Mechanisms Underlying Auditory Hallucinations—Understanding Perception without Stimulus

Abstract

Auditory verbal hallucinations (AVH) are a common phenomenon, occurring in the “healthy” population as well as in several mental illnesses, most notably schizophrenia. Current thinking supports a spectrum conceptualisation of AVH: several neurocognitive hypotheses of AVH have been proposed, including the “feed-forward” model of failure to provide appropriate information to somatosensory cortices so that stimuli appear unbidden, and an “aberrant memory model” implicating deficient memory processes. Neuroimaging and connectivity studies are in broad agreement with these with a general dysconnectivity between frontotemporal regions involved in language, memory and salience properties. Disappointingly many AVH remain resistant to standard treatments and persist for many years. There is a need to develop novel therapies to augment existing pharmacological and psychological therapies: transcranial magnetic stimulation has emerged as a potential treatment, though more recent clinical data has been less encouraging. Our understanding of AVH remains incomplete though much progress has been made in recent years. We herein provide a broad overview and review of this.

Unusual perceptions at the end of life: limitations to the diagnosis of hallucinations in palliative medicine

“…Sometimes these are viewed as similar to outright hallucinations—‘a perceptual experience in the absence of external stimuli that is sufficiently compelling to be considered a true perception’ to the percipient9—but has no underlying evidence of organic pathology or psychopathology. There is some debate over whether idiosyncratic perceptions are a subcategory of hallucinations, or that it is hallucinations that are the subcategory of idiosyncratic perceptions 10. The epidemiological evidence tends to support the latter, more ‘normalising’, position.…”

“…These do not conform to any ‘standard definition’ of hallucination or pseudohallucination,7 and many clinicians consider these as ‘normal’ experiences 10. Moreover, the prevalence of this type of perception is very high, from 30%,7 or 49%37 to 80%16 or even 90% of Japanese samples of bereaved respondents 15…”

“…In fact, the mere presence of hallucinations does not predict mental health deterioration nor progression into contact with professional or clinical services. The biggest factors that predict and lead to clinical attention and scrutiny are the personal reactions to these unusual perceptions; their loss of a sense of personal control or the level of personal distress 10 15 56 57…”

“…There are two main contradictions that criss-cross the field of hallucination studies, and they remain unresolved. First, the main cognitive and psychodynamic theories of hallucination fit awkwardly in a pharmacological model enjoying some success in managing hallucinations of a psychopathological or organic origin, especially those working with the dopamine hypothesis 10. Second, although the psychiatric definitions of delusions, illusions and hallucinations are precise and clear, they are not supported by similarly discreet physiological signatures in stereoelectroencephalograph studies 62…”

“…Whatever models are favoured, they all come down to a rather general and modest style of explanation about ‘neural connectivity or suppression’ and this, in the end, is a vague and imprecise set of ‘facts’10 that would not allow a botanist to differentiate even a cactus from another type of flowering plant.…”