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Aphantasia
Published in Alexander R. Toftness, Incredible Consequences of Brain Injury, 2023
In theory, damage to specific layers of the visual cortex that are responsible for either sending out or taking in neural information may lead to these more specific impairments. For example, damaging your “input layer of the visual cortex” coming from the eyes while keeping the input to the visual cortex that comes from your inferior temporal lobes intact might lead to a person losing bottom-up perception (actually seeing objects) while allowing them to retain top-down imagery (imagining the objects)—though much more work is needed to determine such relationships (Djikstra et al., 2019, p. 425).
The nervous system and the eye
Published in C. Simon Herrington, Muir's Textbook of Pathology, 2020
James A.R. Nicoll, William Stewart, Fiona Roberts
The visual cortex lies on the medial surface of the occipital lobe. Symptoms and signs of occipital lobe disease include: Unilateral cortical lesion producing homonymous hemianopiaCortical blindness occurring with extensive bilateral lesions of striate cortexVisual hallucinationsVisual agnosia
Technologies for vision impairment
Published in John Ravenscroft, The Routledge Handbook of Visual Impairment, 2019
Lauren N. Ayton, Penelope J. Allen, Carla J. Abbott, Matthew A. Petoe
To understand how a vision prosthesis works, it is beneficial to review how natural human vision works. In a healthy eye, light is transmitted through the clear cornea at the front, focused through the intraocular crystalline lens, and then directed on to the retina, which lines the back of the eye. The photoreceptor cells (which are located at the posterior retinal layers) translate the light energy to electrical signals, which pass back through the neural pathways in the retina and are transmitted via the optic nerve back to the brain (via the lateral geniculate nucleus). Final processing of the signal occurs in the visual cortex, which results in a complex image with colour, spatial, temporal, edge and motion information.
Bilateral Vision Loss and Visual Hallucinations in Subacute Sclerosing Panencephalitis: A Case Report
Published in Neuro-Ophthalmology, 2023
Ravi Uniyal, Ravindra Kumar Garg, Hardeep Singh Malhotra, Neeraj Kumar, Shweta Pandey, Imran Rizvi, Amita Jain, Nidhi Tejan, Rupesh Singh kirar
This patient also developed visual hallucinations. Simple visual hallucinations occur due to hyperactivity or irritation of the primary visual cortex, while complex visual hallucinations can occur due to the involvement of visual association cortices.5 Visual hallucinations can be associated with various psychiatric, neurologic, and ophthalmologic conditions. In patients with severe vision loss, visual deafferentation may cause cortical release phenomenon, in the form of visual hallucinations, which is typically known as Charles Bonnet syndrome (CBS).5 CBS can be associated with vision loss due to any cause. Visual hallucinations in CBS tend to involve people, animals, faces, and even inanimate objects.5 The appearance of visual hallucinations in this patient can be explained by CBS secondary to vision loss; however, the occurrence of seizures suggests that brain parenchyma per se is also contributory.
Neuro-Ophthalmic Literature Review
Published in Neuro-Ophthalmology, 2023
David A. Bellows, John J. Chen, Hui-Chen Cheng, Panitha Jindahra, Collin McClelland, Michael S. Vaphiades, Xiaojun Zhang
The authors report a randomised, double-masked, placebo-controlled crossover trial of 16 individuals diagnosed with Charles Bonnet syndrome (CBS) secondary to visual impairment caused by eye disease who experienced recurrent visual hallucinations. All participants received four consecutive days of active or placebo transcranial direct current stimulation (tDCS) to the visual cortex over two defined treatment weeks, separated by a 4-week washout period. When compared with placebo treatment, active inhibitory stimulation of visual cortex resulted in a significant reduction in the frequency of visual hallucinations. Participants who demonstrated greater occipital excitability on electroencephalography assessment at the start of treatment were more likely to report a positive treatment response. Stimulation was found to be tolerable in all participants, with no significant adverse effects reported, including no deterioration in pre-existing visual impairment. The authors concluded that inhibitory tDCS of the visual cortex may reduce the frequency of visual hallucinations in people with CBS, particularly individuals who demonstrate greater occipital excitability prior to stimulation.
Giant cell arteritis
Published in Postgraduate Medicine, 2023
Visual Hallucinations: The Charles Bonnet Syndrome, first described by the Swiss philosopher and naturalist Charles Bonnet in the 1700s, is a phenomena in which psychologically normal individuals experience chronic visual hallucinations, often of well formed images like flowers, animals or small people [70] This occurs in patients who are visually impaired as would be the case with vision loss in GCA and in most cases patient are aware the images are not real. The cause of the hallucinations is postulated to either be from a ‘release theory’ where a lesion of the visual pathway results in abnormal signals being sent to the visual cortex or the ‘deprivation theory’ where reduction in sensory input leads to production of spontaneous images from the visual cortex [71]. Charles Bonnet Syndrome is felt to be a rare manifestation of GCA but some propose hallucinations are an underrecognized part of GCA [70].