Why people see things that are not there: A novel Perception and Attention Deficit model for recurrent complex visual hallucinations

Abstract

As many as two million people in the United Kingdom repeatedly see people, animals, and objects that have no objective reality. Hallucinations on the border of sleep, dementing illnesses, delirium, eye disease, and schizophrenia account for 90% of these. The remainder have rarer disorders. We review existing models of recurrent complex visual hallucinations (RCVH) in the awake person, including cortical irritation, cortical hyperexcitability and cortical release, top-down activation, misperception, dream intrusion, and interactive models. We provide evidence that these can neither fully account for the phenomenology of RCVH, nor for variations in the frequency of RCVH in different disorders. We propose a novel Perception and Attention Deficit (PAD) model for RCVH. A combination of impaired attentional binding and poor sensory activation of a correct proto-object, in conjunction with a relatively intact scene representation, bias perception to allow the intrusion of a hallucinatory proto-object into a scene perception. Incorporation of this image into a context-specific hallucinatory scene representation accounts for repetitive hallucinations. We suggest that these impairments are underpinned by disturbances in a lateral frontal cortex-ventral visual stream system. We show how the frequency of RCVH in different diseases is related to the coexistence of attentional and visual perceptual impairments; how attentional and perceptual processes can account for their phenomenology; and that diseases and other states with high rates of RCVH have cholinergic dysfunction in both frontal cortex and the ventral visual stream. Several tests of the model are indicated, together with a number of treatment options that it generates.

Cognitive phenotype of psychotic symptoms in Alzheimer's disease: evidence for impaired visuoperceptual function in the misidentification subtype

“…Our findings are also consistent with integrative theories regarding the origins of visual hallucinations within the context of neurodegenerative disease (reviewed by Collerton and Taylor, 2013), which suggest that deficits in attentional control and perceptual processing are necessary for visual misperceptions and hallucinations to occur (Collerton et al ., 2005; Diederich et al ., 2009; Shine et al ., 2011). On the basis of the present findings, it is tempting to speculate that psychotic symptoms in AD are underpinned by disruption of the cholinergic/dopaminergic axis within frontostriatal circuits, with additional pathology in the ventral visual pathway in patients with the misidentification subtype.…”

Parkinson's disease, visual hallucinations and apomorphine: A review of the available evidence

“…Collerton and colleagues have proposed the perception and attention deficit (PAD) model, suggesting that VH are caused by disruption of either the bottom-up and/or top-down visual processing mechanisms [13]. In bottom-up processing, visual stimuli reaching the primary cortex are coupled to specific content e.g.…”

“…Central to the PAD modelVisual processing and VH have been linked to dysfunctional neurotransmitter systems is the connection between visual processing, VHs and cholinergic dysfunction [13,16]. Although the majority of the pathophysiological evidence for VH involves disruption of the cholinergic system, there is also support for a dysbalance in mono-aminergic neurotransmitter systems, especially related to dopamine and serotonin.…”

The Subcortical Confinement Hypothesis: A Neurological Model for Schizotypal Hallucinations.

“…Healthy hallucinations tend to be non-invasive, single-modal and generally affectively positive [47]. Hallucinations in dementia have similar profile but are generally less ephemeral and more complex [48]. Manic hallucinations appear to be triggered by events and opportunities that have a real basis [6] and schizotypal hallucinations are distinguished by their intensity, multimodality and by the repetitive and delusional nature of their themes and narratives.…”