Visual Hallucination
"To see is to believe"… until it isn't
Concepts — We assume:
-Vision = direct access to reality
-What we see = the external world
-Perception = truth
VH break this link
VH show the brain can generate convincing experiences w/o external input
Perception becomes internally driven, not sensory‑driven
Clinical importance
-Pure VH→ neuro > psych cause
-Especially common in PD/LBD due to cholinergic deficits, which reduce sensory precision & increase VH likelihood
Pathophysiology
VH emerge when:
-Sensory input is too weak
-Predictions are too strong
-Prediction errors are ignored or down‑weighted
→ A failure of perceptual updating
Expectancies
-Based on memory, context, intentions, emotions
>Hospital corridor at night→ expect a person
>Forest→ expect movement/animals
-If sensory data can’t correct predictions→ VH dominate
Sensory data
-If eyes/visual pathways provide strong data→ predictions get corrected
-If sensory data is weak→ predictions “win”
>Visual deprivation: darkness, eye disease, clutter→ major triggers
Attention sets the precision of sensory signals
-If attention leans toward expectations→ VH strengthens
-If attention is weak→ poor correction of faulty predictions
>Insight often decreases when attention and arousal drop
Object attention
-We choose which object to process deeply
>Dim room + chair + strong expectation of a person→ poor sensory input + poor object attention→ you see someone on the chair
Spatial attention — Orienting
-You move eyes/head toward expected location
-Strong expectation→ you “see” someone there even with minimal sensory evidence
Spatial attention — Modulating
-You boost peripheral sensitivity (often with anxiety)
-Ambiguous motion→ becomes an animal or figure (common in delirium)
Arousal
-Both very high and very low arousal increase VH risk
-Seen in sleep transitions, delirium, LBD fluctuations
Thalamocortical Synchrony
When disrupted:
-Prediction errors fail
-Updating fails
-Perception becomes unstable
→ fertile ground for VH
Trait & State
-VH = long‑term vulnerability + moment‑to‑moment instability
-Trait: memory‑based expectations, network degeneration, chronic attentional bias
-State: current predictions, sensory precision, arousal level, attention allocation
Why content varies (faces, animals, people)
-Because different higher‑level priors dominate:
>Faces→ fusiform face area bias (brain is hardwired to detect faces)
>Animals→ motion + biological form priors
>People→ social memory networks
Why patients often believe VH are real
-The same perceptual systems activate as if the object were truly present
-It’s not imagination — it’s perception without external input
Why VH are episodic
-VH need the “perfect storm”
-Weak input + strong predictions + attention shift + arousal fluctuation
→ explains waxing/waning in PD/LBD and delirium
Practical bedside triggers
-Dim lighting
-Fatigue
-Sensory deprivation
-Too much visual clutter
-Rapid changes in arousal
-Social isolation
-Small changes can flip the system from stable→ VH