page_title: Visual release hallucinations
section_title: Pathophysiology
hierarchical_section_title: Visual release hallucinations / Pathophysiology
caption_attribution_description: Anatomical illustration of the human nervous system from the 1960 book A functional approach to neuroanatomy
context_page_description: Visual release hallucinations, also known as Charles Bonnet syndrome or CBS, are a type of psychophysical visual disturbance and the experience of complex visual hallucinations in a person with partial or severe blindness.
  First described by Charles Bonnet in 1760, the term Charles Bonnet syndrome was first introduced into English-speaking psychiatry in 1982. A related type of hallucination that also occurs with lack of visual input is the closed-eye hallucination.
context_section_description: There is no general consensus on the definition of CBS. Predominant factors correlated with CBS are a decrease of visual acuity, visual field loss, and elderly age. While characteristic features of visual hallucinations are not specifically linked to the anatomical site of the ocular injury, they usually match to the location of visual loss. The most commonly accepted theory for Charles Bonnet Syndrome proposes that extreme visual impairment promotes sensory deafferentation, leading to disinhibition, thus resulting in sudden neural firings of the visual cortical regions. A few studies record that visual hallucinations are likely to be concentrated in the blind regions. Functional magnetic resonance imaging (fMRI) of Charles Bonnet Syndrome patients display a relationship between visual hallucinations and activity in the ventral occipital lobe. A connection between age-related macular degeneration (AMD) and colored visual hallucinations has been presented. Color vision signals travel through the parvocellular layers of the lateral geniculate nucleus (LGN), later transmitting down the color regions of the ventral visual pathway. Due to cone photoreceptor damage located in the macula, there is a significant reduction of visual input to the visual association cortex, stirring endogenous activation in the color areas and thus leading to colored hallucinations. Patients with CBS alongside macular degeneration exhibit hyperactivity in the color areas of the visual association cortex (as shown in fMRI’s). Those with significant ocular disease yet maintain visual acuity may still be susceptible to CBS. 
  The Deep Boltzmann Machine (DBM) is a way of utilizing an undirected probabilistic process in a neural framework. Researchers argue that the DBM has the ability to model features of cortical learning, perception, and the visual cortex (the locus of visual hallucinations). Compelling evidence details the role homeostatic operations in the cortex play in regards to stabilizing neuronal activity. By using the DBM, researchers show that when sensory input is absent, neuron excitability is influenced, thus potentially triggering complex hallucinations.
  A short-term change in the levels of feedforward and feedback flows of information may intensely affect the presence of hallucinations. In periods of drowsiness, CBS related hallucinations are more prone to arise. Disrupting cortical homeostatic processes after vision has been lost may prevent or setback the emergence of hallucinations. At varying stages of the cortical grading, acetylcholine (ACh) may impact the balance of thalamic and intracortical inputs as well as the balance in between bottom-up and top-down. Particularly in CBS, a shortage of acetylcholine at cortical locations should correspond to the onset of hallucinations.
  The syndrome can also develop after bilateral optic nerve damage due to methyl alcohol poisoning.
caption: Anatomical illustration of human neuroanatomy
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