a 46-year-old man who had been given a diagnosis of schizophrenia when he was 25. He reports hearing constant voices that command him to kill himself. He hears 5 different voices, all unfamiliar and male, that comment on his thoughts and actions. He also hears singing voices and has functional hallucinations through machinery and radio. Joel thinks some of the voices may come from his body (near his stomach). The voices interfere significantly with his life, but their frequency has decreased with medication.

The content of voices is usually highly personalized. The voices frequently express what the person is feeling or thinking and speak about his or her fears or worries. Psychiatric patients view the content of voices to be meaningful and to have personal relevance. The voices are interpreted to be the manifestation of real people or entities, and this experience contributes to the intense emotional response to the voices. The personalized content and subjective reality of voices play a role in the development of strong beliefs about the intent and power of voices, and a complicated and intense relationship frequently ensues between patients and their voices.¹⁶

CASE VIGNETTE

Mary is a 40-year-old woman with mild intellectual impairments and a long history of psychosocial stressors that include sexual abuse from the age of 10 and the loss of her children to state care. Mary reports hearing voices from the age of 11—"I thought it was normal." There are 2 male and 2 female voices, primarily coming from inside her head. The voices command her to commit suicide and homicide, and she has difficulty in resisting—she once tried to jump in front of a bus. The voices are abusive and derogatory ("you are a pig," "you're worthless," "why don't you go kill yourself?") and wake her up at night. Functional (through music) and nonverbal (birds chirping, glass smashing) hallucinations are also present.

Studies in cross-cultural psychiatry show that auditory hallucinations occur in similar forms in all societies around the world but that there are cultural differences in the content and interpretation of voices.¹⁷ In cultures where they are understood in the context of local beliefs and practices, auditory hallucinations have a positive value. This arises because the interpretation is embedded in a strong cultural framework that there is less emphasis on the distinction between imaginary and real experiences.

Occasionally, the semantic meaning plays a greater role than the acoustic characteristics of the voices. For example, some people may know what is being said even though the auditory hallucinations may only consist of nonverbal sounds.

Stress and other negative emotions

Stress has been implicated in provoking episodes of auditory hallucinations. For instance, the frequency of auditory hallucinations is particularly high in the context of bereavement and sensory deprivations. Studies that have examined auditory hallucinations in real-life situations using experience sampling methods show that negative emotional states contribute to the modulation of hallucination intensity. In a study by Delespaul and colleagues,¹⁸ participants with schizophrenia were asked to report experiences of hallucinations as well as negative mood states throughout the day for 1 week. In that study, self-reported anxiety levels were associated with, and often predicted, increased intensity of hallucinations. These findings suggest a causal association between levels of anxiety and hallucinatory experiences.

Auditory hallucinations, in turn, cause high levels of stress. The content and the experience of intrusive and personal voices can cause distress. Patients may feel that they are unable to escape from the experience, and this feeling is persistent and beyond voluntary control. Affective symptoms, including depression, anxiety, fear, and anger, have been found in 25% to 40% of patients.^18-21 Some patients have committed suicide to escape from the voices.²² Further evidence for the role of negative mood comes from studies that show that the development of a depressed mood can predict the onset of a psychotic disorder and can increase the risk of transition from subclinical auditory hallucinations to a psychotic episode.²⁰

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Theoretical frameworks of auditory hallucinations

The exact processes that underlie auditory hallucinations remain largely unknown. There are 2 principal avenues of research: one focuses on neuroanatomical networks using techniques such as positron emission tomography and functional MRI. The other focuses on cognitive and psychological processes and the exploration of mental events involved in auditory hallucinations.

A common formulation suggests that auditory verbal hallucinations represent an impairment in language processing and, particularly, inner speech processes, whereby the internal and silent dialogue that healthy people engage in is no longer interpreted as coming from the self but instead as having an external alien origin. There is support for this language hypothesis of auditory hallucinations from neuroimaging studies. These show that the experience of auditory hallucinations engages brain regions, such as the primary auditory cortex and Broca area (Figure), that are associated with language comprehension and production. This suggests that hallucinatory experiences are associated with listening to external speech in the absence of external sounds.^23,24

An explanation of why these experiences are not perceived as self-generated posits that audi-tory hallucinations arise because persons who have the hallucinations fail to distinguish between internal and external events. According to Frith's self-monitoring theory,²⁵ this arises because of deficits in internal self-monitoring mechanisms that compare the expected with the actual sensations that arise from the patient's intentions. This abnormality also applies to inner speech processes and leads to the misclassification of internal events as external and misattribution to an external agent.

By contrast, Bentall and Slade²⁶ have proposed that individuals with hallucinations use a different set of judgment criteria from healthy people when deciding whether an event is real, and they are more willing to accept that a perceptual experience is true. This bias essentially involves a greater willingness to believe that an event is real on the basis of less evidence.

According to the context memory hypothesis of auditory hallucinations, the failure to identify events as self-generated arises because of specific deficits in episodic memory for remembering the details associated with particular past memory events. These specific deficits in memory cause confusion about the origins of the experience.^27-29 In support of this hypothesis, findings indicate that patients with auditory hallucinations tend to misidentify the origins and source of stimuli during ongoing events and during memory events.^27-30 In addition, imaging studies have shown abnormalities in brain regions associated with memory integration in individuals with schizophrenia.^31-33

The lack of voluntary control over the experience is a key feature of auditory hallucinations, which might explain why self-generated inner speech is classified as external in origin.³³ According to this proposal, hallucinations are experienced when verbal thoughts are unintended and unwanted. Because deficits in cognitive processes, such as inhibitory control, are thought to render people more susceptible to intrusive and recurrent unwanted thoughts, studies have linked auditory hallucinations with deficits in cognitive inhibition.²⁹

From a neuroanatomical point of view, deficits in the prefrontal cortex of patients with auditory hallucinations are consistent with the hypothesis of cognitive inhibitory deficits. Functional disconnectivity between the frontal and posterior areas of the brain in hallucinating patients may result in a lack of modulatory control of the frontal cortex over activities generated by the posterior brain areas so that events that arise from the temporal/parietal areas are not regulated normally.^32,34

Recent advances in the neurosciences provide clues to why patients report an auditory experience in the absence of any perceptual input. Spontaneous activity in the early sensory cortices may in fact form the basis for the original signal. Early neuronal computation systems are known to interpret this activity and engage in decision-making processes to determine whether a percept has been detected.³⁵ A brain system that is abnormally tuned in to internal acoustic experiences may therefore report an auditory perception in the absence of any external sound. Ford and colleagues³⁶ recently suggested that patients with auditory hallucinations may have excessive attentional focus toward internally generated events—the brains of persons who have auditory hallucinations may therefore be overinterpreting spontaneous sensory activity that is largely ignored in healthy brains.

Cognitive impairments are not the only factors responsible for auditory hallucinations. Psychological factors such as metacognitive biases, beliefs, and attributions concerning the origins and intent of voices also play a critical modulatory role in shaping the experience of hallucinations.^16,19,37 The role of environmental cues and reinforcement factors through avoidance strategies must also be incorporated in any explanations of auditory hallucinations. These factors do not explain how hallucinations occur in the first place, but they have strong explanatory power when accounting for individual differences in how the voices are experienced.

Treatment of auditory hallucinations

The presence of hallucinations does not necessarily imply a need for medical treatment if the experience is not intrusive and does not interfere with everyday activities. When treatment is required, antipsychotic medication is usually the treatment of choice in organic and psychiatric conditions. Clinicians should provide information and discuss the benefits and adverse effects of each drug, including a drug's potential to cause symptoms that include the extrapyramidal syndrome and metabolic syndrome. In view of such adverse effects, clinicians need to monitor the physical health of patients regularly.

Few studies have compared the efficacy of different neuroleptic treatments, and hallucinations often persist despite intensive and prolonged psychopharmacological treatment.³⁸ Another biological method that has been researched in recent years is repetitive transcranial magnetic stimulation (rTMS), which plays a role in altering neural activity over language cortical regions. Used as an adjunct to antipsychotic medication, studies show that rTMS can reduce the frequency and severity of auditory hallucinations in treatment-resistant cases.^39,40

Many psychological treatments target the idiosyncratic ways that individuals respond to an abnormal perceptual experience, based on the understanding that this influences their coping strategies and emotional response.^16,19,37 Studies show that some patients respond well to cognitive-behavioral therapy, where the focus is on evaluating and monitoring one's perceptions, beliefs, and reasoning; promoting alternative ways of coping; and reducing distress. Anxiety reduction strategies are particularly effective in reducing the impact of voices.^41-43 Evidence also suggests that a combination of family and psychological interventions, as well as medication, may be the most beneficial treatment for auditory hallucinations.⁴⁴

There is increasing evidence that peer support groups (voice-hearers networks; http://www.intervoiceonline.org) can help alleviate the impact of voices. Self-help groups often encourage patients to take responsibility for their hallucinatory experience, to accept the voices, and to cope with them. A series of investigations showed that accepting hallucinations as an aspect of the normal human condition is one of the most difficult steps to take, but that the acceptance process and lack of resistance lead to successful adaptation to hearing voices and a change in the relationship with the voices.⁴⁵

Because cognitive dysfunctions have been shown to underlie auditory hallucinations, cognitive deficits are becoming targets of treatment with cognitive remediation strategies, although these interventions are at a very early stage of development. By focusing on deficits found to be linked to auditory hallucinations, recent trials have focused on the convergence between theory and practice.^46-48 For example, in their study, Favrod and colleagues⁴⁸ taught patients techniques to help them recognize the source of the voices; beneficial outcomes were maintained at 1-month follow-up.

Conclusion

Auditory hallucinations are much more than false perceptions. The combination of personalized contents and interpretational processes contributes to a dynamic and emotionally charged experience that can be better described as a belief system rooted in a perceptual experience. Auditory hallucinations are most likely to arise because of an interaction between perceptual, cognitive, and biological vulnerability as well as affective factors and contextual influences. In addition, the interpretation of these experiences combined with delusional elaboration makes auditory hallucinations a complex and truly individualized phenomenon. Understanding their complexity can lead to useful insights for therapy.

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