People with vision loss caused by damage to the sensory pathways to the brain may develop visual hallucinations. Charles Bonnet syndrome (CBS). The main feature of CBS is complex visual hallucinations: fully formed animate or inanimate objects that are not seriously related. mental disorders (Altieri & Battaglini, 2026; Pang, 2016). In fact, people with CBS typically exhibit full or partial “awareness of the reality of their hallucinations.”
For about two years after the onset of blindness, I was hallucinating people, bicycles, pillows, and plants. The hallucinations that interested me the most were the ones that occurred repeatedly while performing certain activities. For example, when I was learning to use the white cane, I sometimes found myself in a crowd of hallucinating students. And for a while, when I used my laptop, I hallucinated the keyboard in front of me.
Why did I keep having these particular hallucinations? And why don’t I have them anymore? In this post, I will discuss current thinking in vision science to explore possible answers to these questions.
How the visual system works
Visual perception in sighted people begins when light activates sensory receptors in the eye. Produces activated receptors nerve signals that are transmitted to visual areas in the cerebral cortex – the large outer layer of the brain (Mars et al., 2025).
The visual system of the brain is hierarchically organized (Powers et al., 2016). The “early visual cortex” (ie, the first areas to receive sensory input from the eye) is specialized in processing the basic features of incoming sensory signals. The “posterior visual cortex” (ie, visual areas at higher levels of the hierarchy) are specialized for processing more abstract and complex features of sensory input. For example, Dominic Ffitche and colleagues (1998) have shown that the content of CBS hallucinations corresponds to perceptual features typically activated by higher cortical regions during hallucinations. For example, when someone hallucinates a face, an area specialized for perceiving faces was activated.
But no visual area is ever solely responsible for making “visual perception” (conscious visual perception). The visual system functions as a highly integrated network. Information flows in both directions, with many “back and forth” interactions occurring between visual areas at different levels of the hierarchy (Powers et al., 2016).
The two-way flow of information serves as the basis for two interrelated processes of perception:
- Bottom-up processing analyzes incoming sensory information into its component perceptual features.
- Top-down processing refers to the use of prior knowledge to reduce uncertainty in input sensory signals.
This view of perception holds that processing sensory input involves interpreting rather than reproducing what the eye sees.
For example, after hearing about a person’s encounter with a venomous desert snake, you may become sensitive to the possibility while hiking a desert trail. You might even misinterpret a fallen tree branch as a snake.
An approach known as “predictive processing” may help explain this experience (Clark, 2024; Peelen et al., 2024). Your brain may have used past experience to predict the sensory input it would receive if the object were a snake. Your brain then evaluates the degree of inconsistency between the expected sensory evidence and the sensory evidence. If the mismatch is minimal, your visual system can see the snake. A second look might make you reconsider your prediction, so you’ve now seen the branch of the tree.
Predictive processing theory views conscious visual perception as the brain’s best estimate of what the eyes are looking at.
Anticipatory processing and CBS hallucinations
CBS hallucinations can be explained by a combination of two theories: the predictive processing theory and the “deafferentation theory” (Altieri & Battaglini, 2026; Marshall et al., 2020).
Charles Bonnet Syndrome Critical Reading
“Defferentation” refers to the loss of sensory input to perceptual areas of the brain. CBS hallucinations have long been attributed to deafferentation (Burke, 2002; Painter et al., 2018).
Some have hypothesized that deafferentation hallucinations are due to changes in the relative influence of top-down and top-down processing (Marshall et al., 2020). It is possible that the visual system compensates for the loss of sensory input by becoming more easily activated. Changes required to lower the activation threshold may cause instability in the network. For example, it can cause the early visual cortex to become “hyper-excitable” and perhaps even generate its own activity without external sensory input. This spontaneous activity can cause hallucinations.
However, deafferentation alone cannot explain the content of visual hallucinations (Altieri & Battaglini, 2026). How could hyperexcitable visual areas explain why I kept repeating a keyboard hallucination every time I used my laptop? Why didn’t I hallucinate a piano, a cash register, or a blind kitten? Perhaps post-deafferentation instability created conditions that facilitated hallucinations.
Anticipatory processing theory may help to explain the pattern of deafferentation-induced activity (Altieri & Battaglini, 2026).
Anticipatory processing and hyperexcitability
Copywriting is a well-learned craft. Getting ready to type may have created the context that made my brain think the keyboard was in front of me. From these expectations and my many experiences with typing, my brain could predict the sensory cues (ie, tactile, auditory, visual, and proprioceptive signals) that should accompany the typing behavior.
My brain can then assess the degree of discrepancy between the predicted and actual sensory evidence. If the inconsistency was minimal, my brain would assume the keyboard was in front of me. This conclusion, combined with the act of turning my face into a keyboard, can influence the visual network to shape the internally generated sensory signals to the hallucinatory keyboard.
This proposed explanation is consistent with the claim of Albert Powers and colleagues that “we understand what must be present for our emotions to have meaning.”
My CBS hallucinations eventually went away, as they do for most people. The brain exhibits neural “plasticity” – it is able to reorganize itself and thus adapt to disruptions in its structural and functional organization. In many cases, this reorganization reduces the instability that may make hallucinations possible.
