People whose vision loss is caused by damage to the sensory pathways to the brain may experience visual hallucinations. Charles Bonnet syndrome (CBS). The main feature of CBS is complex visual hallucinations: fully formed animate or inanimate objects that are not related to anything serious. psychiatric disorders (Altieri & Battaglini, 2026; Pang, 2016). In fact, people with CBS usually show “full or partial awareness of the unreality of their hallucinations.”
For about two years after the onset of my blindness, I hallucinated people, bicycles, pillows, and plants. The hallucinations that interested me the most were the ones that happened repeatedly when I performed certain actions. For example, when I was learning to use the white cane, I sometimes found myself in the midst of hallucinating white cane students. And for a while I hallucinated the keyboard in front of me when I used the laptop.
Why did I experience specific hallucinations several times? And why don’t I have them anymore? In this post, I discuss current thinking in vision science to explore possible answers to these questions.
How the visual system works
In sighted people, visual perception begins when light activates sensory receptors in the eyes. Creates activated receptors nervous signals transmitted to visual areas in the cerebral cortex—a large outer layer of the brain (Mars et al., 2025).
The brain’s visual system is hierarchically organized (Powers et al., 2016). The “early visual cortex” (that is, the first areas that receive sensory information from the eye) specializes in processing the main features of incoming sensory signals. The “subsequent visual cortex” (ie, visual areas at higher levels of the hierarchy) is specialized for processing increasingly abstract and complex features of sensory input. For example, Dominic Ffytche and colleagues (1998) showed that the content of CBS hallucinations corresponds to perceptual features processed by higher-order cortical regions that are typically active during hallucinations. For example, when someone hallucinated a face, the area specialized for perceiving faces was active.
But no visual field is responsible for creating a certain “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 perceptual processes:
- Bottom-up processing analyzes incoming sensory information into its component perceptual properties.
- Top-down processing refers to using prior knowledge to reduce uncertainty in incoming sensory signals.
This view of perception involves interpreting rather than reproducing what the eyes see.
For example, after hearing about a person’s encounter with a venomous desert snake, you may be sensitive to the possibility while hiking a desert trail. You may even briefly mistake a fallen tree branch for 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 experiences to predict the sensory input that would occur if the object were a snake. Your brain evaluates the degree of discrepancy between predicted sensory evidence and actual sensory evidence. If the discrepancy is minimal, your visual system can see the snake. A second glance may cause you to revise your prediction so that you now see a tree branch.
Predictive processing theory views conscious visual perception as the brain’s best guess of what the eyes are looking at.
Predictive processing and CBS hallucinations
CBS hallucinations can be explained by combining two theories: the predictive processing theory and the “deafferentation theory” (Altieri & Battaglini, 2026; Marschall et al., 2020).
Basic Readings on Charles Bonnet Syndrome
“Deafferentation” refers to the loss of sensory input to perceptual areas of the brain. CBS hallucinations have long been associated with deafferentation (Burke, 2002; Painter et al., 2018).
Some have suggested that deafferentation hallucinations are due to changes in the relative influence of bottom-up and top-down processing (Marschall et al., 2020). The visual system compensates for the loss of sensory input by easily activating it. Changes required to lower the activation threshold may cause instability in the network. For example, this may cause the early visual cortex to become “hyper-excitable”, perhaps generating its own activity without external input. This spontaneous activity can cause hallucinations.
But deafferentation alone cannot explain the content of visual hallucinations (Altieri & Battaglini, 2026). How can high excitability visual fields explain why I repeatedly experience the same keyboard hallucination every time I use my laptop? Why didn’t I hallucinate a piano, a cash register, or a furry kitten? Perhaps the post-deafferentation instability created conditions that facilitated hallucinations.
Predictive processing theory may help explain the pattern of activity resulting from deafferentation (Altieri and Battaglini, 2026).
Anticipatory processing and hyperarousal
Typing is a well-learned activity. Preparing to type may have created a context in my mind that led me to expect the keyboard to be right in front of me. Based on this anticipation and my many experiences with typing, my brain could predict the sensory evidence (ie, tactile, auditory, visual, and proprioceptive cues) that should accompany the act of writing.
My brain may have overestimated the degree of discrepancy between predicted and actual sensory evidence. If the discrepancy was minimal, my brain would have concluded that the keyboard was in front of me. This conclusion, combined with the act of turning my face toward the location of the keyboard, may have influenced the visual network to transform the sensory signals generated internally to the hallucinatory keyboard.
This proposed explanation is consistent with the claim of Albert Powers and his colleagues that “we understand what must happen for our intuitions to make sense.”
As it does for many, my CBS hallucinations eventually went away. The brain exhibits neural “plasticity” – it is able to reorganize itself and thereby adapt to disturbances in its structural and functional structure. In many cases, this reorganization reduces the instability that can cause hallucinations.
